Classification of elements

Classification of Elements : 

Classification of Elements

In the early part of the 19th century, many chemists noted that chemical properties of elements showed some similarities. The elements could be formed into groups. In 1817, Dobereiner showed that elements came in groups of three, now known as Dobereiner’s triads. In 1863, a 44 year old French geologist, A. E. Béguyer de Chancourtois created a list of the elements, arranged by increasing atomic weight. The list was wrapped around a cylinder so that several sets of similar elements lined up, creating the first geometric representation of the periodic law. In England, a 32 year old analytical chemist John A. R. Newlands was also wrapping the elements, noting that chemical groups repeated every eight elements. He named this the octave rule, and compared it to a musical scale. Chemists Dmitrii I. Mendeleev, a Russian, and German Lothar Meyer, were working independently in 1868 and 1869 on the arrangement of elements into seven columns, corresponding to various chemical and physical properties. Their tables were similar - Meyer's table was an accurate accounting of the known facts about each element, such as melting point and atomic volume : 

In the early part of the 19th century, many chemists noted that chemical properties of elements showed some similarities. The elements could be formed into groups. In 1817, Dobereiner showed that elements came in groups of three, now known as Dobereiner’s triads. In 1863, a 44 year old French geologist, A. E. Béguyer de Chancourtois created a list of the elements, arranged by increasing atomic weight. The list was wrapped around a cylinder so that several sets of similar elements lined up, creating the first geometric representation of the periodic law. In England, a 32 year old analytical chemist John A. R. Newlands was also wrapping the elements, noting that chemical groups repeated every eight elements. He named this the octave rule, and compared it to a musical scale. Chemists Dmitrii I. Mendeleev, a Russian, and German Lothar Meyer, were working independently in 1868 and 1869 on the arrangement of elements into seven columns, corresponding to various chemical and physical properties. Their tables were similar - Meyer's table was an accurate accounting of the known facts about each element, such as melting point and atomic volume

What we will study in this chapter : 

What we will study in this chapter 1. Dobereiner’s triads2. Newlands’ Law of octaves3. Lothar Mayer’s atomic volume curves4. Mendeleev’s periodic table5. Modern periodic table

1. Dobereiner’s triads : 

1. Dobereiner’s triads

Dobereiner in 1817 observed that certain elements, which had similar chemical properties, could be grouped together. When these elements were arranged in increasing order of their atomic masses, they generally occurred in groups of three. These groups were called triads. He noticed that the atomic mass of the middle element of the triad was the arithmetic mean of the other two elements of the triad. This was known as the Dobereiner’s law of triads. The law states that : when elements are placed in order of the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean of the atomic masses of the other two elements of the triad. Drawbacks of Deberneir’s law of triad, was that it was valid only for a few groups of elements known during that time. Also with more accurate measurements of atomic masses showed that the mid element of the triad did not really have the mean value of the sum of the other two elements of the triad. : 

Dobereiner in 1817 observed that certain elements, which had similar chemical properties, could be grouped together. When these elements were arranged in increasing order of their atomic masses, they generally occurred in groups of three. These groups were called triads. He noticed that the atomic mass of the middle element of the triad was the arithmetic mean of the other two elements of the triad. This was known as the Dobereiner’s law of triads. The law states that : when elements are placed in order of the ascending order of atomic masses, groups of three elements having similar properties are obtained. The atomic mass of the middle element of the triad is equal to the mean of the atomic masses of the other two elements of the triad. Drawbacks of Deberneir’s law of triad, was that it was valid only for a few groups of elements known during that time. Also with more accurate measurements of atomic masses showed that the mid element of the triad did not really have the mean value of the sum of the other two elements of the triad.

