Ionisation Energy / Ionisation Enthalpy (I.E.)

It is amount of energy required to remove most loosely bound electron from gaseous atom to form gaseous cation

Example

It is expressed in Kj/mole , Kcal/mole , en/mol , en/atom

Successive Ionisation energy are the energies required to remove I^st, II^nd and III^rd electron one by one.

I.E₃ > I.E₂ > I.E₁

Second Ionisation energy. is always greater than first Ionisation energy because after the loss of one electron, the size decreases and remaining electrons are tightly held to the nucleus so more energy is required to remove them.

Factors affecting Ionisation Energy.

(1) Size:-Smaller the size of atom, higher is the Ionisation Energy.

(2) Effective Nuclear Charge:-  greater the electro negative charge greater is Ionisation Energy.

(3) Penetration Effect (Closeness to the Nucleus):- greater this effect, greater Ionisation Energy.

Example-  S > P > D > F ( decreasing order of penelvation  effect.

        IE_(s) > IE_p > IE_(d) > IE_f. for the same shell.

(4) Half filled / Fully filled configuration:- There type of configurations are extra – stable ionisation Energy are tightly held to the nucleus, so greater is the Ionisation Energy.

(5) Screening Effect / Shielding Effect:- greater the Screening Effect, grater the Ionisation Energy. Electro Nuclear charge Decreases for outermost Shell.

Trend of Ionisation Energy along the Periods and Groups

Periods

Groups

Trend:- Along the periods Ionisation Energy increases Reason:- Size of atom Decreases. Exceptional case:- Ionisation Energy of Be > Ionisation Energy Boron Ionisation Energy of Mg > Ionisation Energy of Al. Because ­4Be:- 1s2 2s2 (EΘ unfiguration)                  5B- :- 1s2 2s2 2p1 Because of fully filled s-orbital in Beryllium or greater penetration of seΘ in Be than B. so more energy is required to remove  last eΘ .   Mg - 1s2 2s2 2p6 3s2   Al   - 1s2 2s2 2p6 3s2 3p1                 Same reason.

Along the groups Ionisation Energy decreases Reason:-Size of atom increases. Exceptional Cases:- Ionisation Energy of Ga > Ionisation Energy of Al Because of smaller size of gallium and higher effective Nuclear charge in gallium, it has more Ionisation Energy than Aluminium

 

Another Exceptional Case In Periods

Ionisation Energy of Nitrogen > Ionisation Energy of Oxygen

Ionisation Energy of phosphorous > Ionisation Energy of Sulpher.

Same Reason    7^N - 1s² 2s² 2p³

                            8^O - 1s² 2s² 2p⁴

Nitrogen has half filled p orbital , which is extra stable so more energy is required to remove last e^Θ .

Important Question and Answer

Q(1) First Ionisation Energy of Na is < that of mg. but second Ionisation Energy of Na is higher than that of Mg?

Ans. 71^Na- 1s² 2s² 2p⁶ 3s¹          Mg^- 1s² 2s² 2p⁶ 3s²

I case:- Mg  has fully filled extra stable configuration. So more energy is required

II case:- Configuration will be

Na⁺ - 1s² 2s² 2p⁶               Mg⁺ - 1s² 2s² 2p⁶ 3s¹

Now Na⁺ has fully filled extra stable configuration. So more energy is needed to remove outermost election

Q. Why isolated gaseous atom is taken in determination of ionisation Energy?

Ans. e^Θ is always lost from the gaseous isolated atom not in the liquid / solid state and also for the compassion purposes, e^Θ lost from isolated gaseous atom.