2. Mass Number Atomic Number equals the # of... NUCLEUS ELECTRONS PROTONS NEUTRONS NEGATIVE CHARGE POSITIVE CHARGE NEUTRAL CHARGE ATOM

3. Scale of the atom

5. atomic mass unit (amu) = the mass of one proton 1.66  10 -27 g

6. Sum of the protons and neutrons in the nucleus of an atom Always a whole number # of neutrons = mass # - atomic # The number of protons within the nucleus

7. Atoms of the same element with different numbers of neutrons Mass # Atomic# Isotope symbol: “Carbon-12” 6 C

8. How to calculate the average atomic mass of an element: List all isotopes, mass numbers, and percent relative abundance of an element Multiply the mass number of each isotope by its relative abundance Add all the products together = atomic mass

9. An example: Chlorine-35 ( 35 Cl) has 17 protons and 18 neutrons and has a relative abundance of 75.8% Chlorine-37 ( 37 Cl) has 17 protons and 20 neutrons and appears 24.2% of the time. Multiply the mass number of each isotope by its relative abundance (Chlorine-35) 35 amu × 0.758 = 26.53 amu (Chlorine-37) 37 amu × 0.242 = 8.95 amu Add all the products together = atomic mass 26.53 amu + 8.95 amu = 35.48 amu