Examples of Dobereiner Triads : In the alkali metal group, consider elements lithium (Li), sodium (Na) and potassium (K). All these elements are metals, they are highly reactive and they show valency of +1. The Dobereiner’s triad for alkali metal group can be shown as: Elements   Symbol   A (atomic mass)            Lithium Li   7          Sodium Na   23            Potassium   K 39   From the Dobereiner’s law of triads, the atomic mass of the middle element, in this case Na, should be the arithmetic mean of Li and K. Thus                                                    arithmetic mean of Li and K = 7 + 39 =   23 2                    It can be seen that Arithmetic mean of atomic masses of Li and K = atomic mass of Na. : 

Examples of Dobereiner Triads : In the alkali metal group, consider elements lithium (Li), sodium (Na) and potassium (K). All these elements are metals, they are highly reactive and they show valency of +1. The Dobereiner’s triad for alkali metal group can be shown as: Elements   Symbol   A (atomic mass)            Lithium Li   7          Sodium Na   23            Potassium   K 39   From the Dobereiner’s law of triads, the atomic mass of the middle element, in this case Na, should be the arithmetic mean of Li and K. Thus                                                    arithmetic mean of Li and K = 7 + 39 =   23 2                    It can be seen that Arithmetic mean of atomic masses of Li and K = atomic mass of Na.

Now consider elements in the halogen group : chlorine (Cl), bromine (Br) and iodine (I). All these elements are non-metallic, they are very reactive and form acids with water, they have a valency of –1. Due to their similar chemical properties, these three elements formed another of Dobereiner’s triad. So see if the Cl, Br, I obey the Dobereiner’s law of triad, consider the following table. Elements   Symbol   A (atomic mass)            ChlorineCl 35.5         BromineBr 80          Iodine I127 For Dobereiner’s law to be valid A (Br) =  A (Cl) + A (I) = 35.5 + 127 =      81.2              2                                         2        The actual atomic mass of Br is 80. Thus the atomic mass of the middle element of the triad, is nearly equal to the arithmetic mean of the atomic masses of the other two elements of the triad. Hence the Dobereiner’s law holds true for halogen triads. : 

Now consider elements in the halogen group : chlorine (Cl), bromine (Br) and iodine (I). All these elements are non-metallic, they are very reactive and form acids with water, they have a valency of –1. Due to their similar chemical properties, these three elements formed another of Dobereiner’s triad. So see if the Cl, Br, I obey the Dobereiner’s law of triad, consider the following table. Elements   Symbol   A (atomic mass)            ChlorineCl 35.5         BromineBr 80          Iodine I127 For Dobereiner’s law to be valid A (Br) =  A (Cl) + A (I) = 35.5 + 127 =      81.2              2                                         2        The actual atomic mass of Br is 80. Thus the atomic mass of the middle element of the triad, is nearly equal to the arithmetic mean of the atomic masses of the other two elements of the triad. Hence the Dobereiner’s law holds true for halogen triads.

Consider another group of elements : sulphur (S), selenium (Se) and tellurium (Te). All these elements are non-metals, tending to show metallic behavior. When you arrange them in the ascending order of their atomic masses, they obey Dobereiner’s law. See the table given below.   Elements   Symbol   A (atomic mass)            SulphurS 32         SeleniumSe 79          Tellurium Te128   We can verify that                      A (Se) =  A (S) + A (Te)           2   Dobereiner’s law of triads failed for the following reasons :   all the then known elements could not be arranged in the form of triads. for very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br. as the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly validThe only advantage of Dobereiner’s research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered. : 

Consider another group of elements : sulphur (S), selenium (Se) and tellurium (Te). All these elements are non-metals, tending to show metallic behavior. When you arrange them in the ascending order of their atomic masses, they obey Dobereiner’s law. See the table given below.   Elements   Symbol   A (atomic mass)            SulphurS 32         SeleniumSe 79          Tellurium Te128   We can verify that                      A (Se) =  A (S) + A (Te)           2   Dobereiner’s law of triads failed for the following reasons :   all the then known elements could not be arranged in the form of triads. for very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br. as the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly validThe only advantage of Dobereiner’s research was that it made chemists look at elements in terms of groups of elements with similar chemical and physical properties. This eventually led to rigorous classification of elements and the modern periodic table of elements, as we now know it, was discovered.

