). So, the valency of all the alkali metals is ‘ ’. As the elements present in a group have identical valence shell electronic configurations, they possess similar chemical properties. The physical properties of the elements in a group such as melting point, boiling point and density vary gradually.
The atoms of the 'group ' elements have stable electronic configuration in their valence shells and hence they are unreactive. . PERIODIC TRENDS IN PROPERTIES The electronic configurations of elements help us to explain the periodic recurrence of physical and chemical properties. Anything which repeats itself after a regular interval is called periodic and this behaviour is called periodicity.
Some of the atomic properties of the elements are periodic. Properties such as atomic radius, ionic radius, ionisation energy, electronegativity, electron affinity, show a regular periodicity and hence they are called periodic properties . The main significance of the modern periodic table is that it gives a clear understanding of the general properties and trends within a group or a period to predict with considerable accuracy, the properties of any element, even though that element may be unfamiliar to us. Let us discuss the periodic trend of some of the properties.
. . Atomic Radius Atomic radius of an atom is defined as the distance between the centre of its nucleus and the outermost shell containing the valence electron. Direct measurement of the radius of an isolated atom is not possible.
Except for noble gases, usually the atomic radius is referred to as covalent radius or metallic radius depending on the nature of the bonding between the concerned atoms. Atomic radius in metal atoms is known as metallic radius. It is defined as half the distance between the nuclei of adjacent metal atoms (Figure . Figure .
(a) Metallic Radius (b) Covalent Radius (a) (b) (a)). In non-metallic elements, their atomic radius is known as Covalent radius. It is defined as half the distance between the adjacent nuclei of two covalently bonded atoms of the same element in a molecule ( Figure . (b)).
For example, let us