In a given group, the number of filled shells increases. The number of valence electrons is the same in all the elements of a given group. However, these valence electrons but they are present in higher shells which are farther away from the nucleus. In view of this, decreases the force of attraction between the outermost shell and the nucleus as we move downwards in a group.
In a given period, the nuclear charge and the number of valence electrons in a particular shell increase from left to right. This increases the force of attraction between the valence electron and nucleus as we move across a period from left to right.
These affect various properties which show gradual variations in groups and periods, and they repeat themselves after certain intervals of atomic number. They are called periodic properties.
Atomic size is the distance between the centre of nucleus and the outermost shell of an isolated atom. It is also known as atomic radius. It is measured in picometre, pm (1 pm = 10-12 m). Atomic size is a very important property of atoms because it is related to many other properties.
Variation of atomic size in periodic table
The size of atoms decreases from left to right in a period but increases from top to bottom in a group. In a period the atomic number and therefore the positive charge on the nucleus increases gradually. As a result, the electrons are attracted more strongly and they come closer to the nucleus. This decreases the atomic size in a period from left to right.
In a group as one goes down, a new shell is added to the atom which is farther away from the nucleus. Hence electrons move away from the nucleus. This increases the atomic size in a group from top to bottom.
The tendency of an element to lose electrons to form cations is called electropositive or metallic character of an element. Alkali metals are most electropositive. The tendency of an element to accept electrons to form anions is called electronegative or non-metallic character of an element.
Variation of Metallic Character in a Group
Metallic character increases from top to bottom in a group as tendency to lose electrons increases. This increases the electropositive character and metallic nature.
Variation of Metallic Character in a Period
Metallic character decreases in a period from left to right. It is because the ionization energy increases in a period. This decreases the electropositive character and metallic nature.