Electron arrangements and the periodic table

The electron arrangement of an element is related to its position on the periodic table.

Electronic structure featureLink to the periodic table
Number or numbers of shellsPeriod number
Number of electrons in outermost shellGroup number
Total number of electrons in all shellsAtomic number

The electron arrangement of sodium (2.8.1) shows that sodium, Na:

  • is in period 3
  • is in group 1
  • has an atomic number of (2 + 8 + 1) = 11

The number of electron shells is therefore the same across a period but increases when moving down a group.

Electron arrangements and properties of elements

The electron arrangements of atoms help explain the properties of elements and the structure of the periodic table. When substances react, it is only the outer electrons in the atoms that are involved - the nature of the reactions which occur depend upon the number of outer shell electrons in the reacting atoms.

So, elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.

The electron arrangements of the first three elements in group 1:

Table showing electron arrangements of group 1 elements, lithium, sodium and potassium. Group 1 elements have similar properties and reactions as they all have one electron in their outer shell.

The atoms of all group 1 elements have similar chemical properties and reactions because they all have one electron in their outer shell. Similarly, the atoms of all group 7 elements have similar chemical properties and reactions to each other, because they all have seven electrons in their outer shell.