Molecules, Ions and Formulae: Quick Revision

A brief note on Molecules, Ions and Formulae. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• A molecule is a group of joined atoms; its atomicity is the number of atoms in it.
• An ion has a charge: a cation lost electrons (positive), an anion gained them (negative).
• Polyatomic ions are groups with one charge: $\text{OH}^-$, $\text{NO}_3^-$, $\text{NH}_4^+$, $\text{CO}_3^{2-}$, $\text{SO}_4^{2-}$, $\text{PO}_4^{3-}$.
• Valency is the combining capacity. Criss-cross: cross the valencies over, simplify, and bracket a group used more than once.

Valencies at a glance

A worked example

Write the formulas of sodium phosphate and aluminium carbonate, and count the atoms in one unit of aluminium carbonate.

1. Sodium phosphate. $\text{Na}$ is $1$ and $\text{PO}_4$ is $3$: $\text{Na}_3\text{PO}_4$.
2. Aluminium carbonate. $\text{Al}$ is $3$ and $\text{CO}_3$ is $2$: $\text{Al}_2(\text{CO}_3)_3$.
3. Atoms. $2$ Al $+ 3$ C $+ 3 \times 3$ O $= 14$ atoms.

Practice with new numbers

A program to try

The program counts the atoms of the worked example.

Mistakes to avoid

• No bracket: $\text{CaOH}_2$ is wrong; write $\text{Ca(OH)}_2$.
• Not simplifying: $\text{Ca}_2(\text{CO}_3)_2$ is $\text{CaCO}_3$.
• Mixing up the charges: losing electrons makes a positive ion.
• Calling salt a molecule: it is a pattern of ions.

Check yourself

1. What is the formula of potassium sulphate?

2. How many atoms are there in one molecule of glucose, $\text{C}_6\text{H}_{12}\text{O}_6$?

3. A chlorine atom, with $17$ protons, gains one electron. What is the ion, and how many electrons does it have?

4. What is the valency of iron in $\text{FeCl}_3$?