2. Newlands’ Law of octaves : 

2. Newlands’ Law of octaves

After Dobereiner’s ideas were ruled out, an English chemist John Alexander Reina Newlands in 1864 noted that every eighth element showed similar physical and chemical properties, when the elements are placed in the increasing order of their atomic masses. This was called as the Newlands’ law of octaves. The law states that when elements are placed in the increasing order of atomic masses, the properties of the eight element are repeated. Newlands arranged the elements then known in the following manner. Li Be B C N O F Na Mg Al Si P S Cl K Ca Row of elements had seven elements and the eighth fell under the first element. In those days, the number of elements known were very limited and no elements from the noble or inert gas elements such as helium (He), neon (Ne), argon (Ar), etc. were known. First let us see the elements in the first column. Li is the first element. The eighth element after Li is Na. Similarly, the eighth element after Na is K. So from the Newlands' law of octaves, we should expect the elements Li, Na and K to have similar chemical properties. This they do have. All the elements are metallic, highly reactive and show a valence of +1. They are known as alkali elements.Next, if we take beryllium (Be) as the first element, the eighth element from Be is magnesium (Mg). If we continue in the similar fashion, the eighth element after Mg is calcium Ca. According to Newlands’ law, the elements Be, Mg and Ca should display similar chemical and physical properties. They do. The elements Be, Mg, Ca fall under the group of alkali-earth metals. All these elements are metallic in nature, their oxides are alkaline in nature and they have a valence of +2. : 

After Dobereiner’s ideas were ruled out, an English chemist John Alexander Reina Newlands in 1864 noted that every eighth element showed similar physical and chemical properties, when the elements are placed in the increasing order of their atomic masses. This was called as the Newlands’ law of octaves. The law states that when elements are placed in the increasing order of atomic masses, the properties of the eight element are repeated. Newlands arranged the elements then known in the following manner. Li Be B C N O F Na Mg Al Si P S Cl K Ca Row of elements had seven elements and the eighth fell under the first element. In those days, the number of elements known were very limited and no elements from the noble or inert gas elements such as helium (He), neon (Ne), argon (Ar), etc. were known. First let us see the elements in the first column. Li is the first element. The eighth element after Li is Na. Similarly, the eighth element after Na is K. So from the Newlands' law of octaves, we should expect the elements Li, Na and K to have similar chemical properties. This they do have. All the elements are metallic, highly reactive and show a valence of +1. They are known as alkali elements.Next, if we take beryllium (Be) as the first element, the eighth element from Be is magnesium (Mg). If we continue in the similar fashion, the eighth element after Mg is calcium Ca. According to Newlands’ law, the elements Be, Mg and Ca should display similar chemical and physical properties. They do. The elements Be, Mg, Ca fall under the group of alkali-earth metals. All these elements are metallic in nature, their oxides are alkaline in nature and they have a valence of +2.

Now look at another vertical column that has carbon (C) as the first element. The eighth element from C is silicon (Si). It is seen that C and Si are similar in properties. Both of them show tetra-valency. They are non-metals and form oxides easily. Thus Newlands' law of octaves hold good.Similarly lets see the last group of halogens starting with flourine (F). The eighth element after F is chlorine (Cl). As we have already seen that F and Cl display similar properties. Both of them are highly reactive, when dissolved in water, form acids, and have valence of -1. Thus the Newlands' law of octaves was obeyed.Newlands' Law of octaves failed for the following reasons :1. It was not valid for elements that had atomic masses higher than Ca.2. When more elements were discovered, such as elements from the noble gases such as He, Ne, Ar, they could not be accommodated in his table.But the most important contribution in the process of classification of elements was the periodicity he saw in every eighth element. The modern periodic table, that we shall study in later sections, drew heavily from the concept of periods of eight. Also it must be noted that Dobereiner’s triads occurred in the octaves of Newlands. : 

Now look at another vertical column that has carbon (C) as the first element. The eighth element from C is silicon (Si). It is seen that C and Si are similar in properties. Both of them show tetra-valency. They are non-metals and form oxides easily. Thus Newlands' law of octaves hold good.Similarly lets see the last group of halogens starting with flourine (F). The eighth element after F is chlorine (Cl). As we have already seen that F and Cl display similar properties. Both of them are highly reactive, when dissolved in water, form acids, and have valence of -1. Thus the Newlands' law of octaves was obeyed.Newlands' Law of octaves failed for the following reasons :1. It was not valid for elements that had atomic masses higher than Ca.2. When more elements were discovered, such as elements from the noble gases such as He, Ne, Ar, they could not be accommodated in his table.But the most important contribution in the process of classification of elements was the periodicity he saw in every eighth element. The modern periodic table, that we shall study in later sections, drew heavily from the concept of periods of eight. Also it must be noted that Dobereiner’s triads occurred in the octaves of Newlands.

