The mass number is given at the top left of the element's symbol, for example, sodium has a mass number of 23.
We know that the atomic number of sodium is 11. This tells us that sodium has 11 protons and because it is neutral it has 11 electrons.
The mass number of an element tells us the number of protons AND neutrons in an atom (i.e. the two particles that have a measureable mass).
Sodium has a mass number of 23amu. Since sodium has 11 protons, the number of neutrons is given by mass number – number of protons (23 - 11) = 12 neutrons.
In some periodic tables there is a dark zigzag line that separates the metal elements from the non-metal elements.
To the left of the zigzag line are the metals and on the right hand side of the zigzag line are the non-metals.