AP Chemistry – Chapter 2

Atoms, Molecules and Ions

The Atomic Theory• John Dalton’s Postulates:

– Each element is composed of atoms.– All atoms of an element are identical to each

other, but different than atoms of other elements.

– Atoms of one element cannot be changed into atoms of different elements by chemical reactions; atoms are neither created or destroyed in reactions.

– Compounds are formed when atoms of different elements combine.

Laws!• Law of Constant Composition – In a given

compound, the relative numbers and kinds of atoms are constant. (CO2 and H2O)

• Conservation of Mass – The total mass of products equals the mass of the reactants. Mass is not created or destroyed in a reaction.

• Law of Multiple Proportions – If 2 elements combine to form more than one compound, the masses exist in small whole number ratios.

Developing the Current Atomic Theory

• J. J. Thomson and the cathode ray tube lead to the discovery of the electron and its charge to mass ratio.

• Millikan’s Oil Drop experiment calculated the mass of an electron.

• Rutherford’s Gold Foil Experiment determined the atom is mostly empty space with a small dense nucleus at the center of the atom.

Thomson’s Plum Pudding

Model

Atomic Structure

Average Atomic Mass

The average atomic mass is determined by multiplying the abundance by the mass, and adding these values for all isotopes of that given atom.

The Periodic Table

• Metals lose electrons and become positive ions (cations)

• Non-metals gain electrons and become negative ions (anions)

The Periodic Table of Elements