Group 1: The Alkali Metals

Introduction to Alkali Metals

Alkali metals are the elements found in Group 1 of the periodic table. This group includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). They share similar properties:

• Soft metals that can be cut easily with a knife.
• Low density; lithium, sodium, and potassium are less dense than water and will float.
• Shiny and metallic when freshly cut but quickly tarnish in air.
• Stored under oil to prevent reaction with air or moisture.

Physical Properties

Alkali metals have distinctive physical properties that set them apart from other metals:

• They have low melting and boiling points compared to most metals.
• They are good conductors of electricity due to their metallic bonding.
• They have low density; for example, lithium, sodium, and potassium float on water.

Element	Melting Point ($^\circ\mathrm{C}$)	Density ($\mathrm{g\,cm^{-3}}$)
Lithium (Li)	$180^\circ\mathrm{C}$	$0.53\,\mathrm{g\,cm^{-3}}$
Sodium (Na)	$98^\circ\mathrm{C}$	$0.97\,\mathrm{g\,cm^{-3}}$
Potassium (K)	$63^\circ\mathrm{C}$	$0.86\,\mathrm{g\,cm^{-3}}$

These low melting points and densities make alkali metals unique compared to many other metals, which are harder and denser.

Chemical Properties

Alkali metals are very reactive, especially with water and oxygen:

• They react vigorously with water to form alkaline hydroxides and hydrogen gas:

$$\mathrm{2M (s) + 2H_2O (l) \rightarrow 2MOH (aq) + H_2 (g)}$$

where $M$ represents an alkali metal.

For example, sodium reacts with water:

$$\mathrm{2Na (s) + 2H_2O (l) \rightarrow 2NaOH (aq) + H_2 (g)}$$

This reaction produces sodium hydroxide, which is alkaline, and hydrogen gas, which may ignite due to the heat released.

Alkali metals also react with oxygen in the air to form metal oxides:

$$\mathrm{4M (s) + O_2 (g) \rightarrow 2M_2O (s)}$$

For example, potassium reacts with oxygen to form potassium oxide.

Because of their high reactivity, alkali metals must be handled carefully and stored under oil to prevent unwanted reactions.

Reactivity Trends

The reactivity of alkali metals increases as you go down Group 1. This is because:

• The atoms get larger due to more electron shells.
• The outermost electron is further from the nucleus and is less strongly attracted.
• It becomes easier for the atom to lose this outer electron and react.

For example, lithium reacts less vigorously with water than sodium, which reacts less vigorously than potassium.

This trend is important for predicting how alkali metals behave in reactions and their practical uses.

For instance, lithium reacts slowly with water, producing bubbles of hydrogen gas gently, while potassium reacts explosively, often igniting the hydrogen produced.

Uses and Safety

Alkali metals have practical uses due to their unique properties:

• Lithium: Used in lithium-ion batteries found in phones, laptops, and electric cars.
• Sodium: Used in street lighting; sodium vapour lamps produce a bright yellow light.

Because alkali metals are highly reactive, especially with water and air, they must be handled with care:

• Always store under oil to prevent reactions with moisture or oxygen.
• Wear safety goggles and gloves when handling.
• Keep away from water and damp environments.

Learning Example: Reacting Sodium with Water

When sodium reacts with water, it produces sodium hydroxide and hydrogen gas. The reaction is vigorous and exothermic.

The balanced equation is:

$$\mathrm{2Na (s) + 2H_2O (l) \rightarrow 2NaOH (aq) + H_2 (g)}$$

If 46 g of sodium (2 moles) react with excess water, how many moles of hydrogen gas are produced?

From the equation, 2 moles of sodium produce 1 mole of hydrogen gas.

So, 2 moles of sodium produce:

$$\frac{1}{2} \times 2 = 1 \text{ mole of } H_2$$

Therefore, 46 g of sodium produces 1 mole of hydrogen gas.