Gaining and losing electrons, and the oxides

Losing and gaining

In Chapter 7, we saw that a metal atom loses electrons and a non-metal atom gains them. The table tells us how easy each one is.

Across a period, from left to right:

• The tendency to lose electrons decreases: sodium loses its one electron very easily, and magnesium loses two with a little more difficulty.
• The tendency to gain electrons increases: chlorine gains one electron very easily.

Down a group:

• The tendency to lose electrons increases, since the outer electron is farther away. So potassium is more reactive than sodium.
• The tendency to gain electrons decreases, since the nucleus is farther from the new electron. So fluorine is more reactive than chlorine.

The oxides

An oxide is a compound of an element with oxygen. The nature of the oxide follows the nature of the element.

• The oxides of metals are basic. They react with an acid, and many dissolve in water to make a base (an alkali).

• The oxides of non-metals are acidic. They dissolve in water to make an acid.

• The oxide of aluminium, $\text{Al}_2\text{O}_3$, is amphoteric. It reacts with both acids and bases, as we saw in Chapter 7.

Across period $3$, the oxides go from basic ($\text{Na}_2\text{O}$, $\text{MgO}$), to amphoteric ($\text{Al}_2\text{O}_3$), to acidic ($\text{SiO}_2$, $\text{P}_2\text{O}_5$, $\text{SO}_3$, $\text{Cl}_2\text{O}_7$).

Down a group, the oxides are more basic. The oxides of the metals at the bottom of group $1$ are more strongly basic.

You should find that each step to the right adds half an oxygen atom for each atom of the element, while the oxide turns from basic to acidic.

The formulae of the oxides follow the valency

The valency of oxygen is $2$. An element of valency $1$ makes $\text{X}_2\text{O}$, as in $\text{Na}_2\text{O}$. An element of valency $2$ makes $\text{XO}$, as in $\text{MgO}$. An element of valency $3$ makes $\text{X}_2\text{O}_3$, as in $\text{Al}_2\text{O}_3$.

Practise with new numbers

Try it in Python

This program lists the oxides of period 3 with their nature.

Remember these

• Across a period: the tendency to lose electrons falls, and the tendency to gain rises. Down a group: the tendency to lose rises.
• Metal oxides are basic. Non-metal oxides are acidic. $\text{Al}_2\text{O}_3$ is amphoteric.
• Across period $3$, the oxides change from basic to acidic.

Check yourself

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

1. What kind of oxide does sulphur form, and what does it make with water?

2. Magnesium oxide dissolves a little in water. What does the solution do to red litmus?

3. Write the formula of the oxide of an element X with a valency of $1$.

4. Across period 3, where do the oxides change from basic to acidic?