Group 1 — The Alkali Metals
The alkali metals are the elements in Group 1: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). At GCSE we focus mainly on Li, Na and K.
Physical Properties
| Property | Detail |
|---|---|
| Density | Low density — Li, Na and K are all less dense than water and float on the surface |
| Melting point | Low melting points compared to most other metals; melting points decrease going down the group |
| Appearance | Shiny when freshly cut; rapidly tarnish as the surface oxidises in air |
| Hardness | Soft — can be cut with a knife |
| Storage | Stored under oil to prevent reaction with air and moisture |
Reactions with Water
Alkali metals react with water to produce a metal hydroxide and hydrogen gas. The hydroxide dissolves to give an alkaline solution (hence the group name).
alkali metal + water → metal hydroxide + hydrogen
| Metal | Balanced symbol equation | Observations |
|---|---|---|
| Lithium | 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g) | Floats and moves around the surface. Steady fizzing. |
| Sodium | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) | Moves rapidly. Melts into a ball. More vigorous fizzing. |
| Potassium | 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) | Very vigorous. Burns with a lilac/violet flame. May spit. |
Hold a lit splint near the mouth of the test tube. Hydrogen burns with a squeaky pop.
📌 Spec note: The gas tests (hydrogen, oxygen, carbon dioxide, chlorine) are formally assessed in C8 — Chemical Analysis. They're included here because hydrogen gas is produced in this reaction — but you'll cover all four tests together later in the course.
Pick an alkali metal to drop onto the water and watch how vigorously it reacts.
Reactions with Oxygen
Alkali metals react with oxygen in the air to form metal oxides. This is why they tarnish rapidly when freshly cut — the shiny surface turns dull as the oxide layer forms.
| Metal | Balanced symbol equation | Product |
|---|---|---|
| Lithium | 4Li(s) + O₂(g) → 2Li₂O(s) | Lithium oxide |
| Sodium | 4Na(s) + O₂(g) → 2Na₂O(s) | Sodium oxide |
| Potassium | 4K(s) + O₂(g) → 2K₂O(s) | Potassium oxide |
The oxides formed are ionic compounds (metal oxide ion, O²⁻, combined with the metal ion). All three metals are stored under oil to prevent reaction with oxygen and moisture in the air.
Reactions with Chlorine
Alkali metals react vigorously with chlorine gas to form metal chlorides — white, ionic solids that dissolve in water to give a neutral solution:
| Metal | Balanced symbol equation | Product |
|---|---|---|
| Lithium | 2Li(s) + Cl₂(g) → 2LiCl(s) | Lithium chloride |
| Sodium | 2Na(s) + Cl₂(g) → 2NaCl(s) | Sodium chloride |
| Potassium | 2K(s) + Cl₂(g) → 2KCl(s) | Potassium chloride |
Ionic Compounds with Non-metals
When alkali metals react with non-metals they form ionic compounds. The alkali metal loses its one outer electron to achieve a full outer shell, forming an ion with a charge of +1. These compounds are:
- Always white solids
- Soluble in water — they dissolve to give colourless solutions
- Held together by electrostatic attraction between oppositely charged ions
Example: sodium reacting with chlorine to form sodium chloride:
2Na(s) + Cl₂(g) → 2NaCl(s)
Reactivity Trend Down Group 1
Reactivity increases going down Group 1: Li → Na → K → Rb → Cs
Going down Group 1, each successive element has an extra electron shell. The outer electron is further from the nucleus and shielded by more inner electron shells. The attraction between the outer electron and the positive nucleus is therefore weaker, so the outer electron is more easily lost when the metal reacts → greater reactivity.
Do not say reactivity increases because the atom "has more electrons." The correct explanation is that the outer electron is further from the nucleus and less strongly attracted to it — so it is lost more easily.
A student places a small piece of rubidium (Rb) into a trough of water. Rubidium is below potassium in Group 1. Predict two observations the student would make, and write a word equation for the reaction.
Show answer
Two observations (any two):
- Very vigorous/violent reaction — more vigorous than potassium
- Floats on the water surface
- Fizzing (hydrogen gas produced)
- Burns/catches fire
Word equation: rubidium + water → rubidium hydroxide + hydrogen
When a question asks for an observation, write only what you could actually see, hear or measure. "Gas is produced" is an inference, not an observation — the creditworthy observations are fizzing/bubbling, the metal moving on the surface, melting into a ball, or a flame/colour. Vague phrases like "a more violent reaction" score nothing unless paired with a specific visible change.
🧪 Exam-style questions
How does the reactivity of the Group 1 metals change going down the group?
Explain why reactivity increases going down Group 1.
Show answer
- Going down the group the atoms get bigger / have more shells 1 mark
- so the outer electron is further from the nucleus 1 mark
- the force of attraction between the nucleus and the outer electron is weaker (accept: more shielding / attracts less) 1 mark
- so the outer electron is lost more easily 1 mark
Examiner tip: the word outer must be used in the correct context at least once, or this answer is capped at 2 marks.
Group 7 — The Halogens
The halogens (Group 7) are non-metals: fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). They all have 7 electrons in their outer shell and all exist as diatomic molecules — pairs of atoms bonded together (F₂, Cl₂, Br₂, I₂).
