Periodic Table

The Periodic Table is a vital tool used by chemists to predict the way in which elements react during chemical reactions. It is a method of categorising elements according to their properties.

The period number tells you how many shells there are. All the elements in a group have the same number of electrons in their outer shells. So Group I elements have 1, Group II have 2, and so on. These outer-shell electrons are also called the valency electrons. The group number is the same as the number of outer-shell electrons. The valency electrons dictate how an element reacts. So the elements in Group I all have similar reactions, for example.

Elements with similar chemical properties are found in the same groups. There are eight groups of elements. The first column is called Group I; the second Group II; and so on up to Group VII. The final column in the Periodic Table is called Group 0 (or Group VIII).

Group Number
Period 111H11H24He
Period 237Li49Be511B612C714N816O919F1020Ne
Period 31123Na1224Mg1327Al1428Si1531P1632S1735.5Cl1840Ar
Period 41939K2040Ca

Group I – the alkali metals

They include Lithium, sodium and potassium. They are all very reactive metals and they are stored under oil to prevent them coming into contact with water or air.

Of these three metals, potassium is the most reactive towards water, followed by sodium and then lithium. The further down the group you go the more reactive the metals become. Francium is, therefore, the most reactive Group I metal

Electronic structure of the first three elements of Group I.
ElementSymbolProton numberElectronic structure

Sodium atom

Potassium atom

Group II – the alkaline earth metals

Magnesium and calcium are the most common group 2 metals.

The further down the group you go, the more reactive the elements

Electronic structure of the first three elements of Group II.
ElementSymbolProton numberElectronic structure

Magnesium atom

Calcium atom

Group VII – the halogens

Group VII includes elements such as fluorine, chlorine, bromine and iodine.

These elements are coloured and darken going down the group.

Colours of some halogens.
ChlorinePale greenChlorine
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IodinePurple–black vapour/ Silver-grey solidIodine
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Electronic structure of the first three elements in Group VII
ElementSymbolProton numberElectronic structure

Group 0 – the noble gases

Also known as group 8, includes elements such as Helium, neon, argon, krypton, xenon.

Their atoms all have 8 outer-shell electrons, except for helium, which has 2 because it has only one shell. This stable arrangement of electrons has a very important result because it makes the Group 0 elements unreactive.

Electronic structure of helium, neon and argon.
ElementSymbolProton numberElectronic structure