Atoms and Molecules

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Atom :

Atom An atom is the smallest particle of an element that may or may not exist independently and retains all its chemical properties. Atoms are very small in size and smaller than anything we can imagine or compare with. Atomic radius is measured in nanometers (nm) 1 nanometer = 10 -9 m or 1 meter = 10 9 nm E.g. :- The atomic radius of an atom of hydrogen is 10 -10 m. The radius of a molecule of water is 10 -9 m.

Fundamental particles of an atom:

Fundamental particles of an atom There are three fundamental particles of an atom. They are: Electron Proton Neutron Position of electron , proton and neutron: Electron: It revolves round the nucleus. Proton: Inside the nucleus. Neutron: Inside the nucleus.

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Why atom as a whole is regarded as neutral? Atom as a whole is regarded as neutral because it consists of equal number of positive charge ( proton ) and negative charge ( electron ) . Both equal and opposite charges cancel each other and atom become neutral. Where the whole mass of an atom is concentrated? In the nucleus of an atom. Atomic mass= n+p=A Atomic no.= e=p=Z


NUCLEUS Nucleus of an atom is the dense part of an atom where whole mass of an atom is concentrated. It consists of proton and neutron.

Laws of chemical combination :

Laws of chemical combination a) Law of conservation of mass :It states that ‘Mass can neither be created nor destroyed in a chemical reaction’. This means that during a chemical reaction the sum of the masses of the reactants and products remain unchanged and it is conserved.

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Activity : Take some sodium sulphate solution in a conical flask and some barium chloride solution in an ignition tube. Hang the ignition tube in the flask by a thread and put a cork on the flask. Find the mass of the flask on a balance. Then tilt the flask. A chemical reaction takes place and sodium chloride and barium sulphate are formed. Then find the mass of the flask again. It will be seen that the sum of the masses of the reactants and products remain unchanged.

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It states that ‘ In a chemical compound the elements are always present in a definite proportion by mass’. E.g. : Water ( H 2 O) always contains two elements, hydrogen and oxygen combined together in the same ratio of 2:16 or 1:8 by mass. If 9 g of water is decomposed we get 1 g of hydrogen and 8 g of oxygen. Ammonia (NH3) always contains two elements, nitrogen and hydrogen combined together in the same ratio of 14:3 by mass. b) Law of constant proportions :

Dalton’s atomic theory ::

Dalton’s atomic theory : a) Matter is made up of tiny particles called atoms. b) Atoms are indivisible and cannot be created or destroyed in a chemical reaction. c) Atoms of a given element are identical in mass and chemical properties. d) Atoms of different elements has different masses and properties. e) Atoms combine in the ratio of small whole number to form compounds . f) In a given compound the relative number and kind of atoms are constant.

Symbols of atoms of different elements :

Symbols of atoms of different elements The symbols of elements are represented by letters. The symbols of some elements are represented by one letter and the symbols of some elements are represented by two letters. If the symbol has only one letter it should be written in capital letter and if the symbol has two letters then the first letter should be capital letter and the second letter should be small letter . E.g. C, Hg etc.

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Symbol of elements proposed by Dalton

Symbols of some common elements :

Symbols of some common elements Element Symbol Element Symbol Aluminium Al Copper Cu Argon Ar Fluorine F Barium Ba Gold Au Boron B Hydrogen H Bromine Br Iodine I Calcium Ca Iron Fe Carbon C Lead Pb Chlorine Cl Magnesium Mg Cobalt Co Neon Ne

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Element Symbol Element Symbol Nitrogen N Lithium Li Oxygen O Beryllium Be Potassium K Helium He Silicon Si Tin Sn Silver Ag Mercury Hg Sodium Na Radium Ra Sulphur S Thorium Th Uranium U Antimony Sb Zinc Zn Chromium Cr

Atomic mass :

Atomic mass Since atoms are very small in size its mass is very small and determining its mass is very difficult. So the mass of an atom is compared with the mass of a standard atom. The atom which is considered as a standard atom for comparing the masses of other atoms is carbon 12 atom whose atomic mass is 12 u (atomic mass unit). One atomic mass unit (u) is the mass of 1/12 th the mass of a carbon 12 atom. The atomic mass of an element is defined as the average mass of one atom of the element compared with 1/12 th the mass of a carbon 12 atom.

