The energy required to remove an electron from its outermost orbit is called Ionization Energy.
Ionization Energy: The minimum amount of energy required to remove an electron from its
outermost orbit of an atom or molecule in gaseous state is called Ionization Energy.
Units : KJ/mol or
Kcal/mol.
1st ionization energy
X ⟶ X+ + e−
The first ionization energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to
produce 1 mole of gaseous ions each with a charge of 1+.
2nd Ionization Energy
X+ ⟶ X2+ + e−
The second ionization energy is the energy it takes to remove another electron to form an M2+ ion in the gas phase.
3rd Ionization energy
X2+ ⟶ X3+ + e−
The third ionization energy is the energy it takes to remove another electron to form an M3+ion in the gas phase.
Explanation:
A simple analogy is that when 50 rows of persons are sitting in front of a screen showing movie. The persons sitting in front
rows will be able to listen audio and visualize properly when compared with the 50th row of people. So, here the rows
of persons are compared to electrons and screen to nucleus. The amount of energy we require to deviate a person sitting in the
50 row is very less as he is unable to visualize properly and hear properly. The amount of energy we should spend to deviate a
first row person concentration is more. So as the atom, if the size of the atom is more the valence electrons are loosely held,
so, ionization energy is less. If the size of the atom is less, valence electrons are tightly held and the Ionization energy is
more.
Graph showing Variations in Ionization energy in Period 3 elements.
In a group: As you go down a group, the ionization energy decreases. This is due to the “shielding effect” which is basically when there is repulsion between the electrons in the inner shell and the valence electrons.
Explanation: As we already know that nucleus which is positively charged attracts the negatively charged electrons. Just like unlike poles attract each other and like poles repel each other. There exists repulsions between the electrons in the inner shells and valence electrons. The electrons in the inner shells acts like a ‘shield’ and as a result the valence electrons are less attracted by the nucleus. So, the electrons are loosely held and needs less energy to remove an electron from valence shell.
Several exceptions arise in trend of ionization energies across a period
Above shown is the 2nd period in which IE increases from Li to Be, as Be has fully filled outer shell, then
decreases to boron followed by increase at ‘N’ due to stable half filled configuration which followed by decrease to
oxygen. ‘Ne’ has highest value as it is difficult to remove the electron from its shell.
In a period: As we go across a period, ionization energy increases. This occurs because as the number of protons increase in the nucleus, the attraction to the valence electrons also increases.
Explanation: As we move across the period the electrons are added to the same shell, as a result the size of the atom decreases. The valence electrons are tightly held by the nucleus. So, the ionization energy increases.