chapter 7.  ELEMENTS OF GROUPS 16, 17 AND 18 class 12 chemistry textbook solution

Answer:-

The trend in first ionization enthalpies across sulfur (S), chlorine (Cl), and argon (Ar) can be explained by the following factors:

1. Effective Nuclear Charge (Zeff): The first ionization enthalpy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state to form cations. It depends on the effective nuclear charge experienced by the outermost electrons. The effective nuclear charge is the positive charge felt by the outermost electrons, which is equal to the actual nuclear charge (the number of protons) minus the shielding effect of inner electrons.

2. Atomic Radius: Atomic size or radius also plays a significant role in determining ionization enthalpies. As you move across a period (from left to right) in the periodic table, the atomic radius generally decreases. This is because as you add more protons to the nucleus, the electrons are pulled closer to the nucleus, resulting in a smaller atomic size.

Now, let’s explain the trend:

1. Sulfur (S) is in the third period of the periodic table, with 16 protons. Its atomic number is 16. The effective nuclear charge experienced by its valence electrons is less than the actual nuclear charge due to the shielding effect of the inner electrons. Sulfur has three electron shells, and its valence electrons are in the third shell. The first ionization enthalpy of sulfur is 1000 kJ/mol^-1.

2. Chlorine (Cl) is also in the third period of the periodic table, with 17 protons. Its atomic number is 17. Chlorine has one more proton in its nucleus compared to sulfur. This extra proton results in a higher effective nuclear charge experienced by the valence electrons, making it more difficult to remove an electron from chlorine. Additionally, the atomic radius of chlorine is smaller than that of sulfur due to the increased nuclear charge. Therefore, the first ionization enthalpy of chlorine is higher than that of sulfur, at 1256 kJ/mol^-1.

3. Argon (Ar) is also in the third period of the periodic table, but it has 18 protons. Its atomic number is 18. Argon has the highest effective nuclear charge among the three elements because it has the most protons, and it has the smallest atomic radius. It is very stable and has a full valence shell (eight electrons in the third shell). As a result, it requires the most energy to remove an electron from argon. Therefore, the first ionization enthalpy of argon is the highest among the three, at 1520 kJ/mol^-1.

In summary, the observed trend in first ionization enthalpies of S, Cl, and Ar can be explained by the increasing effective nuclear charge and decreasing atomic radius as you move from left to right across the periodic table. Elements with higher effective nuclear charges and smaller atomic radii have higher first ionization enthalpies because it is more energetically unfavorable to remove electrons from them.