3. Lothar Meyer’s atomic volume curves : 

3. Lothar Meyer’s atomic volume curves

A German chemist Julius Lothar Meyer in 1869 was looking at physical properties of elements along with their valence states. He made a table that contained a preliminary tabulation of 28 elements. The table showed how the integral valence changed as the atomic weight of elements increased. Meyer considered the volume taken up by fixed weights of the various elements. Under such conditions, each weight contained the same number of atoms of its particular element (Avogadro’s number). This meant that the ratio of the volumes ofLothar Meyerthe various elements was equal to the ratio of the volumes of single atoms of the various elements. Thus Lothar Meyer could determine the atomic volumes of elements.If the atomic volumes of the elements were plotted against the atomic weight, a series of peaks were produced. The peaks had alkali metals: sodium, potassium, rubidium, and cesium. Each fall and rise to a peak, corresponded to a period like the waves. In each period a number of physical properties other than atomic volume also fell and rose, such as valence and melting point. Figure below shows the curve obtained by Lothar Meyer when he plotted the atomic masses and the respective atomic volumes.   . : 

A German chemist Julius Lothar Meyer in 1869 was looking at physical properties of elements along with their valence states. He made a table that contained a preliminary tabulation of 28 elements. The table showed how the integral valence changed as the atomic weight of elements increased. Meyer considered the volume taken up by fixed weights of the various elements. Under such conditions, each weight contained the same number of atoms of its particular element (Avogadro’s number). This meant that the ratio of the volumes ofLothar Meyerthe various elements was equal to the ratio of the volumes of single atoms of the various elements. Thus Lothar Meyer could determine the atomic volumes of elements.If the atomic volumes of the elements were plotted against the atomic weight, a series of peaks were produced. The peaks had alkali metals: sodium, potassium, rubidium, and cesium. Each fall and rise to a peak, corresponded to a period like the waves. In each period a number of physical properties other than atomic volume also fell and rose, such as valence and melting point. Figure below shows the curve obtained by Lothar Meyer when he plotted the atomic masses and the respective atomic volumes.   .

Hydrogen, the first in the list of elements is a special case and can be considered as making up the first period all by itself. The second and third period in Meyer's table included seven elements each, and duplicated Newlands's law of octaves.Li Be B C N O F Na Mg Al Si P S Cl However, the next wave had more than seven elements. The third wave had about 17 to 18 elements. This clearly showed where Newlands’ law had failed. One could not force the law of octaves to hold strictly throughout the table of elements, with seven elements in each row. After the first two periods, the length of the period had to be longer.If we see the atomic mass versus atomic volume curve, we will notice the following features :Alkali metals such as Na. K, Rb that have similar properties, occur as peaks of the curve. Halogen elements like F, Cl, Br, that have similar properties, occur at the rising or the ascending part of the curve. Noble gasses such as Ne, Ar, Kr , that have similar properties, occur just before the alkali elements. H, He seem to be exception to the rule. Meyer published his work in 1870, but before that Dimitrii Medeleev has shown a neat version of how elements can be arranged in periods and the periods increased with atomic masses. Mendeleev retained Mayer’s observation that H is an exception to the periodic behaviour. : 

