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.
Practice This ConceptMain explanation
Teacher 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.
Example
Sodium has distribution 2, 8, 1, so its valency is 1. Oxygen has 2, 6, so its valency is 2.
Simple analogy
Valency is the gap to stability, not always the outer electron count.
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.
Exam tip
If valence electrons are more than 4, subtract from 8 to find valency for basic examples.
Answer writing and exam use
1-mark use
Write the exact meaning of valency and combining capacity in one clean line.
2-mark use
Define valency and combining capacity and add one example or condition.
3-mark use
Explain valency and combining capacity, show the method or example, and mention the common mistake.
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