Consider the following statements regarding the calculation of valency: 1. For elements like hydrogen and sodium, valency is equal to the number of electrons in the outermost shell. 2. For elements like fluorine and oxygen, valency is equal to the number of valence electrons plus two. 3. Magnesium has a valency of 2 because it loses two electrons to complete its octet. Which of the statements given above is/are correct?

NCERT Class 9 Science — Chapter 4: Structure of the atom

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Consider the following statements regarding the calculation of valency:

For elements like hydrogen and sodium, valency is equal to the number of electrons in the outermost shell.
For elements like fluorine and oxygen, valency is equal to the number of valence electrons plus two.
Magnesium has a valency of 2 because it loses two electrons to complete its octet.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C1 and 3 only
D1, 2 and 3

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Solution

The correct option is (c) 1 and 3 only.

Explanation

Valency is the combining capacity of an element, determined by the electronic configuration of its atoms. It represents the number of electrons an atom must lose, gain, or share to achieve a stable noble gas configuration (octet or duplet).

Statement 1 is Correct:
For elements having 1, 2, 3, or 4 electrons in their outermost shell, the valency is typically equal to the number of valence electrons.

Hydrogen: Has 1 valence electron; its valency is 1.
Sodium (Atomic No. 11): Configuration is 2, 8, 1. It has 1 valence electron; its valency is 1.

Statement 2 is Incorrect:
For elements with 5, 6, or 7 electrons in the outermost shell (typically non-metals), valency is determined by subtracting the number of valence electrons from 8 (Valency = 8 – Valence Electrons).

Oxygen (Atomic No. 8): Configuration is 2, 6. Valency is \(8 - 6 = 2\).
Fluorine (Atomic No. 9): Configuration is 2, 7. Valency is \(8 - 7 = 1\).

The statement incorrectly claims valency is equal to valence electrons plus two.

Statement 3 is Correct:
Magnesium (Atomic No. 12) has an electronic configuration of 2, 8, 2. Being a metal, it achieves stability by losing the 2 electrons in its outermost shell to attain the stable configuration of Neon (2, 8). Consequently, its valency is 2.

Key Takeaway:
Valency equals the number of valence electrons for atoms with 1 to 4 valence electrons, whereas for atoms with 5 to 7 valence electrons, it is calculated as 8 minus the number of valence electrons.

Your note

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With reference to 'Valency', consider the following statements: 1. It is the combining capacity of the atoms of elements. 2. Atoms react to achieve a duplet in the outermost shell in all cases. 3. If the outermost shell has 8 electrons, the valency is 8. Which of the statements given above is/are correct?

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With reference to the definition of an element, consider the following statements: 1. Elements are defined by the number of protons they possess. 2. Atoms of a given element can have different numbers of protons. 3. The atomic number is characteristic of a particular element.

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The correct option is (c) 1 and 3 only.

Explanation

Statement 1 is Correct:
For elements having 1, 2, 3, or 4 electrons in their outermost shell, the valency is typically equal to the number of valence electrons.

Hydrogen: Has 1 valence electron; its valency is 1.
Sodium (Atomic No. 11): Configuration is 2, 8, 1. It has 1 valence electron; its valency is 1.

Oxygen (Atomic No. 8): Configuration is 2, 6. Valency is \(8 - 6 = 2\).
Fluorine (Atomic No. 9): Configuration is 2, 7. Valency is \(8 - 7 = 1\).

The statement incorrectly claims valency is equal to valence electrons plus two.