Question

An element X with atomic number 12 forms a compound with element Y with atomic number 17. The formula of the compound formed is:

• A. XY
• B. XY₂
• C. X₂Y
• D. X₂Y₃

Answer 1

The correct option is B

XY₂

The explanation for the correct option:

Step-1: Calculate the valency

The atomic number of element X= 12

Electronic configuration: The distribution of electrons in the various energy shells around the nucleus of an atom.

• Electronic configuration of X= 2, 8, 2
• For atoms with valence electrons less than 4, valency= number of valence electrons
• For X, valency= 2 as number of valence electrons= 2 i.e.; less than 4

Atomic number of element Y= 17

• Electronic configuration of Y= 2, 8, 7
• For atoms with valence electrons equal to more than 4, valency= 8- the number of valence electrons
• For Y, valency = 1 as a number of valence electrons is 6 i.e. more than 4. So, 8-7= 1

Step-2: Use the criss-cross method to calculate the formula

In this method, the valency of one element is transposed with the valency of the other element and vice versa.

$$\begin{gathered} \mathrm{Symbol}\mathrm{X}\mathrm{Y} \\ \searrow \swarrow \\ \mathrm{Valency}21 \end{gathered}$$

Therefore, the formula is ${\mathrm{XY}}_2$

Hence, option (B) is correct.