1. Reviewing Subatomic Particles
Nucleus contains:
1. Protons -
2. Neutrons -
Positively charged, mass of 1 amu
No charge, mass of 1 amu
Electrons -
Negatively charged, mass of 1/1840 amu
Atomic number =
# of Protons =
# of Electrons
Mass Number =
# of protons + # of neutrons

2. Distinguishing Between Atoms
Atomic Number (Z) – The number of protons in the nucleus of the atom.
In a neutral atom the # of protons (+) = the # of electrons (-)
Nuclear Charge = # of protons

3. The Nucleus Protons (positive charge) Neutrons (No Charge)
Electrons (negative charge) are found in a cloud outside of the nucleus.

4. Mass Number (A) = number of protons + the number of neutrons
Most of the mass of an atom is found in the nucleus.
The mass number is given after the name of the element (example: Carbon – 12)
Isotope – Atoms that have the same number of protons but different numbers of neutrons.
Example: Carbon – 12 and Carbon – 14

5. U Atomic Symbols 238 92 Mass Number = protons + neutrons
Ion Charge (if there is one)
Symbol = Element
Atomic Number = Protons

6. Atomic Masses
Atomic Mass Unit (AMU) – Using carbon-12 as a base unit, the mass is defined as 1/12 the mass of the carbon-12 isotope.
mass of a proton = 1 amu
mass of a neutron = 1 amu.
Atomic Mass – The weighted average mass of the isotopes of an element.

7. How to Calculate the Average Atomic Mass
Change the percentages back to the decimal values
Example: % = .9994
Multiply the mass number by the decimal of the percent.
Example: Carbon – 12 (.9994 x 12)
Add all of the products of the mass numbers and the decimal percentages