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Element Atomic mass (u) Element Atomic mass (u) Hydrogen 1 Magnesium 24 Carbon 12 Aluminium 27 Nitrogen 14 Sulphur 32 Oxygen 16 Chlorine 35.5 Sodium 23 Calcium 40 ATOMIC MASS OF SOME ELEMENTS

Molecule :

Molecule A molecule is the smallest particle of an element or compound which exists independently and shows all the properties of that substance. A molecule is a group of two or more elements that are held together by attractive forces. Atoms of the same element or different elements can join together to form molecules.

Molecule of elements :

Molecule of elements Molecule of an element contains atoms of the same element. Molecules of some elements contain only one atom and molecules of some elements contain two or more atoms. Atomicity of an element is the number of atoms present in one molecule of the element.

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Type of element Name Atomicity Non metal Argon Ar 1 – Mono atomic Non metal Helium He 1 – Mono atomic Non metal Oxygen O 2 2 – Diatomic Non metal Hydrogen H 2 2 – Diatomic Non metal Nitrogen N 2 2 – Diatomic Non metal Chlorine Cl 2 2 – Diatomic Non metal Phosphorus P 4 4 – Phosphorus Non metal Sulphur S 8 8 - Poly atomic Metal Sodium Na 1 – Mono atomic Metal Iron Fe 1 – Mono atomic Metal Aluminium Al 1 – Mono atomic Metal Copper Cu 1 – Mono atomic Atomicity of some elements

Chemical formula:

Chemical formula The representation of a molecule in terms of symbols of the constituting atom is called chemical formula. or The chemical formula of a compound is the symbolic representation of its composition (different elements).

Characteristics of formula of a compound:

Characteristics of formula of a compound The formula of a compound is made up of group of atoms carrying positive and negative charge called radicals. Radicals made up of a single atom are called simple radicals. e.g. He, Au, Ca, Cl, Zn, Ne etc. Radicals made up of more than two atoms are called compound radicals. e.g. CO 3 , NO 3 , PO 4 , SO 4 . Radicals with positive charges are called as cations. e.g. Na + , Mg +2 , Ca +2 etc. Radicals with negative charges are called anions.e.g. Cl - ,S - , O -2 etc. Radicals may be monovalent, bivalent, trivalent, tetravalent, polyvalent etc.

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Valency Name of ion Symbol Name of ion Symbol Name of ion Symbol 1 Sodium Na + Hydrogen H + Ammonium NH 4 - 1 Potassium K + Hydride H - Hydroxide H - 1 Silver Ag + Chloride Cl - Nitrate NO 3 - 1 Copper (I) Cu + Bromide Br - Hydrogen carbonate HCO 3 - 2 Magnesium Mg 2 + Iodide I - 2 Calcium Ca 2 + Oxide O 2- 2 Zinc Zn 2+ Carbonate CO 3 2- 2 Iron (II) Fe 2+ Sulphite SO 3 2- 2 Copper (II) Cu 2+ Sulphate SO 4 2- 3 Aluminium Al 3+ Nitride N 3- 3 Iron (iii) Fe 3+ Phosphate O 4 3- Some common ions and their valencies

Molecule of compounds :

Molecule of compounds Molecule of a compound contains atoms of two or more different types of elements. Molecules of some compounds : Compound Combining elements Number of atoms of each elements Water – H 2 O Hydrogen, Oxygen 2 - Hydrogen, 1 - Oxygen Ammonia – NH 3 Nitrogen, Hydrogen 1 - Nitrogen, 3 - Hydrogen Carbon dioxide CO 2 Carbon, Oxygen I - Carbon, 2 - Oxygen Hydrochloric acid HCl Hydrogen, Chlorine 1 - Hydrogen, 1 - Chlorine Nitric acid HNO 3 Hydrogen, Nitrogen, Oxygen 1 - Hydrogen, 1 - Nitrogen, 3 - Oxygen Sulphuric acid H 2 SO 4 Hydrogen, Sulphur, Oxygen 2 - Hydrogen, 1 - Sulphur, 4 - Oxygen

Ions :

Ions Compounds containing metal and non metal elements contain charged particles called ions. An ion is a charged particle having positive or negative charge. A positively charged ion is called ‘cation’ and a negatively charged ion is called ‘anion ’. Polyatomic atomic ion: The cluster of atoms forming charge is called polyatomic ions/radicals. Valency : It is the combining capacity of an element .

