Modern Periodic Law And The Present Form Of The Periodic Table

Correct Answer: Y would have an ionization potential between those of X and Z

Explanation:
• The elements X (Z = 19), Y (Z = 37), and Z (Z = 55) are all alkali metals:
→ X = Potassium (K), Y = Rubidium (Rb), Z = Cesium (Cs)

• In a group, as atomic number increases (going down the group), the atomic size increases.
• As a result, the outermost electron is farther from the nucleus and is more easily removed.
• Therefore, ionization potential decreases down the group.

So the correct order is:
Ionization Potential: K (X) > Rb (Y) > Cs (Z)

Hence, Y (Rubidium) would have an ionization potential between those of X and Z.

The property of halogens which is not correctly matched is:

(electron gain enthalpy)

Explanation:

• Density: Density increases down the group due to increasing atomic mass. Hence, is correct.
• Ionization Enthalpy: Ionization enthalpy decreases down the group due to increasing atomic size. Hence, is correct.
• Electron Gain Enthalpy: Electron gain enthalpy generally becomes less negative (less exothermic) down the group. However, fluorine has an exceptionally small size. Due to this, there are strong interelectronic repulsions in the relatively compact 2p subshell of fluorine. Consequently, it does not accept the additional electron as readily as chlorine does. Thus, chlorine has a more negative electron gain enthalpy than fluorine. The correct order is . Therefore, is incorrect.
• Electronegativity: Electronegativity decreases down the group due to increasing atomic size. Hence, is correct.

Therefore, the incorrect match is:

(electron gain enthalpy)