Q12

1 markMCQSection A

On the basis of electronic configuration of $\frac{9}{5}\mathrm{X}$ , the group number and period of the element 'X' is:

Metals and Non-metals

Chemical Properties of Metals

Options

(a)Group 15 period 2

(b)Group 13 period 2

(c)Group 9 period 5

(d)Group 13 period 5

Official Answer

The correct option is (b). The formula of the compound formed is $\text{X}_2\text{Y}$ .

X2Yvalency 1valency 2electronic configurationSodium Oxide

Marking Scheme

1Writing electronic configurations of X and Y (0.5 marks)
2Determining valencies and deriving the formula X2Y (0.5 marks)

Hint

Write the electronic configurations of both elements to find their valencies, then use the criss-cross method to determine the chemical formula.

Quick Oral Answer

Element X with atomic number 11 is Sodium (valency 1) and Y with atomic number 8 is Oxygen (valency 2). They combine to form Sodium Oxide, which has the formula X2Y.

Analysis & Explanation

Element X (Atomic Number 11):

Electronic configuration: $2, 8, 1$ .
It has $1$ valence electron, which it loses to achieve a stable octet, forming a univalent cation: $\text{X}^+$ .
This element is Sodium ( $\text{Na}$ ), an alkali metal.

Element Y (Atomic Number 8):

Electronic configuration: $2, 6$ .
It has $6$ valence electrons and needs $2$ more electrons to complete its octet, forming a divalent anion: $\text{Y}^{2-}$ .
This element is Oxygen ( $\text{O}$ ), a non-metal.

Formula of the Compound:

To form a neutral ionic compound, two $\text{X}^+$ ions combine with one $\text{Y}^{2-}$ ion to balance the charges.
Using the criss-cross method:
Valency of $\text{X} = 1$
Valency of $\text{Y} = 2$
Formula = $\text{X}_2\text{Y}$ (representing Sodium Oxide, $\text{Na}_2\text{O}$ ).

Option (a) is incorrect because it represents a 1:1 ratio, which does not balance the charges of $\text{X}^+$ and $\text{Y}^{2-}$ .
Option (c) is incorrect because it would represent a compound like $\text{XY}_2$ .
Option (d) is incorrect because it represents a 2:3 ratio, typical of trivalent metals like aluminium ( $\text{Al}_2\text{O}_3$ ).

Common Mistakes

1Swapping the valencies to write XY2 instead of X2Y.
2Not balancing the charges of the ions to form a neutral compound.

Interesting Facts

The compound formed is Sodium Oxide ( $\text{Na}_2\text{O}$ ), which is a basic oxide that dissolves in water to form the strong alkali Sodium Hydroxide ( $\text{NaOH}$ ), as described in the NCERT text: $\text{Na}_2\text{O(s)} + \text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)}$ .

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 1 mark in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Metals and Non-metals" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chemical Properties of Metals" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2020 paper.