Metals with Two Valencies

Why it matters

Iron can make two different rust-coloured compounds. The same metal, but it grabs chlorine in two different amounts! Some metals just cannot make up their minds.

Predict first

Iron can form FeCl₂ and also FeCl₃. What does this tell you about iron?

Iron has a variable valency. In FeCl₂ its valency is 2 (ferrous), and in FeCl₃ its valency is 3 (ferric). Some metals can combine in more than one way.

• Iron is not a real metal
• Iron can show more than one valency — correct
• Iron always has a valency of 2
• The two formulae are the same substance

What you do

Sort each element into the correct bin: does it show only one fixed valency, or can it show two or more?

Check yourself

In FeCl₂, iron is called ferrous. What is iron's valency here?

In FeCl₂ each chlorine has valency 1, and there are two of them, so iron must balance them with valency 2. This lower-valency form is named ferrous.

For a metal with variable valency, what does the suffix -ous mean compared with -ic?

The old naming system uses -ous for the lower valency (ferrous = Fe with valency 2) and -ic for the higher valency (ferric = Fe with valency 3).

Copper also has variable valency. Which two valencies does copper show?

Copper shows valency 1 (cuprous, as in Cu₂O) and valency 2 (cupric, as in CuO). Like iron, it can combine in more than one way.

• 1
• 2 — correct
• 3
• 4
• -ous is the lower valency and -ic is the higher valency — correct
• -ous is the higher valency and -ic is the lower valency
• They both mean exactly the same valency
• -ous means the metal is a gas
• 1 and 2 — correct
• 2 and 3
• Only 2
• 3 and 4