Hydrogen, the first in the list of elements is a special case and can be considered as making up the first period all by itself. The second and third period in Meyer's table included seven elements each, and duplicated Newlands's law of octaves.Li Be B C N O F Na Mg Al Si P S Cl However, the next wave had more than seven elements. The third wave had about 17 to 18 elements. This clearly showed where Newlands’ law had failed. One could not force the law of octaves to hold strictly throughout the table of elements, with seven elements in each row. After the first two periods, the length of the period had to be longer.If we see the atomic mass versus atomic volume curve, we will notice the following features :Alkali metals such as Na. K, Rb that have similar properties, occur as peaks of the curve. Halogen elements like F, Cl, Br, that have similar properties, occur at the rising or the ascending part of the curve. Noble gasses such as Ne, Ar, Kr , that have similar properties, occur just before the alkali elements. H, He seem to be exception to the rule. Meyer published his work in 1870, but before that Dimitrii Medeleev has shown a neat version of how elements can be arranged in periods and the periods increased with atomic masses. Mendeleev retained Mayer’s observation that H is an exception to the periodic behaviour.

4. Mendeleev’s periodic table : 

4. Mendeleev’s periodic table

Working on the research done by Newlands, in 1869 a 35 year old Russian chemist named Dimitry Mendeleev presented a much bolder and more scientifically useful table of elements. In it, the periodic relationship between chemical groups is clearly illustrated. Mendeleev saw that there is a periodicity occurring in the physical and chemical properties, if the elements were arranged in order of their atomic weights. The periodic table gave order to the large amount of data available for all the elements. In Mendeleev’s time there were about 60 to 70 known elements.  Dimitry Mendeleev The periodic table thus gave a chart of elements grouped in such a manner that elements showing similar properties occur in the same vertical group. Mendeleev’s periodic law states that the properties of elements are a periodic function of their atomic mases. Figure below shows the chart of elements initially made by Mendeleev. The elements are arranged in such a manner that the vertical columns are called groups and horizontal columns are called periods. Elements in each group have similar physical and chemical properties (valence, melting point). The periods are made with elements written in rows of increasing atomic masses. As one goes vertically downwards in a group, the elements show increase in atomic volume. The first two periods are similar to Newlands law of octaves. The feature within each group thus explained Lothar Meyer’s observations also.Mendeleev did something quite ingenious. He placed elements that had similar properties under vertical columns, even if other elements were not found. For example he placed titanium (Ti) under silicon (Si) as he saw that Ti and Si had similar properties. Thus there was a gap below aluminum (Al) in the group and after calcium (Ca) in the horizontal period. As if this were not enough, he also found it necessary to leave gaps altogether in his table. Rather than considering these gaps as imperfections in the table, Mendeleev said that they represented elements as yet undiscovered. : 

Working on the research done by Newlands, in 1869 a 35 year old Russian chemist named Dimitry Mendeleev presented a much bolder and more scientifically useful table of elements. In it, the periodic relationship between chemical groups is clearly illustrated. Mendeleev saw that there is a periodicity occurring in the physical and chemical properties, if the elements were arranged in order of their atomic weights. The periodic table gave order to the large amount of data available for all the elements. In Mendeleev’s time there were about 60 to 70 known elements.  Dimitry Mendeleev The periodic table thus gave a chart of elements grouped in such a manner that elements showing similar properties occur in the same vertical group. Mendeleev’s periodic law states that the properties of elements are a periodic function of their atomic mases. Figure below shows the chart of elements initially made by Mendeleev. The elements are arranged in such a manner that the vertical columns are called groups and horizontal columns are called periods. Elements in each group have similar physical and chemical properties (valence, melting point). The periods are made with elements written in rows of increasing atomic masses. As one goes vertically downwards in a group, the elements show increase in atomic volume. The first two periods are similar to Newlands law of octaves. The feature within each group thus explained Lothar Meyer’s observations also.Mendeleev did something quite ingenious. He placed elements that had similar properties under vertical columns, even if other elements were not found. For example he placed titanium (Ti) under silicon (Si) as he saw that Ti and Si had similar properties. Thus there was a gap below aluminum (Al) in the group and after calcium (Ca) in the horizontal period. As if this were not enough, he also found it necessary to leave gaps altogether in his table. Rather than considering these gaps as imperfections in the table, Mendeleev said that they represented elements as yet undiscovered.

