Topic 1, Atomic Structure
Topic 1, Atomic Structure
(Key concepts)

3 main subatomic particles:
protons: +1, 1 amu, in nucleus
neutrons: 0, 1 amu, in nucleus
electrons: -1, 0 amu (1/1836 amu), surrounding nucleus

Atomic structure:

Same element, same # protons, same # electrons
Different # neutrons, therefore different mass #

Historical development of the atomic model:
Dalton: Atom is an indivisible dense sphere

Thomson:  Discovered e- with cathode ray tube; atom is a + sphere with e- embedded throughout; plum pudding model
Rutherford: Fired alpha particles at gold foil; most went through = atom mostly empty space; a few deflected = small + nucleus

Bohr: Place e- in orbits

Modern ( wave mechanical, quantum model): protons and neutrons in small nucleus surrounded by electron cloud; orbitals within e- cloud are most probable locations of e-


Atomic mass:

Atomic mass is defined as the weighted average of the masses of the naturally occurring isotopes of an element.  Atomic mass is listed on the periodic table.  
Example:  About 75% of the Cl in nature has a mass of 35 amu, and about 25% of the Cl in nature has a mass of 37 amu.  To find the atomic mass: (.75 x 35) + (.25 x 37) = 35.5 amu.  
Atomic mass takes into account the abundances (75% & 25%) and masses (35 & 37) of the naturally occurring isotopes of an element.