Group 1: soft, reactive alkali metals
Definition
Outer shell
The highest occupied electron shell of an atom, containing the electrons involved when the atom forms an ion.
- Group 1 contains the alkali metals: lithium, sodium, potassium and those below them.
- Each atom has one electron in its outer shell.
- They are soft (easy to cut), have low density and low melting points for metals.
- They react by losing their one outer electron to gain a full outer shell, forming +1+1+1 ions.
Key Idea
All Group 1 metals form ions with a +1+1+1 charge, because each loses one electron.
Reactions of the Group 1 metals
- With oxygen, the alkali metals form metal oxides, reacting more vigorously down the group.
- Lithium reacts slowly with oxygen in the air to form lithium oxide.
- Sodium reacts more readily and quickly tarnishes in air, forming sodium oxide.
- Potassium reacts very rapidly, producing a lilac flame, and forms potassium oxide.
- 4Na+O2→2Na2O4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}4Na+O2→2Na2O
- With water, they form a soluble metal hydroxide and hydrogen gas, reacting more vigorously down the group.
- Lithium fizzes steadily and gradually disappears.
- Sodium fizzes rapidly, melts into a ball and disappears quickly.
- Potassium ignites, burning with a lilac flame and sparks, and disappears very quickly.
- 2Na+2H2O→2NaOH+H22\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_22Na+2H2O→2NaOH+H2
- The hydroxide makes an alkaline solution, which is why they are called the alkali metals.
- With chlorine, they form white metal chloride salts, reacting more vigorously down the group.
- Lithium reacts with chlorine to form white lithium chloride.
- Sodium burns vigorously with a bright yellow flame to form white sodium chloride.
- Potassium reacts very vigorously to form potassium chloride.
Note
- In every one of these reactions the vigour increases from lithium to sodium to potassium.
- This is the visible sign that reactivity increases down the group.
Reactivity increases down Group 1
Definition
Reactivity
A measure of how readily a substance takes part in a chemical reaction.
- Reactivity of the alkali metals increases down the group.
- Reacting means losing the outer electron, so a metal is more reactive when that electron is easier to lose.
- Down the group, the outer electron is in a shell further from the nucleus.
- There are also more inner shells between the nucleus and the outer electron, which shield it from the nucleus.
- Both effects weaken the attraction on the outer electron, so it is lost more easily and the metal is more reactive.
Self review
- Describe how lithium, sodium and potassium differ in their reaction with water.
- Answer: lithium fizzes steadily, sodium fizzes rapidly and melts into a ball, and potassium ignites with a lilac flame.
- What is made when a Group 1 metal reacts with chlorine?
- Answer: a white metal chloride salt, such as sodium chloride.
- Write the equation for sodium reacting with water.
- Answer: 2Na+2H2O→2NaOH+H22\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_22Na+2H2O→2NaOH+H2.
- What is the trend in reactivity down Group 1, and why?
- Answer: reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily.
