Classification of Elements and Periodicity in Properties · Question 1

The correct statement(s) regarding the periodic properties of elements is(are)

1. Second ionization enthalpy of carbon atom is less than that of boron atom.
2. Increasing order of ionic radii: Al3+<Mg2+<Na+
3. Under identical conditions, in solid state, the density of potassium metal is more than density of sodium metal.
4. The H–H bond is weaker than F–F bond.

Answer

(1) Second ionization enthalpy of carbon atom is less than that of boron atom., (2) Increasing order of ionic radii: Al3+<Mg2+<Na+

Solution

(A) B+ is 1s2 2s2 (stable filled 2s, more penetrating), while C+ is 2s2 2p1 and loses an easily removed 2p electron. So iH2(C) < iH2(B): correct.

(B) Isoelectronic ions: larger nuclear charge, smaller size; Al3+<Mg2+<Na+: correct.

(C) Density of K (0.86 g cm-3) is less than that of Na (0.97 g cm-3): wrong.

(D) H–H bond enthalpy (436 kJ mol-1) is much greater than F–F (158 kJ mol-1, lone-pair repulsion): wrong.

Answer: (A), (B)