1. 1 Atomic weight vs. Mass number (Isotopes)

2. Mass Number Mass Number = Protons + Neutrons What about electrons? Electrons are so small relative to the other subatomic particles we do not include them in an atom’s mass number.

3. Atomic Mass Units A unit used to compare the masses of atoms, has the symbol amu Approximately equal to the mass of a single proton or neutron

4. 4 Atomic Symbols Show the mass number and atomic number Give the symbol of the element 23 Na 11 mass number sodium-23 atomic number

5. 5 More Atomic Symbols 163165 O P Zn 81530 8 p + 15 p + 30 p + 8 n16 n35 n 8 e - 15 e - 30 e -

6. 6 Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35 Cl 37 Cl17 chlorine - 35 chlorine - 37

7. 7 Atomic Mass Listed on the periodic table Gives the mass of “average” atom of each element Average atom based on all the isotopes and their abundance % Atomic mass is not a whole number Na 22.99