Question

Which of the following pairs of elements belong to the same period of the periodic table?

• A. C, Mg
• B. N, Ar
• C. Ca, Cl
• D. K, Cu

Answer 1

The correct option is D

K, Cu

Explanation for correct option:

(D) The electronic configuration of Potassium (K) and Copper (Cu) is:

$$\begin{gathered} \mathrm{K}\left(19\right)=2,8,8,1 \\ \mathrm{Cu}\left(29\right)=2,8,18,1 \end{gathered}$$

• The number of shells present in an atoms of an element determines the period of the element.
• Since both potassium (K) and copper (Cu) possess 4 number of shells thus, they must belong to the same period that is period 4.

Explanations for incorrect options:

(A) The electronic configuration of Carbon and Magnesium are:

$$\begin{gathered} \mathrm{C}\left(6\right)=2,4 \\ \mathrm{Mg}\left(12\right)=2,8,2 \end{gathered}$$

• Since carbon (C) has 2 shells whereas, Mg has 3 shells thus, carbon (C) must belong to the period 2 and magnesium (Mg) belongs to the period 3.

(B) The electronic configuration of Nitrogen and Argon are:

$$\begin{gathered} \mathrm{N}\left(7\right)=2,5 \\ \mathrm{Ar}\left(18\right)=2,8,8 \end{gathered}$$

• Since nitrogen (N) has 2 shells whereas, argon (Ar) has 3 shells thus, nitrogen (N) must belong to the period 2 and argon (Ar) belongs to the period 3.

(C) The electronic configuration of Calcium and Chlorine are:

$$\begin{gathered} \mathrm{Ca}\left(20\right)=2,8,8,2 \\ \mathrm{Cl}\left(17\right)=2,8,7 \end{gathered}$$

• Since calcium (Ca) has 4 shells whereas, chlorine (Cl) has 3 shells thus, calcium (Ca) must belong to the period 4 and chlorine (Cl) belongs to the period 3.

Therefore, the correct option is (D). K and Cu belongs to the same period.