Group 1 — The Alkali Metals

This lesson covers the Group 1 elements (the alkali metals), as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 6: Groups in the Periodic Table. You need to know the properties of the alkali metals, their reactions with water, and how to explain the trend in reactivity down the group using electron configuration.

The Group 1 Elements

Group 1 is found on the far left of the periodic table. The elements in Group 1 are called the alkali metals because they react with water to form alkaline solutions (metal hydroxides).

Element	Symbol	Atomic Number	Electron Configuration	Melting Point (°C)	Density (g/cm³)
Lithium	Li	3	2, 1	181	0.53
Sodium	Na	11	2, 8, 1	98	0.97
Potassium	K	19	2, 8, 8, 1	63	0.86
Rubidium	Rb	37	2, 8, 18, 8, 1	39	1.53
Caesium	Cs	55	2, 8, 18, 18, 8, 1	28	1.87

Exam Tip: You only need to know the properties and reactions of lithium, sodium and potassium in detail for the Edexcel GCSE exam, but you should be able to predict the properties of rubidium and caesium by extrapolating the trends.

Physical Properties of Group 1 Metals

The alkali metals share several distinctive physical properties:

Soft — they can be cut with a knife, revealing a shiny surface that quickly tarnishes in air.
Low density — lithium, sodium and potassium are less dense than water (lithium is the least dense metal).
Low melting and boiling points — compared with most other metals. Melting points decrease down the group.
Shiny when freshly cut — but rapidly oxidise in air to form a dull oxide layer.
Good conductors of heat and electricity (as with all metals).

Trends in Physical Properties

As you go down Group 1:

Property	Trend	Explanation
Melting point	Decreases	Metallic bonds become weaker as the atoms get larger and the delocalised electrons are further from the positive nuclei
Boiling point	Decreases	Same reason as melting point
Density	Generally increases	Atoms become heavier (greater atomic mass) faster than they increase in volume
Atomic radius	Increases	Each element has one more electron shell than the one above
Hardness	Decreases	Weaker metallic bonding makes the metal softer

Exam Tip: When asked to explain a trend in physical properties, always link your answer to the structure and bonding. For melting point, refer to the strength of the metallic bonds and the distance of delocalised electrons from the nucleus.

Reactions of Group 1 Metals with Water

All Group 1 metals react vigorously with cold water. The general word equation is:

alkali metal + water → metal hydroxide + hydrogen

The general symbol equation is:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g)

where M represents any Group 1 metal.

Specific Reactions

Lithium with water:

Word equation: lithium + water → lithium hydroxide + hydrogen

Symbol equation: 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)

Observations:

Lithium floats on the water (low density)
Fizzes steadily as hydrogen gas is produced
Gradually gets smaller and eventually disappears
The solution becomes alkaline (turns universal indicator blue/purple)
No flame is observed

Sodium with water:

Word equation: sodium + water → sodium hydroxide + hydrogen

Symbol equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Observations:

Sodium floats on the water
Melts into a shiny silver ball (low melting point, exothermic reaction)
Moves rapidly across the surface of the water, fizzing vigorously
May ignite with a yellow/orange flame if it gets trapped at the edge
The solution becomes strongly alkaline
The sodium gets smaller and eventually disappears

Potassium with water:

Word equation: potassium + water → potassium hydroxide + hydrogen

Symbol equation: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Observations:

Potassium floats on the water
Immediately catches fire, burning with a lilac/purple flame
Moves rapidly across the surface
Fizzes violently — may spit and crackle
The solution becomes strongly alkaline
Reacts much more vigorously than sodium

Exam Tip: In the exam, you must describe what you would see (observations), not just the products. Saying "hydrogen is produced" is not an observation — saying "fizzing/bubbling is observed" is. Saying "potassium burns with a lilac flame" is a good observation.

Trend in Reactivity

Reactivity increases as you go down Group 1. Potassium is more reactive than sodium, which is more reactive than lithium.

Explaining the Trend

All Group 1 metals have one electron in their outer shell. When they react, they need to lose this one outer electron to form a positive ion (M⁺) with a stable noble gas electron configuration.

As you go down the group:

The atoms get larger (more electron shells).
The outer electron is further from the nucleus.
There is more shielding from inner electron shells.
The attraction between the nucleus and the outer electron is weaker.
The outer electron is lost more easily.
Therefore the metal is more reactive.

Metal	Shells	Distance of Outer Electron from Nucleus	Ease of Losing Outer Electron	Reactivity
Li	2	Closest	Hardest	Least reactive
Na	3	Further	Easier	More reactive
K	4	Even further	Even easier	Most reactive (of the three)

Exam Tip: When explaining the trend in reactivity for Group 1, you MUST mention: (1) the outer electron is further from the nucleus, (2) there is increased shielding, and (3) the attraction between the nucleus and the outer electron is weaker — so the electron is lost more easily. All three points are needed for full marks.

Storage and Safety

Because of their high reactivity, Group 1 metals must be stored carefully:

Stored under oil to prevent them reacting with oxygen and moisture in the air.
Handled with forceps — never touched with bare hands.
Cut with a knife inside a fume cupboard.
Only small pieces are used in experiments due to the vigorous reactions.

Predicting Reactions of Rubidium and Caesium

Using the trend in reactivity, we can predict:

Rubidium would react even more vigorously with water than potassium — likely to explode.
Caesium would react extremely violently with water — an explosive reaction.
Both would float (rubidium may just about sink as its density is 1.53 g/cm³).
Both would produce a hydroxide and hydrogen gas.

Flame Colours

Each Group 1 metal produces a characteristic flame colour when heated:

Metal	Flame Colour
Lithium	Crimson red
Sodium	Yellow/orange
Potassium	Lilac/purple

These flame colours are used in flame tests to identify the metal ions in compounds.

Summary

Group 1 metals are called the alkali metals because they form alkaline hydroxides when they react with water.
They have low melting points, low densities and are soft.
They react with water to form a metal hydroxide and hydrogen gas.
Reactivity increases down the group because the outer electron is further from the nucleus, there is more shielding, and the electron is lost more easily.
The metals are stored under oil due to their high reactivity.
Each element has one electron in its outer shell, which it loses to form a 1+ ion.

Exam Tip: A 6-mark question on Group 1 trends is very common. Structure your answer: state the trend, describe the observations for two or three metals, then explain using electron configuration (number of shells, distance from nucleus, shielding, ease of electron loss).