1. 4.3: HOW ATOMS DIFFER ATOMIC NUMBER
Atomic number = number of protons
= number of electrons
Atomic number defines what the element is
Periodic Table is arranged by atomic number

2. MASS NUMBER Mass Number = atomic number + number of neutrons
Mass number is the sum of the protons and neutrons in the nucleus

3. ISOTOPES Isotopes - atoms of the same element - same number of protons
But different numbers of neutrons (so different mass numbers)

4. CALCULATING ATOMIC MASS
Atomic mass is the weighted average mass of the isotopes of an element
This is the mass that is given on the periodic table
Formula for average atomic mass:
(mass of isotope 1 x % abundance) + (mass of isotope 2 x % abundance) + …
Remember x% = 0.0x