Chemistry

Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air. The following are the major gaseous and particulate pollutants present in the troposphere:

1. Gaseous air pollutants: These are oxides of sulphur, nitrogen and carbon, hydrogen sulphide, hydrocarbons, ozone and other oxidants.

2. Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.

3.25. Isotopes have same number of electrons and protons, only the number of neutrons is different. The atomic number remains the same and only atomic mass differs. Hence the ionization energy of the isotopes of a chemical element remains the same.

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.

1.21 Two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical neutrality. Hence some cation vacancies are created. For example : In an ionic solid 'NaCl', is impurity of Sr2+ is added (as SrCl2) then two Na+ ions left thus lattice sited. To maintain electrical neutrality one lattice site is occupied by Sr2+ ion while other lattice site will remain vacant.

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