Periodic Properties of the Elements
Periodic properties are the properties of the elements which depend on their electronic configuration and these properties changes on moving down in the group and on moving left to right in the period of the periodic table. Some periodic properties are atomic size, electron affinity, ionization energy, metallic and non metallic properties etc. are the main periodic properties.
Size of any atom is explained with the help of atomic radius.
The distance between outermost shell electrons and centre of nucleus of isolated atom is called atomic radius.
There are two hypotheses for the calculation of atomic radius i.e.
1. Van der Waals Radii
It is the one half of the distance between the two adjacent atoms nuclei centres, is known as Van der Waals radii.
Van der Waals radii is calculated as follows, first calculate the distance from the centre of nuclei of one atom to centre of nuclei of another adjacent atom in solid state of same substance. Then divide this distance by two. Van der Waals distance is represented by Angstrom A.
Where, 1 A = 10-8 cm.
2. Covalent Radii
It is the one half of the distance between the diatomic molecule nuclei centres, is known as Covalent Radii.
Covalent radii is calculated as follows, first calculate the distance from the centre of nuclei of one atom to centre of nuclei of another atom in diatomic molecule containing covalent single bond. Then divide this distance by two.
Isolated atom is an absolutely alone single atom.
Periodicity in Atomic Size
In Period: On moving left to right in any period atomic size decreases because atomic radii decreases, As electrons are attracted towards nucleus due to increasing nuclear charge.
In Group: On moving down in any group atomic size increases because atomic radii increase, as more electrons shells added.
An ionic radius of cation is smaller than its normal atom while that of anion is larger than its normal atom.
Cations are formed by loss of electron or electrons and carry positive charge.
Anions are formed by gain of electron or electrons and carry negative charge.
The energy required for the removal of one electron from the isolated atom in gaseous state is called ionization energy.
Ionization Energy is represented by I.E.
Ionization Energy is expressed in Electron Volts per Atom or Kilo Joules per Mole or Kilo Calorie per Mole. Ionization Energy is also known as Ionization Potential.
Equation for I.E.
Atom (g) + Ionization Energy (I.E.) ---à Cation + Electron (e-)
Removal of further electron from cation is difficult so value of Ionization Energy kept on increasing.
I.E.1 < I.E.2 < I.E.3 < I.E.4 …………………………………..
Periodicity in I.E. (Ionisation Energy)
In Period: On moving left to right in any period ionization energy increases because nuclear charge increases with the increasing atomic number, so more energy required to remove electron. Therefore value of I.E. increases on moving left to right in any period.
In Group: On moving down in any group ionisation energy decreases because atomic size increase, as more electrons shells added.
The amount of energy release as a result of addition of electron in any atom in gaseous state to form anion is known as Electron Affinity.
Electron Affinity is represented by E.A.
Electron Affinity is expressed in Electron Volts per Mole or Kilo Joule per Mole
Values of all electron affinity are negative except first Electron Affinity which is positive.
Periodicity in E.A. (Electron Affinity)
In Period: On moving left to right in any period the value of Electron Affinity increases because atomic size decreases so amount of energy released is more.
In Group: On moving down in any group the value of Electron Affinity decreases with some irregularities.
Metallic and Non-Metallic characters
In Period: On moving left to right in any period Metallic character decreases and Non Metallic character increases.
In Group: On moving down in any group Metallic character increases and Non Metallic character decreases.
In Periodic Table: Metallic elements are found on left hand side and Non Metallic elements are found on right hand side.
The elements which are neither metals nor non metals are known as metalloids.
Example of Metalloids: B, Si, As, Te etc.