Unit 3 – Atomic Theory & Structure

Chemistry – King William High School

1.Atomic Theory Foundations… Law of conservation of mass – mass

is neither created nor destroyed Law of definite proportions – a

compound contains the same elements in the same proportions regardless of sample size

Law of multiple proportions – elements can form compounds in different ratios

Dalton’s Atomic Theory

All matter is composed of atoms Atoms of the same element are

identical Atoms cannot be subdivided, created

or destroyed Atoms of different elements combine

in small whole number ratios to form compounds

In a reaction…atoms are combined, separated or rearranged (NOT destroyed!)

Modern Atomic Theory

What do we still take from Dalton…

All matter is composed of atoms

Atoms of one element differ in properties from atoms of another element

2.Structure of the atom

Discovery of the electron JJ Thompson Plum Pudding model (electron’s = plums and protons = pudding)

Mass and charge of the electron Millikan

Discovery of atomic nucleus Rutherford Gold foil experiment (nucleus has a dense, positive center)

2. Structure continued…

Nucleus = protons and neutrons (both with large mass)

Electrons outside the nucleus (very small mass…but large volume)

3. Counting atoms Atomic number = the # of protons in

the nucleus (and the # of electrons in a neutral atom)

Mass number = protons + neutrons EX:

Isotopes Same element with different number

of neutrons Do not differ significantly in their

chemical behavior EX:

Average atomic mass

Weighted average of the atomic masses of the naturally occurring isotopes of an element

The decimal # on the periodic table EX: 10

5B has an abundance of 19.91% and 11

5B has an abundance of 80.09%. What is the atomic mass of boron?

The MOLE!!! Avogadro’s number = 6.02 x 1023

EX: How many moles of gold are in 5.64 x 1036 atoms of gold?

EX: How many atoms of gold are in a sample that contains 0.259 mol?

Molar mass The mass of one mole of a pure

substance EX: What is the molar mass of

potassium?

EX: What is the molar mass of water?

More molar mass…

EX: What is the mass in grams of 6.21 x 10-2 mol of sodium?

EX: How many moles of copper are present in a 0.125 g sample?