Isotopes

Isotopes are copies of atoms but with different numbers of neutrons. Because they have different numbers of neutrons, they have different atomic masses.

An example of a pair of isotopes would be carbon-12 and carbon-14. The mass of an atom is the number of protons and neutrons added together. As carbon has 6 protons, to make carbon-12 (which has a weight of 12) it must also have 6 neutrons. In Carbon 14, it also has 6 protons but must also have 8 neutrons to weigh 14. The following table may make this easier to understand...

Isotope
Protons
Neutrons
Electrons (not counted as part of the weight)
Mass of isotope (Protons and Neutrons)
Carbon-12
6
6
6
12
Carbon-14
6
8
6
14

 

Molecular Masses

Molecular masses are basically the masses of the atoms in a molecule added together. For example a molecule of Hydrogen Gas (H2) is made of two hydrogen atoms. The atomic mass of each hydrogen atom is 1, so the atomic mass of a hydrogen molecule (H2) is 2 (1+1)

Other examples get more complicated as they involve different several different atoms. Some examples are given below using the atomic masses given in the table.

Atom H C O N Cl Na
Atomic Weight 1 12 16 14 35 23

 

- Water - H2O = (H - 2 x 1) + (O - 1 x 16) = 18
- Ammonia - NH3 = (N - 1 x 14) + (H - 3 x 1) = 17
- Salt - NaCl = (Na - 1 x 23) + (Cl - 1 x 35) = 58
- Ethanol - C2H6O = (C - 2 x 12) + (H - 6 x 1) + (O - 1 x 16) = 46