chapter2 notes

Atoms, Molecules and Ions: 

Atoms, Molecules and Ions Chapter 2 Dr. Daniel E. Autrey August 24th, 2005 CHEM 140

Dalton’s Atomic Theory (1808): 

Dalton’s Atomic Theory (1808) Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

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2

Slide4: 

8 X2Y

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J.J. Thomson, measured charge/mass of e- (1906 Nobel Prize in Physics)

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e- charge = -1.60 x 10-19 C Thomson’s charge/mass of e- = -1.76 x 108 C/g e- mass = 9.10 x 10-28 g Measured mass of e- (1923 Nobel Prize in Physics)

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(Uranium compound) Rutherford Experiment (1902)

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Shown to be incorrect by Rutherford’s experiments

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atoms positive charge is concentrated in the nucleus proton (p) has opposite (+) charge of electron (-) mass of p is 1840 x mass of e- (1.67 x 10-24 g) particle velocity ~ 1.4 x 107 m/s (~5% speed of light) (1908 Nobel Prize in Chemistry)

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atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m Rutherford’s Model of the Atom

Chadwick’s Experiment (1932): 

Chadwick’s Experiment (1932) H atoms - 1 p; He atoms - 2 p mass He/mass H should = 2 measured mass He/mass H = 4 neutron (n) is neutral (charge = 0) n mass ~ p mass = 1.67 x 10-24 g

Subatomic Particles (Table 2.1): 

Subatomic Particles (Table 2.1) mass p = mass n = 1840 x mass e-

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Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

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6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons Do You Understand Isotopes?

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A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4

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An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion.

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A monatomic ion contains only one atom A polyatomic ion contains more than one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- OH-, CN-, NH4+, NO3-

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13 protons, 10 (13 – 3) electrons 34 protons, 36 (34 + 2) electrons Do You Understand Ions?

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A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance An empirical formula shows the simplest whole-number ratio of the atoms in a substance H2O C6H12O6 CH2O O3 O N2H4 NH2

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ionic compounds consist of a combination of cations and an anions the formula is always the same as the empirical formula the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl

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Formula of Ionic Compounds Al2O3 Al3+ O2- CaBr2 Ca2+ Br- Na2CO3 Na+ CO32-

Some Polyatomic Ions (Table 2.3): 

Some Polyatomic Ions (Table 2.3)

Chemical Nomenclature: 

Chemical Nomenclature Ionic Compounds often a metal + nonmetal anion (nonmetal), add “ide” to element name BaCl2 barium chloride K2O potassium oxide Mg(OH)2 magnesium hydroxide KNO3 potassium nitrate

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Transition metal ionic compounds indicate charge on metal with Roman numerals FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide

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Molecular compounds nonmetals or nonmetals + metalloids common names H2O, NH3, CH4, C60 element further left in periodic table is 1st element closest to bottom of group is 1st if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom last element ends in ide

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HI hydrogen iodide NF3 nitrogen trifluoride SO2 sulfur dioxide N2Cl4 dinitrogen tetrachloride NO2 nitrogen dioxide N2O dinitrogen monoxide Molecular Compounds

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An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water. HCl Pure substance, hydrogen chloride Dissolved in water (H+ Cl-), hydrochloric acid An oxoacid is an acid that contains hydrogen, oxygen, and another element.

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A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water.