Periodic table

shows all 118 types of atom (elements)

The period (row) tells us the number of electron shells (rings).

The group number (column) tells us the number of electrons in an atoms outer shell (ring)

Elements in the yellow box are transition metals. We don't learn much about these.

Everything left of these stairs are metals. Everything right are not metals.

mass number = number of protons + neutrons

Sodium has 11 protons and (23-11=12 neutrons).

atomic number = number of electrons or number of protons. Sodium has 11 protons and 11 electrons.

The atomic number is not always on the bottom. To know which number is which look at the periodic table. The number that rises by one from left to right and top to bottom is the atomic number.