Question

An element ‘X’ with atomic number 11 forms a compound with element ‘Y’ with atomic number 8. The formula of the compound formed is:

• A. XY
• B. X₂Y
• C. XY₂
• D. X₂Y₃

Answer 1

The correct option is B

X₂Y

Explanation

Step-1: Calculate the valency

The atomic number of element X= 11

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

• Electronic configuration of X= 2, 8, 1
• For atoms with valence electrons less than 4, valency = number of valence electrons
• For X, valency= 1 as number of valence electrons=1 i.e.; less than 4

The atomic number of element Y= 8

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

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.

$$\begin{gathered} \mathrm{Symbol}\mathrm{X}\mathrm{Y} \\ \searrow \swarrow \\ \mathrm{Valency}12 \end{gathered}$$

Therefore, the formula is ${\mathrm{X}}_2\mathrm{Y}$

Hence, option (b) is correct.