Periodic Table organisation

The Periodic Table arranges elements in order of increasing proton number, also called atomic number. This is the number of protons in the nucleus of an atom. Each element has its own proton number, so its position in the table identifies the element.

The horizontal rows are called periods. Moving from left to right across a period, the proton number increases. The vertical columns are called groups.

An element’s position helps predict its properties. Elements in the same group show related chemical behaviour, although they are not identical. Properties also change in patterns across a period. These group and period patterns make the table useful for predicting how an element may behave.

Metallic character across a period

Metallic character describes how strongly an element shows the properties of a metal. Metals generally conduct electricity well and can be shaped without breaking.

A period is a horizontal row of the periodic table. Moving from left to right across a period, metallic character generally decreases, while non-metallic character increases. Elements towards the left are therefore more likely to be metals, and those towards the right are more likely to be non-metals.

An element’s position across a period helps predict its character. For example, an element further left in the same period generally has greater metallic character than one further right.

Chemical similarity within a group

A group is a vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell.

Outer-shell electrons determine how an atom takes part in chemical reactions: they are the electrons that can be lost, gained or shared. Because elements in a group have the same number of outer-shell electrons, they tend to react in similar ways and have similar chemical properties.

For example, lithium and sodium are both in Group 1 and each has one outer-shell electron. Each can lose that electron to form an ion with a charge of $$+1$$. Knowing that another element is in Group 1 therefore helps predict that it can behave similarly.

Elements in the same group are similar, but their reactions are not necessarily identical.

Group number and ion charge

Atoms can form simple ions by losing or gaining electrons to obtain a full outer shell. Electrons have a negative charge, so losing electrons produces a positive ion, while gaining electrons produces a negative ion. The number lost or gained determines the size of the charge.

For main-group elements that form simple ions, the group number indicates how many electrons an atom tends to lose or gain:

• Group I: loses one electron, forming a $$+1$$ ion.
• Group II: loses two electrons, forming a $$+2$$ ion.
• Group III: commonly loses three electrons, forming a $$+3$$ ion.
• Group V: can gain three electrons, forming a $$-3$$ ion.
• Group VI: can gain two electrons, forming a $$-2$$ ion.
• Group VII: can gain one electron, forming a $$-1$$ ion.

The pattern changes direction after Group IV: atoms in Groups V to VII can reach a full outer shell by gaining fewer electrons than they would need to lose.

Group 0 elements already have full outer shells and do not generally form ions. Group IV elements do not generally have a simple ion charge predicted by this pattern.