What you'll learn

• Why lithium, sodium and potassium are placed together in Group 1.
• How their reactions with water and air show a trend in reactivity.
• How to predict the properties of other Group 1 metals.
• How electronic configurations explain the reactivity trend (Paper 2 only).

The big idea: a chemical family

The elements in Group 1 of the Periodic Table are called the alkali metals. The main ones you need for this topic are lithium, sodium and potassium.

They are called alkali metals because they react with water to form alkaline solutions.

Lithium, sodium and potassium are recognised as a family of elements because they have very similar chemical reactions. Their similarities come from their atoms all having one electron in the outer shell.

Basic physical properties of Group 1 metals

Group 1 metals are:

• soft — they can be cut with a knife
• shiny when freshly cut
• low density compared with most metals
• good conductors of heat and electricity
• stored under oil because they react with oxygen and water vapour in air

They are still metals, but they are much softer and more reactive than typical transition metals such as iron or copper.

Reaction with water: the key family reaction

All three metals react with cold water to produce:

• a metal hydroxide solution
• hydrogen gas

The general word equation is:

Group 1 metal + water → metal hydroxide + hydrogen

For lithium:

$$2Li(s)+2H_{2}O(l)\to 2LiOH(aq)+H_2(g)$$

For sodium:

$$2Na(s)+2H_{2}O(l)\to 2NaOH(aq)+H_2(g)$$

For potassium:

$$2K(s)+2H_{2}O(l)\to 2KOH(aq)+H_2(g)$$

The hydroxides formed are alkaline, so universal indicator would turn blue or purple.

Observations with water

Lithium reacts gently:

• floats on water
• fizzes slowly
• moves slowly on the surface
• gradually disappears

Sodium reacts more vigorously:

• floats and fizzes
• melts into a silvery ball because the reaction releases heat
• moves quickly around the surface
• disappears faster than lithium

Potassium reacts very vigorously:

• floats and fizzes rapidly
• moves quickly
• may produce a lilac flame
• disappears very quickly

Reaction with air: tarnishing and oxides

Freshly cut lithium, sodium and potassium are shiny. In air, they quickly become dull because they react with oxygen and other gases in the air. This is called tarnishing.

The main idea for IGCSE is that the metals react with oxygen to form metal oxides.

For lithium oxide:

$$4Li(s)+O_2(g)\to 2L{i}_{2}O(s)$$

A simplified equation for sodium oxide is:

$$4Na(s)+O_2(g)\to 2N{a}_{2}O(s)$$

Potassium also reacts very quickly with oxygen in air, forming oxide-type products. The exact product can be more complicated, but for this topic the important evidence is the increasing speed of reaction.

Trend in reaction with air

Down Group 1:

• lithium tarnishes slowly
• sodium tarnishes faster
• potassium tarnishes very quickly

This shows that reactivity increases down Group 1.

Comparing reactions with water and air

Both air and water reactions show the same trend.

With water

The reaction becomes more vigorous:

lithium < sodium < potassium

This means:

• fizzing becomes faster
• movement becomes faster
• more heat is released
• flames become more likely

With air

The metals tarnish faster:

lithium < sodium < potassium

So the differences in their reactions are evidence for the trend in reactivity.

Predicting properties of other alkali metals

You can use the Group 1 trend to predict the properties of other alkali metals, such as rubidium and caesium, which are below potassium.

Because reactivity increases down Group 1:

• rubidium is more reactive than potassium
• caesium is more reactive than rubidium
• both would react extremely vigorously with water
• both would tarnish very quickly in air

They would also form hydroxides with water:

rubidium + water → rubidium hydroxide + hydrogen

$$2Rb(s)+2H_{2}O(l)\to 2RbOH(aq)+H_2(g)$$

caesium + water → caesium hydroxide + hydrogen

$$2Cs(s)+2H_{2}O(l)\to 2CsOH(aq)+H_2(g)$$

Explaining the trend using electronic configurations

This explanation is Paper 2 only, but it is a very useful way to understand the chemistry.

The electronic configurations are:

• lithium: 2,1
• sodium: 2,8,1
• potassium: 2,8,8,1

Each atom has one electron in its outer shell. When a Group 1 metal reacts, it loses this outer electron to form a positive ion.

For example:

$$Na\to N{a}^{+}+e^{-}$$

The easier it is to lose the outer electron, the more reactive the metal is.

Why the outer electron is lost more easily down the group

As you go down Group 1:

1. The atoms have more electron shells.
2. The outer electron is further from the positive nucleus.
3. Inner shells reduce the attraction between the nucleus and the outer electron.
4. The outer electron is lost more easily.
5. The metal is more reactive.

Linking structure to reactions

The Group 1 pattern is powerful because it links three ideas:

1. Same group means same outer electrons

Lithium, sodium and potassium all have one outer electron.

2. Same outer electrons means similar reactions

They all form $+1$ ions and react with water to form alkaline hydroxides and hydrogen.

3. More shells down the group means higher reactivity

The outer electron is lost more easily down the group, so reactions become more vigorous.