1. 1 The Atom Atomic Number and Mass Number Isotopes

2. 2 Subatomic Particles ParticleSymbol Charge Relative Mass Electron e - 1- 0 Proton p + + 1 Neutron n0 1

3. 3 Location of Subatomic Particles 10 -13 cm electrons protons neutrons 10 -8 cm nucleus

4. 4 Atomic Number Counts the number of protons in an atom

5. 5 Atomic Number on the Periodic Table 11 Na Atomic Number Symbol

6. 6 All atoms of an element have the same number of protons 11 Na 11 protons Sodium

7. 7 Number of Electrons An atom is neutral The net charge is zero Number of protons = Number of electrons Atomic number = Number of electrons

8. 8 Mass Number Counts the number of protons and neutrons in an atom

9. 9 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

10. 10 Atomic Mass Listed on the periodic table Gives the mass of “average” atom of each element compared to 12 C Average atom based on all the isotopes and their abundance % Atomic mass is not a whole number Na 22.99

11. 11 Calculating Atomic Mass Percent(%) abundance of isotopes Mass of each isotope of that element Weighted average = mass isotope 1 (%) + mass isotope 2 (%) + … 100 100

12. 12 Atomic Mass of Magnesium Isotopes Mass of Isotope Abundance 24 Mg =24.0 amu 78.70% 25 Mg = 25.0 amu 10.13% 26 Mg = 26.0 amu 11.17% Atomic mass (average mass) Mg = 24.3 amu Mg 24.3