Class 11th

Environmental chemistry is a branch of environmental studies. Environmental studies deal with the sum of all social, economical, biological, physical and chemical interrelations with our surroundings. Environmental chemistry deals with the study of the origin, transport, reactions, effects and fates of chemical species in the environment

3.24.  (a) Gain of an electron leads to the formation of an anion. The size of an anion will be larger than that of the parent atom because the addition of one or more electrons would result in increased repulsion among electrons and decrease in effective nuclear charge.

(b) Loss of an electron from an atom results in the formation of a cation. A cation is smaller than its parent atom because it has former electrons while its nuclear charge remains the same.

3.23. On Pauling scale, the electronegativity of nitrogen is 3.0 which indicates that it is quite electronegative. But it is not correct to say that the electronegativity of nitrogen in all the compounds is 3. It varies with the atoms that N is bonded with. For example, in NH₃, N has a different electronegativity than N in NO₂

3.22. Electron gain enthalpy refers to tendency of an isolated gaseous atom to accept an additional electron to form a negative ion. Whereas electronegativity refers to tendency of the atom of an element to attract shared pair of electrons towards it in a covalent bond.

3.21. When an electron is added to oxygen atom, energy is released to form a negative ion. This enthalpy change called the first electron gain enthalpy is thus negative. On adding another electron to the O^? ion, it experiences repulsion from the anion due to which the instability of the ion increases. Thus, the addition of the second electron requires energy due to which the second electron gain enthalpy is positive.

O + e^? ? O^?     ? H₁ = -141KJ

O^? + e^? ? O^2?   ? H₂ = 780 KJ

3.19. On moving down the group, the ionization enthalpy decreases. This is true for B and Al due to the bigger size of Al.

The ionization enthalpy of Ga is unexpectedly higher than Al because Ga contains 10d electrons in inner shell whose shielding is less effective than that of s and p electrons.

The outer electron is held fairly strongly by the nucleus. The ionization enthalpy increases slightly. A similar increase is observed from In to Tl due to presence of 14f electrons in the inner shell of Tl which have poor screening effect.

Atomic size: With the increase in atomic size, the number of electron shells increase. Therefore, the force that binds the electrons with the nucleus decreases. The ionization enthalpy thus decreases with the increase in atomic size.

Screening or shielding effect of inner shell electron: With the addition of new shells, the number of inner electron shells which shield the valence electrons increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionization enthalpy decreases.