Lesson 2 - Simple Formulae

The following link may help with explaining this topic after you have read the notes below. https://www.youtube.com/watch?v=URc75hoKGLY&t=28s

1. Chemical Names

Only two elements present

NaCl - sodium chloride

MgS - magnesium sulfide

AlF₃ - aluminium fluoride

The metal part of the compound's name goes first and then the second element, a non-metal, has its name shortened and 'ide' added.

Sometimes the name can have a little more detail, saying how many atoms of each element there are in the formula. For example CO₂ is called carbon dioxide and CO would be called carbon monoxide

Three elements present

NaClO - sodium chlorate

CaCO₃ - calcium carbonate

LiNO₃ - lithium nitrate

In each of these cases, the third element is always oxygen. This turns the name of the compound into an 'ate' and not an 'ide', like above.

(The only annoying exception to this rule for three elements is when the oxygen is not at the end of the formula.....

e.g. NaOH - sodium hydroxide

Here, even though there are three elements present, we called it hydroxide, not hydroxate)

2. Working Out The Chemical Formula

(a) For elements

The formula for an element on its own is usually just the symbol on the Periodic Table. For example, sodium's formula is Na, aluminium's formula is Al, because they travel around as simple atoms.

However, some elements have molecules that are diatomic and made up of two atoms joined together. These are

Hydrogen - H₂

Oxygen - O₂

Nitrogen - N₂

Any element in group 7 - F₂, Cl₂, Br₂, I₂, At₂

(b) For simple compounds where there are two elements...

There are two different methods that students use to work out formulae and both are valid. Your teacher may have chosen one method over the other. One may use hands and holding hands, the other uses charges (which I will focus on below)

When writing the formula of simple compounds containing two elements, you look at the Periodic Table to see the group the element is in and then see the charge it will have…

e.g. 1. What is the formula for sodium chloride?

Sodium is in group 1 so Na has a +1 charge

Chloride is chlorine from group 7 so Cl has a -1 charge.

Because compounds are neutral overall, we need to work out how many Cl^-1 we need for each Na⁺¹ to balance out at 0. This is a simple example, as we only need one of each...

so the formula is NaCl

e.g. 2. What is the formula for calcium fluoride?

Calcium is in group 2 so Ca has a +2 charge

Fluoride is fluorine from group 7 so F has a -1 charge.

This time, because each F is -1 and each Ca is +2, we will need two of the Fs to cancel out the charge of the one Ca, to give an overall 0.

Ca = +2 2 x F = 2 x -1 = -2

So the answer is CaF₂

e.g. 3. What is the formula for lithium nitride?

Lithium is in group 1, so Li has a charge of +1

Nitride is nitrogen from group 5, so N has a charge of -3

This time we will need three of the Li⁺¹ to cancel out each of the N^3-, to give an overall 0.

So the answer is Li₃N

YOU DON'T PUT ANY OF THE CHARGES IN THE FINAL ANSWER... THEY ARE JUST THERE AS WORKING TO HELP YOU GET TO THE FINAL ANSWER.