The atomic number Z and the outermost shell

The atomic number: the element’s identity card

Every element has an atomic number, denoted by Z, which corresponds to the number of protons in the nucleus of its atoms. It is Z that defines the element: all atoms with Z = 6 are carbon atoms, regardless of their number of neutrons. In a neutral atom, the number of electrons is equal to Z.

The outermost shell: the key to chemical properties

The electrons in an atom are distributed across successive shells around the nucleus. The shell furthest from the nucleus, known as the outermost shell (or valence shell), is the one that takes part in chemical reactions. It is the number of electrons in this outer shell that determines most of an element’s chemical properties, not the total number of electrons.

A practical example

Sodium (Z = 11) has the electron configuration (K)2 (L)8 (M)1: a single electron in its outer M shell. Potassium (Z = 19) also has a single electron in its outer shell. This is why both react violently with water, despite having very different atomic numbers.

Why the periodic table is organised this way

As you move from one element to the next in the table, Z increases by 1 (one more proton and one more electron). When the outermost shell is full, a new period begins with a new shell: this is exactly what creates the columns of similar properties.

Common mistake

Do not confuse the atomic number Z (number of protons) with the mass of the atom (which also depends on the neutrons). Two atoms of the same element may have different masses (isotopes) but they always have the same Z.