What are the oxidation number of $2C{l^ - }$ and $C{l_2}$

Hint :Oxidation number is the number of electrons gained or lost during the oxidation. If the given sign is positive it indicates electron loss. If the sign is negative it indicates electron gain.

Complete Step By Step Answer:
For $2C{l^ - }$ iron it indicates that there are $2$ions of $C{l^ - }$present. That means the oxidation number is $2 \times ( - 1) = - 2$
Whereas for $C{l_2}$ it indicates $2$atoms of $Cl$ present. These to atoms share bonds to form chlorine gas.
 There are total of $7$ valance electrons per chlorine atom making it a total of $14$in $C{l_2}$
These electron share a bond between $2$chlorine atom to form $C{l_2}$.
One atom receives a charge of $ - 1$and the other receives a charge of $ + 1$ making the total of $ - 1( + 1) = 0$

As seen in the above diagram each $Cl$ atom shares one of the valance electrons to form a bond.

The final structure looks like

Therefore the oxidation number of $C{l_2}$is $0$.

Note :
Chlorine gas is used in disinfection, paper producing industries etc. an atom having the highest electronegativity is given negative oxidation number.