Is Dr. H2O2F2Br2I2N2Cl2 in the house??
There are 7 diatomic molecular elements on the periodic table. When they are in the elemental state (all by themselves, not tied up in a compound) they must have a subscript 2 behind them.
Each molecule is made of two atoms of the substance.
I like to think of them as the 7 hip-hop stars of the Periodic Table. Sure... they only come in Two-packs [Tupacs !!!! ; ) ]
Tupac Hydrogen, Tupac Oxygen, Tupac Nitrogen, Tupac Fluorine, Tupac Chlorine, Tupac Bromine, Tupac Iodine. Oh Yeah!
They never exist as single atoms. They are either tied up in a compound, or tied up (chemically bonded) with each other as two packs (diatomic molecules). When balancing equations, you can never have one of these seven elements all by itself as a single atom. Doesn't happen. They can be in a compound however without the subscript two. Once they are in a compound they follow the criss-cross rules.
Example: Suppose we tried to react aluminum with oxygen to make aluminum oxide.