Unit #1Unit #2Unit #3Unit #4Unit #5Unit #6Unit #7Unit #8Unit #9Unit #10
spacerUNIT # 3  

spacerof the Atom

spacerAtomic Number - Number of Protons
spacerNumber of Electrons
spacerMass Number - Number of Neutrons
spacer3.2 Isotopes
spacer3.3 Arrangement
spacerof Electrons

spacerThe Bohr Model
spacerBohr Diagram
spacerThe Lewis Electron-dot Symbol
spacerThe Quantum Mechanical Model of the Atom
spacerElectrons in the sublevels

spacer3.4 Electron

spacerFilling Order of the Sublevels

spacerElectron Configuration Notations
spacerElectron Configuration for Atoms of the First 20 Elements
spacerElectron Configuration and the Periodic Table
spacer3.5 Atomic Mass
spacerCalculation of Atomic Mass
spacerCalculate the Atomic Mass of Carbon

spacer1 | 2 | 3


3.5 - Atomic Mass

3.5.1 - Calculation of Atomic Mass

The atomic mass of the elements as seen on the periodic table is a weighted average of the masses of all naturally occurring isotopes for the element. Most elements have several naturally occurring isotopes in varying abundance. Some elements like hydrogen and carbon have 2 naturally occurring isotopes. Others like Magnesium has 3 naturally occurring isotopes, and Strontium has 4 naturally occurring isotopes.

Atomic masses that are on the periodic table are calculated based on knowledge of the following information:

  1. the mass of all the naturally occurring isotopes for the element
  2. the natural abundance of each of these naturally occurring isotopes (usually expressed as a percentage)

The formula that you need is to calculate the atomic mass of an element is:

atomic mass = (the fraction of isotope 1)(mass of isotope 1) +
(the fraction of isotope 2)(mass of isotope 2) +
(the fraction of isotope 3)(mass of isotope 3) + ...


  • the fractions of the isotopes is the natural abundance of the isotope written as a decimal.
  • the masses of the isotopes are determined accurately to as many as 4 decimal places.


Section 4.5
Isotopes ..p86





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