In 1871, he pointed to three gaps in particular, those falling next to the elements boron, aluminum, and silicon in the table. He named the unknown elements as eka-aluminum, and eka-silicon ("eka" is the Sanskrit word for "one"). He also predicted various properties of these missing elements, such as density, boiling point, judging from what these must be from the properties of the elements above and below the gaps in his table. Much later on, Gallium (Ga) and Germanium (Ge) were found, which had same properties as eka-aluminum, and eka-silicon respectively. This demonstrated the success of Mendeleev's periodic table of elements. Although very successful, Mendeleev’s periodic table had the following problems :   1. The positions of isotopes could not be accommodated within the table. As you well know, isotopes are elements having same properties but different atomic masses (same proton number Z, but different neutron number N. Thus atomic mass A = Z + N differs in isotopes). If one obeys Mendeleev’s periodic law then the variation in chemical properties vertically in a group is not followed strictly. For example, an isotope of carbon is 14C. This would have to be accommodated along with nitrogen (Fig 1). But 14C shows properties same as normal carbon (12C). Mendeleev was thus unable to place isotopes in his table. 2. In order to make the elements fit the requirements, that those in a particular column all have the same valence, Mendeleev was forced in one or two cases to put an element of slightly higher atomic weight ahead of one of slightly lower atomic weight. Thus, tellurium (Te) (atomic weight 127.6, valence 2) had to be put ahead of iodine (I) (atomic weight 126.9, valence 1) in order to keep tellurium in the valence-2 column and iodine in the valence-1 column. Success of Mendeleev’s periodic table :   1. When Mendeleev presented his periodic table, inert gas elements like He, Ne, Ar were not discovered. When they were discovered, they could be neatly put in as the last group of elements, without disturbing the rest of the table. 2. His predictions about unknown elements from gaps in his table were a great success. Scientists who repeatedly discover newer and newer elements follow this feature. Although the periodic table is now complete with about 109 different elements (naturally occurring and artificially made in the lab), scientists still use Mendeleev’s method to predict properties of elements by looking vertically across groups and horizontally across periods of elements. : 

In 1871, he pointed to three gaps in particular, those falling next to the elements boron, aluminum, and silicon in the table. He named the unknown elements as eka-aluminum, and eka-silicon ("eka" is the Sanskrit word for "one"). He also predicted various properties of these missing elements, such as density, boiling point, judging from what these must be from the properties of the elements above and below the gaps in his table. Much later on, Gallium (Ga) and Germanium (Ge) were found, which had same properties as eka-aluminum, and eka-silicon respectively. This demonstrated the success of Mendeleev's periodic table of elements. Although very successful, Mendeleev’s periodic table had the following problems :   1. The positions of isotopes could not be accommodated within the table. As you well know, isotopes are elements having same properties but different atomic masses (same proton number Z, but different neutron number N. Thus atomic mass A = Z + N differs in isotopes). If one obeys Mendeleev’s periodic law then the variation in chemical properties vertically in a group is not followed strictly. For example, an isotope of carbon is 14C. This would have to be accommodated along with nitrogen (Fig 1). But 14C shows properties same as normal carbon (12C). Mendeleev was thus unable to place isotopes in his table. 2. In order to make the elements fit the requirements, that those in a particular column all have the same valence, Mendeleev was forced in one or two cases to put an element of slightly higher atomic weight ahead of one of slightly lower atomic weight. Thus, tellurium (Te) (atomic weight 127.6, valence 2) had to be put ahead of iodine (I) (atomic weight 126.9, valence 1) in order to keep tellurium in the valence-2 column and iodine in the valence-1 column. Success of Mendeleev’s periodic table :   1. When Mendeleev presented his periodic table, inert gas elements like He, Ne, Ar were not discovered. When they were discovered, they could be neatly put in as the last group of elements, without disturbing the rest of the table. 2. His predictions about unknown elements from gaps in his table were a great success. Scientists who repeatedly discover newer and newer elements follow this feature. Although the periodic table is now complete with about 109 different elements (naturally occurring and artificially made in the lab), scientists still use Mendeleev’s method to predict properties of elements by looking vertically across groups and horizontally across periods of elements.