Writing chemical formulae :

Writing chemical formulae Write the symbols / formula of the elements or ions so that the symbol of the metal or positive ion is on the left and symbol / formula of the non metal or negative ion is on the right. Write the valencies of the elements or ions below the elements or ions. Cross over the valencies of the combining ions. Polyatomic ions should be enclosed in bracket before writing the formula.

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Formula of hydrogen chloride Symbol H Cl Valency 1 1 Formula HCl Formula of hydrogen sulphide Symbol H S Valency 1 2 Formula H 2 S

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Symbol : Mg Cl Valency : 2 1 Formula : MgCl 2 Formula of Carbon tetrachloride Symbol : C Cl Valency : 4 1 Formula : CCl 4 Formula of Magnesium chloride

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Formula of Calcium oxide Formula of Aluminium oxide Symbol Ca O Symbol : Al O Valency 2 2 Valency: 3 2 Formula Ca 2 O 2 = CaO Formula: Al 2 O 3 Formula of Sodium nitrate Formula of Calcium hydroxide Symbol/ Na NO 3 Symbol/ Ca OH Formula: Formula : Valency : 1 1 Valency: 2 1 Formula : NaNO 3 Formula: Ca(OH) 2 Formula of Sodium carbonate Formula of Ammonium sulphate Symbol/ Na CO 3 Symbol / Formula : NH 4 SO 4 Formula: Valency: 1 2 Valency : 1 2 Formula: Na 2 CO 3 Formula : (NH4) 2 SO 4

Molecular mass / Formula unit mass:

Molecular mass / Formula unit mass The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. Molecular mass is expressed in atomic mass units (u). E.g.:- Molecular mass of water – H 2 O Atomic mass of H = I u Atomic mass of O = 16 u Molecular mass of H 2 O = 1x2+16= 2+16 = 18 u Molecular mass of Nitric acid – HNO 3 Atomic mass of H = 1 u Atomic mass of N = 14 u Atomic mass of O = 16 u Molecular mass of HNO 3 = 1+14+16x3 = 1+14+48 = 63 u

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Gram atomic mass of an element It is the atomic mass expressed in grams. E.g. Gram atomic mass of Oxygen = O 2 = 16 x 2 = 32g . Gram molecular mass of a compound It is the molecular mass expressed in grams. E.g. Gram molecular mass of water = H 2 O = 1 x 2 + 16 = 2 + 16 = 18g.

Mole concept:

Mole concept A mole of a substance is that amount of the substance which contains the same number of particles (atoms, molecules or ions) that are present in 12g of Carbon 12. The number of particles (atoms) present in 12g of Carbon 12 is 6.022 x 10 23 . This number is called Avogadro Number or Avogadro Constant. A mole represents two things :- It represents a definite number of particles (atoms, molecules or ions) equal to 6.022 x 10 23 . It represents a definite mass of a substance equal to the gram atomic mass of an element or the gram molecular mass of a compound.

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6.022x10 23 atoms of H 6.022x10 23 no. of that particles 6.022x10 23 no. of molecules. 1 Mole of carbon atom 6.022x10 23 atoms of C 12 g of carbon 1 mole of hydrogen atom 1 mole of any particles 1 mole of molecules 1 g of H atom Relative mass of those particles in g Molecular mass in gram

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Relationship between number of moles (n), mass (m), molar mass (M), Number of atoms or molecules (N), and Avogadro number (N O ). Given mass m No. of moles= n = molar mass M No. of atoms/molecules/particles Moles= Avogadro no. N n = No

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Given mass = molar mass x no of moles m=M x n No. of atoms/particles/molecules Given mass = X Avogadro no. molar mass m N = x No M No of atoms = no. of moles x Avogadro no. N=n x No

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