Formulae

Now that I've taught you about bonding, I can teach you about writing formulae! You see, the method you need to use to write formulae depends on what kinds of bonding is present in the substance.

If ionic bonding is present, write down the chemical symbols for the ions, using subscripts to show the ratio between the atoms. (The ratio should be the simplest ratio. If each number has a common factor other than 1, you haven't simplified enough.)

NaCl

CaCl₂

In NaCl, the ratio between Na and Cl atoms is 1:1- that is, for every sodium atom there's one chlorine atom. In CaCl_2, the ratio between Ca and Cl atoms is 1:2- that is, for every calcium atom there are two chlorine atoms.

How did I know that two chlorine atoms were needed? Simple. Calcium is a group 2 atom. It wants to lose two electrons and become a +2 ion. Chlorine is a group 17 atom. It wants to gain one electron and become a -1 ion. However, for the charges to balance out, you need twice as much chlorine as calcium.

If the substance is a covalent molecular substance, simply write down how many of each type of atom is in the substance.

O₂

NH₃

Of course, if the ions contain covalent bonding (e.g. the ammonium ion or the carbonate ion), write down the formulae for the ions as though they are covalent molecular substances. Use subscripts to show the ratios (if a polyatomic ion- an ion containing more than one atom- needs a subscript, surround the polyatomic ion with brackets so that the ratio subscript doesn't get confused with the formula of the ion).

(NH₄)₂CO₃

_{Hopefully I didn't make the whole formula-writing thing sound a lot more complicated than it is. It really is quite simple when you get used to it.}

If the substance has metallic bonding or is a covalent network substance, just write the ratio between atoms (like for ionic bonding).

Now it's time to teach you how to write the standard names for most of these compounds.

For ionically bonded compounds, write the first element normally, and then write the second element with an -ide ending (e.g. chlorine -> chloride, fluorine -> fluoride etc.)

NaCl- sodium chloride

K₃PO_{4- potassium phosphate}

For covalently bonded compounds, do pretty much the same as you would do for an ionic compound, but add prefixes (mono-, di-, tri-, tetra-, penta- etc.) to say how many of each atom you need. (However, the suffix mono- is rarely used for the first element.)

CO- carbon monoxide (NOT monocarbon monoxide)

N₂O₄ - dinitrogen tetroxide

Following these rules, water is technically dihydrogen monoxide, which sounds poisonous, but obviously isn't because the Health Department and whatnot like to encourage people to drink 2 litres of the stuff every day. (I rarely come close to that...)

Which reminds me... a bunch of other compounds have standardised names. For example, H₂O is water and CH₃COOH is ethanoic acid. You have to more or less know these names off by heart, but most of them are so common that you'll have a hard time forgetting them. Here are the ones that you need to know:

NH_{3- ammonia}

H₂O- water

H₂O2- hydrogen peroxide

CH₃COOH- ethanoic/acetic acid

HCl- hydrochloric acid

HNO_{3- nitric acid}

H₂CO_{3- carbonic acid}

H₂SO_{4- sulfuric acid}

H₂SO_{3- sulfurous acid}

H₃PO_{4- phosphoric acid}

Doing the inverse (writing formulae from standard names) is pretty simple. For covalently bonded compounds, just write the elements in the order stated in the formula, adding subscripts depending on the prefixes. For ionic compounds, you need to think about the charges on each ion and how many of each ion is needed in order to balance out the charges. With systematic names, you just need to remember them, but most of the systematic names are reasonably easy to remember anyway. The only hard bit would be remembering which one's sulfuric acid and which one's sulfurous acid.

Next up: calculating formula weights and molecular weights!