Isotopes and Molar mass

Before we start the lesson, its good for us to play this game.  It can help us to get a basic idea of isotopes and atomic mass

http://phet.colorado.edu/zh_TW/simulation/isotopes-and-atomic-mass

So, Isotopes are atoms which have the same atomic number but different mass numbers.   In Other words, they have the same number of protons but different numbers of neutrons.

The number of neutrons in an atom can vary within small limits.
Eg. there are three kinds of carbon atom ¹²C, ¹³C and ¹⁴C. They all have the same number of protons, but the number of neutrons varies.

	protons	neutrons	mass number
carbon-12	6	6	12
carbon-13	6	7	13
carbon-14	6	8	14

These different atoms of carbon are called isotopes.

Molar mass of a substance combined by isotopes
Eg.
Carbon can be formed by these isotopes.
90.% of carbon-12, 5.0% of carbon 13 and 5.0 % of carbon 14

We can calculate this particular substance's atomic mass by:
  12x0.90 + 13x  0.050 +14x0.050
=   12.15  (2 sig fig)
=   12g