Ions

Charged particles

Not all compounds are made of molecules. Common salt is made of ions.

An ion is an atom or a group of atoms that carries an electric charge. An atom is neutral, and it has the same number of positive and negative charges. When an atom loses or gains electrons, it has a charge.

• A cation is a positively charged ion. An atom that loses electrons becomes a cation. Examples: $\text{Na}^+$, $\text{Mg}^{2+}$, $\text{Ca}^{2+}$, $\text{Al}^{3+}$.
• An anion is a negatively charged ion. An atom that gains electrons becomes an anion. Examples: $\text{Cl}^-$, $\text{O}^{2-}$.

To remember which is which: an electron has a negative charge. Take electrons away, and what is left is more positive. Add electrons, and the atom becomes more negative. In the word cation, the letter t looks like a plus sign, so a cation is positive. An anion is "a negative ion".

A sodium atom has $11$ protons and $11$ electrons. If it loses $1$ electron, it has $11$ protons (charge $+11$) and $10$ electrons (charge $-10$). The total is $+1$. That is the ion $\text{Na}^+$. A chlorine atom has $17$ protons and $17$ electrons. If it gains $1$ electron, it has $18$ electrons, and the charge is $17 - 18 = -1$. That is $\text{Cl}^-$.

Sodium chloride

A sodium atom gives one electron to a chlorine atom. Sodium becomes the cation $\text{Na}^+$, and chlorine becomes the anion $\text{Cl}^-$. The two opposite charges attract each other, and they hold the ions together. Salt is a large number of these ions in a regular pattern, with no separate molecules.

Compounds that are made of ions are called ionic compounds. They usually form between a metal and a non-metal. The total charge on the positive ions is equal to the total charge on the negative ions, so the compound is neutral overall.

For $\text{MgCl}_2$: magnesium is $\text{Mg}^{2+}$, and chlorine is $\text{Cl}^-$. To balance $2+$ we need $2$ ions of $\text{Cl}^-$. The charges are $(+2) + 2 \times (-1) = 0$. That is why the formula has $\text{Cl}_2$. For $\text{Al}_2\text{O}_3$: $2 \times (+3) + 3 \times (-2) = +6 - 6 = 0$.

Groups of atoms with a charge

Some ions are groups of atoms, with one charge on the whole group. They are called polyatomic ions. The atoms in the group stay together in chemical changes.

The charge belongs to the whole group. In the sulphate ion, $\text{SO}_4^{2-}$, there is one sulphur atom and four oxygen atoms, and the whole group has a charge of $2-$.

The names of ionic compounds

The name has the metal first, and then the non-metal with the ending ide. So $\text{NaCl}$ is sodium chloride, and $\text{MgO}$ is magnesium oxide. A polyatomic ion keeps its name: $\text{CaCO}_3$ is calcium carbonate.

When the metal ion needs more than one polyatomic ion to balance, we put the group in a bracket: $\text{Ca(OH)}_2$ is calcium hydroxide, because $\text{Ca}^{2+}$ needs two $\text{OH}^-$ ions. And $\text{(NH}_4\text{)}_2\text{SO}_4$ is ammonium sulphate: two $\text{NH}_4^+$ balance one $\text{SO}_4^{2-}$.

You should find that $12$ protons with $10$ electrons give $\text{Mg}^{2+}$, and $17$ protons with $18$ electrons give $\text{Cl}^-$.

Ions in daily life

Ions are why salt water conducts electricity. In solid salt, the ions cannot move. When salt dissolves in water, the ions are free to move, and they carry the electric current. Pure water has almost no ions, so it conducts very badly. This is why you must never touch an electric switch with wet hands.

Practise with new numbers

Try it in Python

The program works out the charge of five particles from their protons and electrons.

Common mistakes

• Thinking that an ion is a different kind of atom. An ion is an atom (or group) that gained or lost electrons. The number of protons does not change.
• Mixing up the charges. Losing electrons makes the ion positive. Gaining electrons makes it negative.
• Forgetting that the compound is neutral. The total positive charge equals the total negative charge.
• Calling the ionic compound a molecule. Salt has no separate $\text{NaCl}$ molecules, only a pattern of ions.
• Changing the small numbers inside a polyatomic ion when balancing. $\text{SO}_4^{2-}$ stays $\text{SO}_4^{2-}$.

Remember these

• An ion is an atom or group of atoms with a charge. A cation is positive, and an anion is negative.
• A cation forms when an atom loses electrons. An anion forms when an atom gains electrons.
• Ionic compounds are made of ions, usually from a metal and a non-metal. They are neutral overall.
• Polyatomic ions, such as $\text{OH}^-$, $\text{CO}_3^{2-}$ and $\text{SO}_4^{2-}$, are groups of atoms with one charge.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A calcium atom loses two electrons. What is the ion, and is it a cation or an anion?

2. A sulphur atom has $16$ protons. How many electrons does the sulphide ion, $\text{S}^{2-}$, have?

3. Why is a crystal of salt neutral, though it is made of charged ions?

4. How many hydroxide ions, $\text{OH}^-$, balance one aluminium ion, $\text{Al}^{3+}$? Write the formula.