medium difficulty5 min

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.

In one line: Valency is the gap to stability, not always the outer electron count.

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Nitrogen has distribution 2, 5.
2It needs 3 electrons to complete 8 in the outer shell, so its valency is 3.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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

Common wrong answer

Writing fluorine valency as 7.

Why it loses marks: Fluorine needs only 1 electron to complete the octet.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

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

1
Enough for 1 mark
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
Add for 2 marks
Example: Sodium has distribution 2, 8, 1, so its valency is 1. Oxygen has 2, 6, so its valency is 2.
3
Add for 3 marks
Atoms tend to reach a stable outer shell arrangement.

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