Question 1

How is valency different from oxidation number?

Accepted Answer

Valency is the combining capacity of an element, commonly represented by the number of bonds it forms in a particular compound. Oxidation number is the charge assigned by the oxidation-number rules, including assigning covalent bond electrons to the more electronegative atom; it is distinct from formal charge.

Question 2

Why does the valency trend rise and then fall across Period 3?

Accepted Answer

The common valencies are Na 1, Mg 2, Al 3 and Si 4, after which they are P 3, S 2 and Cl 1. The fall after silicon reflects the simple outer-shell capacity pattern together with the variable valencies shown by phosphorus, sulfur and chlorine.

Question 3

Which Period 3 elements show variable valency?

Accepted Answer

Phosphorus shows 3 and 5, sulfur shows 2, 4 and 6, and chlorine shows 1, 3, 5 and 7. These common valencies should not be replaced by a single fixed number.

Question 4

Why is oxygen -1 in hydrogen peroxide?

Accepted Answer

Hydrogen peroxide is a peroxide. Each oxygen keeps one electron from the O-O bond because that bond is shared equally, while it receives both electrons from its O-H bond. This gives each oxygen an oxidation state of -1.

Question 5

Why is hydrogen -1 in sodium hydride but usually +1?

Accepted Answer

Hydrogen is usually +1 in compounds with nonmetals because it is generally less electronegative than them, so bonding electrons are assigned away from H. In a metal hydride such as NaH, hydrogen is more electronegative than sodium and receives the transferred electrons, giving it oxidation state -1.

Valency & Oxidation States — The Number Above Every Tile

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