[Sterling]

Just wondering if it is true in equations that Cl or H as gases are written as Cl2 and H2 and why that is.

[MrTeo]

Of course it's true 

Every system in nature tends to reach the less energetic configuration: in our case a single H or Cl atom should be written as H• or Cl•, as it's a radical, an atom (the real definition applies also to atom complexes, such as CH₃•) with an unpaired electron and a high reactivity (and it's very unstable too). That's why more Cl• tend to form Cl₂ lowering their potential energy in the formation of molecule.

[Borek]

Also N₂, O₂, F₂, Br₂ and I₂.

There are more gaseous diatomics, but they require exotic conditions and are not that easy to observe.