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Valency and Combining Capacity

Valency is the combining capacity of an atom, usually decided by the number of electrons lost, gained, or shared to complete the outermost shell.

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Student-friendly explanation

Atoms tend to reach a stable outer shell arrangement. If an atom has 1, 2, or 3 valence electrons, its valency is usually 1, 2, or 3. If it has 5, 6, or 7 valence electrons, its valency is usually 3, 2, or 1 because it needs that many electrons to complete the octet. Noble gases usually have valency 0.

How to write this in exams

  1. 1

    Start with the exact idea

    Valency is the combining capacity of an atom, usually decided by the number of electrons lost, gained, or shared to complete the outermost shell.

  2. 2

    Then show how to use it

    Write electronic distribution, count outermost electrons, decide whether it loses, gains, or shares to complete the shell, then write valency.

  3. 3

    Add one concrete example

    Sodium has distribution 2, 8, 1, so its valency is 1. Oxygen has 2, 6, so its valency is 2.

  4. 4

    Avoid this incomplete answer

    Writing fluorine valency as 7 is wrong because fluorine needs only 1 electron to complete the octet.

Definition

Valency is the combining capacity of an atom, usually decided by the number of electrons lost, gained, or shared to complete the outermost shell.

Example

Sodium has distribution 2, 8, 1, so its valency is 1. Oxygen has 2, 6, so its valency is 2.

Rule to remember

For 1 to 4 valence electrons, valency is usually the same number; for 5 to 7, valency = 8 - valence electrons.

Memory hook

Valency is the gap to stability, not always the outer electron count.

Examples and method

Worked example

Nitrogen has distribution 2, 5. It needs 3 electrons to complete 8 in the outer shell, so its valency is 3.

Method to apply

Write electronic distribution, count outermost electrons, decide whether it loses, gains, or shares to complete the shell, then write valency.

Diagram support

Show outermost shell electrons and mark how many are needed to complete the octet.

How CBSE asks it

Asked by giving electron distribution and asking valency, combining capacity, or formula formation basics.

Avoid common mistakes

Common confusion

Students often write valency as the number of valence electrons for every element, so they write oxygen valency as 6 instead of 2.

Common wrong answer

Writing fluorine valency as 7 is wrong because fluorine needs only 1 electron to complete the octet.

Exam tip

If valence electrons are more than 4, subtract from 8 to find valency for basic examples.

Quick check

Why is the valency of chlorine 1 when it has 7 valence electrons?

Chlorine has valency 1 because it needs one more electron to complete its outer shell octet.

Answer writing and exam use

1-mark answer

Valency is the combining capacity of an atom, usually decided by the number of electrons lost, gained, or shared to complete the outermost shell.

2-mark answer

Valency is the combining capacity of an atom, usually decided by the number of electrons lost, gained, or shared to complete the outermost shell. For 1 to 4 valence electrons, valency is usually the same number; for 5 to 7, valency = 8 - valence electrons. Sodium has distribution 2, 8, 1, so its valency is 1. Oxygen has 2, 6, so its valency is 2.

3-mark answer

Atoms tend to reach a stable outer shell arrangement. If an atom has 1, 2, or 3 valence electrons, its valency is usually 1, 2, or 3. If it has 5, 6, or 7 valence electrons, its valency is usually 3, 2, or 1 because it needs that many electrons to complete the octet. Noble gases usually have valency 0. For 1 to 4 valence electrons, valency is usually the same number; for 5 to 7, valency = 8 - valence electrons. Nitrogen has distribution 2, 5. It needs 3 electrons to complete 8 in the outer shell, so its valency is 3. Asked by giving electron distribution and asking valency, combining capacity, or formula formation basics. Writing fluorine valency as 7 is wrong because fluorine needs only 1 electron to complete the octet.
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