Question

Question #f2b1a

Answer 1

It's an atom that can have different valencies.

Explanation:

Some atoms can have multiple valencies.
Copper is usually `Cu^(2+)`, but `Cu^+` also exists.
Iron can be `Fe^(2+)` or `Fe^(3+)`

Chlorine is even 'worse':
It has valencies `-1,+1,+3,+5,+7` in ions like:
chloride (`Cl^-`), hypochlorite (`ClO^-`), chlorite (`ClO_2^-`), chlorate (`ClO_3^-`), and perchlorate (`ClO_4^-`).

The above are just examples; there are many more.
In the more elaborate versions of the Periodic System the possible valencies are given.