Compounds

Every compound has a characteristic chemical formula. This is essentially the ratio(and to an extent, the structural ordering) of the atoms in a compound. The ratios are represented by the subscripts. Let's take the compound we arguably know the most about, water(H₂O). The formula for water means that there are 2 hydrogen atoms and 1 oxygen atom in one molecule of water. Let's take another example known as benzene. The chemical formula for benzene is C₆H₆, meaning that there are 6 Carbon atoms and 6 Hydrogen atoms in one molecule of benzene.

However, another way to denote the formula of a compound is through its empirical formula. Think of an empirical formula like a reduced fraction in math. If we have a fraction of 2/4, we can reduce it to 1/2. The empirical formula of benzene, C₆H₆, is CH, because the ratio of carbon atoms to hydrogen atoms is 1:1. This may seem equal to the true molecular formula of benzene but it isn't, and this is where the reduced fraction analogy breaks down. Formulae represent compounds, which have distinct masses. A compound with 6 carbon atoms and 6 hydrogen atoms will have a mass six times that of a compound with 1 carbon atom and 1 hydrogen atom. Thus, we can't just say C₆H₆ = CH, since the two have different masses.

The general rule of thumb for writing chemical formulae is to write in alphabetical order by element abbreviation. For example, H₂O has H written first becomes H comes alphabetically before O. When a polyatomic ion is chemically bonded with anything else, be it another polyatomic ion or another atom, you'd write out the polyatomic ion with parentheses and its corresponding subscript.

Polyatomic ions are very similar to compounds except that they have a nonzero net electrical charge(their charge isn't 0 due to the balance of electrons and protons). Think of them as molecules with charge, which is why they're sometimes called molecular ions. Molecules are orderings of various atoms together and polyatomic ions are essentially that except they carry a net charge. This can be exemplified by the bicarbonate ion(HCO₃^-). When a polyatomic ion is chemically bonded with anything else, be it another polyatomic ion or another atom, you'd write out the polyatomic ion with parentheses and its corresponding subscript. However, omit the charge from the formula and stick the net charge at the end. For example, let's take what you get when you put a chlorine anion(Cl^-) and an ammonium cation(NH₄⁺) together, you get (NH₄)Cl, or ammonium chloride. Since the charges of the anion and cation cancel, ammonium chloride has no net electrical charge. Additionally, notice how the charges of neither ion was put in the formula. We omitted the charges of the individual charges and just add them up to represent the total charge of the combined molecule, which in this case is 0.