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.
Atomic Size
Size
of any atom is explained with the help of atomic radius.
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
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.
Ionization Energy
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.
Electron Affinity
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.