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Notes

Chapter 2 Atoms, Molecules, and Ions

Atomic Theory

• Robert Boyle (1627–1691): Provided evidence for the atoms and defined the nature of anelement.

• Joseph Priestley (1733–1804): Isolated oxygen gas from decomposition of mercury(II)oxide.

• Antoine Lavoisier (1743–1794): Showed that mass of products is exactly equal to the

mass of reactants.

• Law of Mass Conservation: Mass is neither created nor destroyed in chemical reactions.

• Law of Definite Proportions: Different samples of a pure chemical substance always

contain the same proportion of elements by mass.

• John Dalton (1766–1844): Proposed explanations for the laws of mass conservation and

definite proportions.

o Postulate # 1 - Each element is composed of extremely small particles called

atoms.

o Postulate # 2 - All atoms of a given element are identical, the atoms of different

elements are different and have different properties (including different masses)

o Postulate # 3 - Atoms of an element are not changed into different types of atoms

  by chemical reactions, atoms are neither created or destroyed in chemical

reaction.

o Postulate # 4 - Compounds are formed when atoms of more than one elements

combine, a given compound always has the same relative number and kind of 

atoms.

•

Law of Multiple Proportions: When two elements form two different  compounds, themass ratios are related by small whole numbers.

The Structure of Atoms

Cathode-Ray Tube (Thomson, 1856–1940):

• Cathode rays consist of tiny negatively charged particles, now called electrons.

• Deflection of electron depends on three factors:

o Strength of electric or magnetic field

o Size of negative charge on electron

o Mass of the electron

• Thomson calculated the electron’s charge to mass ratio as 1.758820 x 108 Coulombs per gram.

Oil Drop Experiment (Millikan, 1868–1953):

• Applied a voltage to oppose the downward fall of charged drops and suspend them.

• Voltage on plates place 1.602176 x 10-19 C of charge on each oil drop.

• Millikan calculated the electron’s mass as 9.109382 x 10-28 grams.

Discovery of Nucleus (Rutherford, 1871 – 1937):

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• Rutherford irradiated gold foil with a beam of alpha (α ) particles to search for positivecharged particles.

• Atom must be mostly empty space except for a central positive mass concentration.

• Isotopes: Atoms with identical atomic numbers, but different mass numbers.

• Average Isotopic Mass: A weighted average of the isotopic masses of an element’s

naturally occurring isotopes.• Atomic Mass: A weighted average of the isotopic masses of an element’s naturally

occurring isotopes.

Atoms, Molecules, and Ions

• Covalent Bonding (Molecules): The most common type of chemical bond is formed

when two atoms share some of their electrons.

• Ionic Bonding (Ionic Solids): These are formed by a transfer of one or more electronsfrom one atom to another.

 Naming Binary Ionic Compounds:

• Identify the positive ion and then the negative ion.• The positive ion uses its elemental name.

• The negative ion substitutes the second half of its elemental name with  –ide.

• Do not use Greek prefixes such as mono– , di– , or tri– . Naming Binary Molecular Compounds:

• The more cationlike element uses its elemental name.

• The more anionlike element substitutes the second half of its elemental name with –ide.

• Use the Greek prefixes to express the number of each element present.

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