5. Modern periodic table : 

5. Modern periodic table

As more and more elements and their isotopes were discovered, Mendeleev’s periodic table that gave a chart of all the elements, was found to be inconsistent in many ways. Henry Moseley, in 1913, found the reason for the inconsistencies. He performed X-ray experiments on the elements and found that each element has an integral positive charge, the atomic number (Z) or the number of protons. Moseley revised the periodic table and made a bold change that removed all inconsistencies. He suggested that instead of arranging elements in the ascending order of their atomic masses, they should be arranged in the ascending order of their atomic numbers. The number of electrons in an atom is equal to the atomic number Z. Thus by making this change, Moseley put the emphasis on electronic configuration of the elements. Also it must be remembered that all physical and chemical properties come about because of the arrangements of electrons.The modern statement of the periodic law is that the properties of the elements are a periodic function of their atomic numbers. In the modern periodic table, atoms with similar electron configurations are placed in the same column. The columns are called groups. Elements across in periods show integral increase in valence. The figure below shows how elements in Group 1, the alkali elements, are arranged. All of them have single valence electron and display similar properties of chemical reactivity, formation of oxides, etc. The elements in period 3 are also shown. The elements in a period show increase in the last electron configuration : 

As more and more elements and their isotopes were discovered, Mendeleev’s periodic table that gave a chart of all the elements, was found to be inconsistent in many ways. Henry Moseley, in 1913, found the reason for the inconsistencies. He performed X-ray experiments on the elements and found that each element has an integral positive charge, the atomic number (Z) or the number of protons. Moseley revised the periodic table and made a bold change that removed all inconsistencies. He suggested that instead of arranging elements in the ascending order of their atomic masses, they should be arranged in the ascending order of their atomic numbers. The number of electrons in an atom is equal to the atomic number Z. Thus by making this change, Moseley put the emphasis on electronic configuration of the elements. Also it must be remembered that all physical and chemical properties come about because of the arrangements of electrons.The modern statement of the periodic law is that the properties of the elements are a periodic function of their atomic numbers. In the modern periodic table, atoms with similar electron configurations are placed in the same column. The columns are called groups. Elements across in periods show integral increase in valence. The figure below shows how elements in Group 1, the alkali elements, are arranged. All of them have single valence electron and display similar properties of chemical reactivity, formation of oxides, etc. The elements in period 3 are also shown. The elements in a period show increase in the last electron configuration

The modern periodic table is a very neat representation of all elements. The chart is easy to read and the arrangement is so accurate, that if you know properties of a few elements, you will be able to make a close guess of other elements close to them. The table also eliminated anomalies in Mendeleev’s periodic table of elements.Removal of anomalies in Mendeleev’s periodic table1. The position of isotopes is taken care of when the elements are arranged in the ascending order of their atomic numbers. The isotopes will occur at the same position as its original element since the electronic configuration of the element and its isotopes is identical [1]. Let us go back to the example of the isotope of carbon, 14C. This would have to be accommodated now along with carbon (12C) itself and not with nitrogen. 2. The anomaly regarding e few elements such as tellurium (Te) – iodine (I) and argon (A) – potassium (K) were solved elegantly. Although Te had a higher atomic mass, Mendeleev was forced to place it ahead of I which had a lower atomic mass. But if one sees their atomic numbers, Te is 52 and I is 53. Thus Te will naturally come before I when atomic numbers are considered. Similarly for the pair of Ar-K. Thus the modern periodic table of elements removed all anomalies of the Mendeleev’s periodic table, by simple considering the atomic numbers of elements. : 

