In short
Valency is the combining power of an element: the number of electrons an atom gains, loses or shares to form a compound. It also tells you how many bonds an atom can form.
For the main groups, the valency is the number of outer electrons for elements with 1 to 4 outer electrons, or minus that number for elements with 4 to 8. Sodium has 1 outer electron, so its valency is 1. Oxygen has 6, so its valency is . Phosphorus can show valencies of 3 and 5. Valency is used to write chemical formulae: the valencies cross over.
Magnesium has valency 2 and chlorine has valency 1. The formula is , not , since each magnesium atom gives up 2 electrons and each chlorine atom takes only 1.
Also searched as: phosphorus valency, why is it mgcl2 and not mgcl.
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