1. Atomic Structure
Chemistry

2. Atomic Number The number of protons in the nucleus of an atom.
Different for every element.
Protons have a mass of 1.67e-24g and a charge of +1.
If no charge on the atom, it is also the number of electrons.

3. Mass Number The sum of protons and neutrons in an atom.
The number of neutrons is the mass number less the atomic number.
It is written as a superscript in front of the chemical symbol.
The atomic number is written as a subscript in front of the symbol.

4. Isotopes
Isotopes have different numbers of neutrons, so different mass numbers.
There are both naturally occurring and synthetic isotopes.
Only the naturally occurring isotopes are used in atomic mass calculations.
Neutrons weigh 1.67e-24g
Electrons weigh 9.11e-28g

5. Atomic Mass 1 amu is defined as 1/12 of the mass of a carbon-12 atom.
The atomic mass of an element is a weighted average of the atoms in a naturally occurring sample of that element.
To calculate the atomic mass: multiply the mass of each isotope by its % occurrence…these should total 100%!