Question

Which of the following sets does not belong to a group?

• A. Mg, Al, Ca
• B. Na, K, Rb
• C. B, Al, Ga
• D. Ne, Ar, Kr

Answer 1

The correct option is A

Mg, Al, Ca

Explanation for the correct option:

1. In a periodic table, groups are vertical columns with the same valence shell electronic configuration. There are 18 groups in the periodic table.
2. To determine if a particular set of elements belongs to the same group or not, first determine their electronic configuration.

$$\begin{gathered} {}_{12}\mathrm{Mg}\to 2,8,2 \\ {}_{13}\mathrm{Al}\to 2,8,3 \\ {}_{20}\mathrm{Ca}\to 2,8,8,2 \end{gathered}$$

Hence, their valence shell electrical configuration is clearly different, indicating that this bunch of elements does not belong to the same group.

Explanation for the incorrect options:

Condition to fall into the same group – the set should have the same valance shell electronic configuration.

1. Option (B) is incorrect because elements belong to the same group 1 (called Alkali metals) as it has the same valance shell electronic configuration.

$$\begin{gathered} {}_{11}Na\to 2,8,1 \\ {}_{19}K\to 2,8,8,1 \\ {}_{37}Rb\to 2,8,18,8,1 \end{gathered}$$

2. Option (C) is incorrect because elements belong to the same group 13 (called Boron family) as it has the same valance shell electronic configuration.

$$\begin{gathered} {}_{5}B\to 2,3 \\ {}_{13}Al\to 2,8,3 \\ {}_{31}Ga\to 2,8,18,3 \end{gathered}$$

3. Option (D) is incorrect because elements belong to the same group 18 (called zero groups) as it has the same valance shell electronic configuration.

$$\begin{gathered} {}_{10}Ne\to 2,8 \\ {}_{18}Ar\to 2,8,8 \\ {}_{36}Kr\to 2,8,18,8 \end{gathered}$$

Hence option (A) is the correct option, where all the elements have different valence shell electronic configurations, so they do not belong to the same group.