Atomic numbers

Each element has its own atomic number.

Sodium (Na) square from the periodic table. The mass number (23) is top-left. The atomic number (11) is bottom-left.

The atomic number of an element tells you how many protons that the element has. This is written at the bottom left hand side of the symbol.

Since atoms are neutral, we know then that sodium atoms must also have 11 negative electrons to cancel the charge from 11 positive protons.

Electron arrangement

The electron arrangement of all atoms can be found in the data booklet. All the electrons are arranged into energy levels. These energy levels can only hold a certain number of electrons.

The first energy level (the one nearest the nucleus) can hold a maximum of two electrons with the others being able to hold up to a maximum of 8 electrons (only true for the first 20 elements).

A sodium atom. Two electrons orbit on a circular path around the nucleus. The next eight electrons all orbit on a wider circle. A final electron orbits on the widest circle of all.A sodium atom contains 11 electrons orbiting a nucleus in 3 levels, 2 in the lowest, then 8, then 1

Sodium has 11 electrons. The first two fill the innermost energy level. The second energy level is also full, holding eight electrons and one electron remaining in the outer energy level.

It is the number of outer electrons that give an element its chemical properties. This is why elements in the same group of the Periodic Table have similar properties.

Mass number

The mass number is given at the top left of the elements symbol, for example, sodium has a mass number of 23.

Sodium (Na) square from the periodic table. The mass number (23) is top-left. The atomic number (11) is bottom-left.

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 (the two particles that have a measurable mass).

Sodium has a mass number of 23amu. Since sodium has 11 protons, the number of neutrons must be 23 – 11 = 12 neutrons.


Magnesium Mg 2+ Mass number 24 (top left) and atomic number 12 (bottom left)
ElementMass numberProtonsNeutrons