Ionization Energy

 

This is the notes for Ionization Energy.

Periodic properties of elements 

Ionization Energy (I.E.) or Ionization Potential(I.P.) 

Ionization energy is defined as the amount of energy required to remove the valence electron from an isolated gaseous atom in its ground state. It can be represented as; 

M + I.E.→ M^+   +   e^-

It is generally expressed in eV per atom or KJ/mole or KCal/mole. 

1 eV/atom = 96.3 KJ/mole = 23.05 Kcal/mole 

Successive ionization energies: 

The energy required to remove 1st , 2nd, 3rd and 4th electrons from the valence shell of an isolated gaseous atom are called first I.E. (I.E.1 ), second I.E. (I.E.2 ), third I.E. (I.E.3 ) and fourth I.E. (I.E.4 ) respectively.

The order of I.E. is: I.E.1 < I.E.2 < I.E.3 < I.E.4

Q. Why is 2nd I.E. higher than 1st I.E. ? 

→ After the removal of an electron, the atom changes into +ve ion, where the no. of electron decreases but the nuclear charge remains same. As a result, the nuclear charge per electron increases and the remaining electrons are held more tightly by the nucleus. Thus, more energy will be required to remove the second electron. Hence, I.E.1 < I.E.2 

Factors affecting ionization energy

i. Atomic size: The I.E. decreases with the increasing atomic size. Larger the distance between the valence electron and the nucleus, less will be the force of attraction between them and hence becomes easier to remove the electron.

ii. Nuclear charge: The attractive force of nucleus increases with the increase in nuclear charge. So, greater the value of nuclear charge, more difficult to remove an electron and hence I.E. increases. 

iii. Screening /shielding effect: The outermost electrons are shielded from the nucleus by the inner electrons. This phenomena is called shielding effect. Due to the shielding effect, the electrons are loosely attracted towards the nucleus and it is easier to remove loosely bound electron. i.e., Ionization energy value decreases.

Electronic configuration: The more stable the E.C., greater will be the I.E. Elements having half-filled and completely-filled orbital are more stable so that it is difficult to remove electron from their valence shells. For e.g.,

• Noble gases having completely filled orbital ns2 np6 posses high I.E. 
• Elements like Be, Mg, etc. having completely filled orbital posses high value of I.E. 
• Elements like N (1s2 2s2 2p3 ) , P , etc. which have half-filled orbital also posses high value of I.E.

Q. Compare the I.E. between N and O. 

Ans: N has higher I.E. than O because N has more stable half-filled E.C. due to which it becomes more difficult to remove electron from such atom. N →(1s2 2s2 2p3 ) 

Variation of I.E. across a period 

• The I.E. increases with the increase in the atomic number in a particular period. This is due to increase in nuclear charge and decrease in atomic size from left to right across a period. As a result, the electrons are more tightly bound to the nucleus which results in gradual increase in I.E. value across a period. There is some irregularity in the trend of increasing I.E. due to presence of half-filled and completely-filled orbital in some elements.

Variation of along the group 

• On moving top to bottom down the group, the I.E. goes on decreasing. This is due to following reasons: 

1. There is increase in atomic size due to addition of extra shell. 
2. There is increase in shielding effect.

[although there is increase in nuclear charge from top to bottom in a group, its effect is outweighed by the effect of increasing atomic size and shielding effect.]

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