Classification of Elements and Periodicity in Properties · Question 27

The incorrect decreasing order of atomic radii is

1. Si > P > Cl > F
2. Mg > Al > C > O
3. Al > B > N > F
4. Be > Mg > Al > Si

Answer

(4) Be > Mg > Al > Si

Solution

Atomic radius decreases from left to right across a period (nuclear charge increases, electrons enter the same shell) and increases down a group (a new shell is added).

(1) Si, P, Cl are in period 3 (left to right) and F is in period 2 Si > P > Cl > F: correct.

(2) Mg, Al are in period 3; C, O are in period 2 Mg > Al > C > O: correct.

(3) Al is below B in group 13; B, N, F are in period 2 (left to right) Al > B > N > F: correct.

(4) Be (period 2) lies above Mg (period 3) in group 2, so Be is smaller than Mg (Be 111 pm, Mg 160 pm).

Be > Mg > Al > Si is incorrect.

Answer: (4)