|Electronic structure feature||Link to the periodic table|
|Number or numbers of shells||Period number|
|Number of electrons in outermost shell||Group number|
|Total number of electrons in all shells||Atomic number|
The electron arrangement of sodium (2.8.1) shows that sodium, Na:
The number of electron shells is therefore the same across a period but increases when moving down a group.
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:
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.