When balancing equations, which elements always have a 2 as their subscript when written alone?

They are called diatomics...bromine, chlorine, nitrogen, fluorine

There is no rule as to which elements have a 2 as their subscript when written alone. There are some elements that are diatomic in their standard states, the form they exist in at 298 K and 1 atm, such as H2(g) for hydrogen or I2(s) for iodine. These are the forms most commonly seen in chemical equations. Note that the standard state specifies both the allotropic form of the element (the subscript) and its state of matter (solid, liquid, or gas). However, it is perfectly acceptable for monatomic forms of these elements, such as H(g) or I(g), to appear in chemical reaction mechanisms. For instance, a reaction mechanism may involve sublimation of I2(s) to I2(g), followed by dissociation of I2(g) to 2I(g), followed by reaction of I(g) with another species. Nevertheless, there is a well-defined list of elements that are known to be diatomic *in their standard state*. These are: H, F, Cl, Br, I, O, and N.