Classification of Elements and Periodicity in Prop

Solution:

Enthalpy in simple terms means chemical energy, the Electron Gain Enthalpy is equal to the energy released or absorbed when an atom gains an electron to form a anion (in gaseous state).

The Negative ion enthalpy is measured when net amount of energy decreases. This happens when atom releases some amount of energy in an Exothermic process.

(i) O or F:

The electronic configuration of O and F are:

O: 1s² 2s² 2p⁴

F: 1s² 2s² 2p⁵

Fluorine (F) has a more negative electron gain enthalpy than oxygen (O) because of its smaller size and its valency.

Due to smaller size, all electrons feels stronger electrostatic attraction force, hence it needs more energy to release the electron. Moreover fluorine needs only one electron to complete its outer shell and become stable unlike oxygen which needs two.

(ii) F or Cl:

The electronic configuration of Cl and F are:

Cl: [Ne] 3s² 3p⁵

F: 1s² 2s² 2p⁵

As per the NCERT "Value of electron gain enthalpy of Cl is more negative than that of F, though F has higher electronegativity. This is because F atom is small, so there is more repulsion in its 2p orbital."

Since Fluorine is very small, its electrons are packed. When an extra electron tries to enter, there's too much repulsion due to lack of space. But chlorine is slightly bigger, so there's more space for the new electron. So less repulsion and more energy released when Cl gains an electron. That's why, chlorine releases more energy, so its electron gain enthalpy is more negative than fluorine.

NCERT Chemistry 11th Solutions

3.17. Electronic configuration of Na and Mg are

Na = 1s² 2s² 2p⁶ 3s¹
Mg = 1s² 2s² 2p⁶ 3s²

First electron in both cases has to be removed from 3s-orbital but the nuclear charge of Na (+ 11) is lower than that of Mg (+ 12) therefore first ionization energy of sodium is lower than that of magnesium.
After the loss of first electron, the electronic configuration of

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

Here electron is to be removed from inert (neon) gas configuration which is very stable and hence removal of second electron from Na⁺ requires more energy in comparison to Mg.
Therefore, second ionization enthalpy of sodium is higher than that of magnesium.

3.16.  (i) In the electronic configuration of Be (1s² 2s²) the outermost electron is present in 2s-orbital while in B (1s² 2s² 2p¹) it is present in 2p-orbital. Since 2s – electrons are more strongly attracted by the nucleus than 2p-electrons, therefore, lesser amount of energy is required to knock out a 2p-electron than a 2s – electron.

Therefore, Be has higher? I H than that of B.

(ii) The electronic configuration of oxygen is 1s² 2s² 2p⁴. The 2p orbital contains 4 electrons out of which 2 are present in the same 2p-orbital. Due to this, the electron repulsion increases. N has stable half-filled configuration. F has higher effective nuclear charge. Hence, the ionization enthalpy of O is lower than that of N and F.

3.14. Significance of the term 'isolated gaseous atom'. The atoms in the gaseous state are far separated in the sense that they do not have any mutual attractive and repulsive interactions. These are therefore regarded as isolated atoms. In this state, the value of ionization enthalpy and electron gain enthalpy are not influenced by the presence of the other atoms. It is not possible to express these when the atoms are in the liquid or solid state due to the presence of inter atomic forces.

Significance of the term 'ground state'. Ground state of the atom represents the normal – energy state of an atom. It means electrons in a particular atom are in the lowest energy state and they neither lose nor gain electron. Both ionisation enthalpy and electron gain enthalpy are generally expressed with respect to the ground state of an atom only.