The modern periodic table is a very neat representation of all elements. The chart is easy to read and the arrangement is so accurate, that if you know properties of a few elements, you will be able to make a close guess of other elements close to them. The table also eliminated anomalies in Mendeleev’s periodic table of elements.Removal of anomalies in Mendeleev’s periodic table1. The position of isotopes is taken care of when the elements are arranged in the ascending order of their atomic numbers. The isotopes will occur at the same position as its original element since the electronic configuration of the element and its isotopes is identical [1]. Let us go back to the example of the isotope of carbon, 14C. This would have to be accommodated now along with carbon (12C) itself and not with nitrogen. 2. The anomaly regarding e few elements such as tellurium (Te) – iodine (I) and argon (A) – potassium (K) were solved elegantly. Although Te had a higher atomic mass, Mendeleev was forced to place it ahead of I which had a lower atomic mass. But if one sees their atomic numbers, Te is 52 and I is 53. Thus Te will naturally come before I when atomic numbers are considered. Similarly for the pair of Ar-K. Thus the modern periodic table of elements removed all anomalies of the Mendeleev’s periodic table, by simple considering the atomic numbers of elements.

Slide 23: 

Neils Bohr prepared the present version of periodic table. Figure below gives the modern periodic table or what is called as the long form of the periodic table

The features of the periodic table are:1. It consists of 7 horizontal periods. The lengths of the periods increase with the order of the period. Elements in a period have consecutive atomic numbers.The 1st period is the shortest period. It consists of just two elements H and He. The 2nd and the 3rd periods have 8 elements each and are called short periods. The 4th and the 5th periods are long periods and have 18 elements each. The 6th period has 32 elements. The period has a 15 element series called Lanthanide series, separated from the table. The lanthanide series are rare-earth elements that show similar properties. The 7th period contains all the rest of the elements. It is incomplete. This period also has a 15 element series called the Actinide series, separated from the table. The actinide series have a separate identity and contains uranium and most of the known transuranic elements. : 

The features of the periodic table are:1. It consists of 7 horizontal periods. The lengths of the periods increase with the order of the period. Elements in a period have consecutive atomic numbers.The 1st period is the shortest period. It consists of just two elements H and He. The 2nd and the 3rd periods have 8 elements each and are called short periods. The 4th and the 5th periods are long periods and have 18 elements each. The 6th period has 32 elements. The period has a 15 element series called Lanthanide series, separated from the table. The lanthanide series are rare-earth elements that show similar properties. The 7th period contains all the rest of the elements. It is incomplete. This period also has a 15 element series called the Actinide series, separated from the table. The actinide series have a separate identity and contains uranium and most of the known transuranic elements.

2. The vertical columns are called groups. There are 18 groups in the periodic table. Elements in a group do not have consecutive atomic numbers. The groups are divided into A and B groups. Group 1A to VIII A has all the normal elements. Group 1B to VIII B holds all the transition metal elements. The other two groups are the lanthanide and the actinide series. They are also known as inner transition elements. 3. The modern periodic table is approximately divided into metals and non-metals. The most metallic elements such as alkalis are on the left-hand side. The non-metals are on the right hand side. The inert gases or the noble gases with their completely filled electronic shells are placed on the extreme right hand side. The transition metals, which are a bridge between highly metallic alkali elements and the non-metals, lie in the centre of the table. Lanthanide and actinide series (or the inner transition elements), which have metal like behavior, are kept separately as their outermost electronic configurations differ from the transition metal elements. A c [1] A chart of isotopes is needed in nuclear physics. This chart follows the principle of the periodic table of elements but N (neutron number) versus Z (proton number) is plotted. : 

2. The vertical columns are called groups. There are 18 groups in the periodic table. Elements in a group do not have consecutive atomic numbers. The groups are divided into A and B groups. Group 1A to VIII A has all the normal elements. Group 1B to VIII B holds all the transition metal elements. The other two groups are the lanthanide and the actinide series. They are also known as inner transition elements. 3. The modern periodic table is approximately divided into metals and non-metals. The most metallic elements such as alkalis are on the left-hand side. The non-metals are on the right hand side. The inert gases or the noble gases with their completely filled electronic shells are placed on the extreme right hand side. The transition metals, which are a bridge between highly metallic alkali elements and the non-metals, lie in the centre of the table. Lanthanide and actinide series (or the inner transition elements), which have metal like behavior, are kept separately as their outermost electronic configurations differ from the transition metal elements. A c [1] A chart of isotopes is needed in nuclear physics. This chart follows the principle of the periodic table of elements but N (neutron number) versus Z (proton number) is plotted.

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