The halogens aren't the only elements that travel in pairs. Seven non-metal elements always exist as diatomic molecules when they appear on their own — two atoms bonded together. You must remember to write the little "2":
H₂ · N₂ · O₂ · F₂ · Cl₂ · Br₂ · I₂
Learn them with the phrase "Have No Fear Of Ice Cold Beer" — the first letter of each word gives you an element:
- Have → Hydrogen (H₂)
- No → Nitrogen (N₂)
- Fear → Fluorine (F₂)
- Of → Oxygen (O₂)
- Ice → Iodine (I₂)
- Cold → Chlorine (Cl₂)
- Beer → Bromine (Br₂)
Physical Properties at Room Temperature
| Halogen | Formula | State | Colour |
|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow |
| Chlorine | Cl₂ | Gas | Yellow-green |
| Bromine | Br₂ | Liquid | Orange-brown |
| Iodine | I₂ | Solid | Grey-black solid purple vapour |
Going down Group 7, melting and boiling points increase. The relative molecular mass of the diatomic molecules increases down the group — larger molecules have stronger intermolecular forces, requiring more energy to separate.
📌 Intermolecular forces — the weak forces between molecules that have to be overcome to melt or boil a simple molecular substance — are covered in detail in C2 — Bonding, Structure and the Properties of Matter.
Down Group 7 the halogens change from gases (F₂, Cl₂) to a liquid (Br₂) to a solid (I₂) as melting and boiling points — and relative molecular mass — increase.
Reactions with Metals — Ionic Compounds
Halogens react with metals to form ionic compounds. Each halogen atom has 7 outer electrons and gains one electron to complete its outer shell, forming a halide ion with a charge of −1.
| Halogen | Ion formed | Ion name |
|---|---|---|
| Fluorine (F) | F⁻ | Fluoride |
| Chlorine (Cl) | Cl⁻ | Chloride |
| Bromine (Br) | Br⁻ | Bromide |
| Iodine (I) | I⁻ | Iodide |
Example: sodium reacting with chlorine to form sodium chloride (ionic compound):
2Na(s) + Cl₂(g) → 2NaCl(s)
Reactions with Non-metals — Molecular Compounds
When halogens react with non-metals, atoms share electrons, forming molecular (covalent) compounds. Example — hydrogen reacting with chlorine to form hydrogen chloride:
H₂(g) + Cl₂(g) → 2HCl(g)
HCl dissolves in water to form hydrochloric acid.
Reactivity Trend Down Group 7
Reactivity decreases going down Group 7: F₂ > Cl₂ > Br₂ > I₂
Going down Group 7, the outer shell (where an electron needs to be gained) is further from the nucleus. Nuclear attraction is weaker and there is more shielding from inner electrons. It is therefore harder to attract and gain an electron — so the element is less reactive.
Note: this is the opposite pattern to Group 1. In Group 1, reactivity increases down the group because losing an outer electron becomes easier. In Group 7, reactivity decreases because gaining an electron becomes harder.
Note: bromine's third shell is drawn with 8 electrons here to keep the picture simple. Its real arrangement is a little more complicated — you'll meet it if you study chemistry to A Level.
Displacement Reactions
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
Chlorine + potassium bromide solution — Cl is more reactive than Br, so it displaces bromine:
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Solution turns orange (bromine produced).
Bromine + potassium iodide solution — Br is more reactive than I, so it displaces iodine:
Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
Solution turns brown (iodine produced).
| Halogen added | Potassium chloride (KCl) | Potassium bromide (KBr) | Potassium iodide (KI) |
|---|---|---|---|
| Chlorine (Cl₂) | No reaction | Orange — Br₂ formed | Brown — I₂ formed |
| Bromine (Br₂) | No reaction | No reaction | Brown — I₂ formed |
| Iodine (I₂) | No reaction | No reaction | No reaction |
A halogen can only displace a halogen that is less reactive than itself. Iodine cannot displace chlorine from potassium chloride — no reaction occurs. The table above shows only six combinations; learn which give a reaction and which do not.
Now try it yourself — pick any halogen and any halide solution to see whether a reaction happens, what you would observe, and the equations.
Test a halogen displacement reaction
Pick a halide solution, then a halogen to add. Both halogens light up in the Group 7 reactivity series so you can compare them — then see whether a reaction happens.
🧪 Exam-style questions
Which row shows the halogens in order of decreasing reactivity (most reactive first)?
Chlorine is bubbled through colourless sodium bromide solution. What happens?
The Group 7 elements all react in similar ways because each atom has…
Chlorine is added to potassium iodide solution and a displacement reaction occurs. Write the balanced symbol equation for this reaction.
Show answer
Cl₂ + 2KI → 2KCl + I₂
- Correct formulae of all reactants and products 1 mark
- Equation correctly balanced (the 2 in front of KI and KCl) 1 mark
Allow: correct state symbols if added, e.g. Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq); the solution turns brown as iodine forms.
The halogens are in Group 7 of the periodic table. Explain the trend in reactivity of the halogens. This is a levels-of-response question — you are marked on how well your ideas are organised and linked, not just the number of points.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a relevant and coherent explanation of the trend that makes logical links and considers both the number of energy levels (shells) and the distance between the nucleus and the outer energy level.
- Level 2 (3–4): linked statements giving a simple explanation using either the number of energy levels or the distance from the nucleus.
- Level 1 (1–2): simple statements about the halogens or the trend in reactivity.
Indicative content — simple statements / descriptions:
- halogens have 7 electrons in the outer shell
- they need to gain one electron when they react
- they form ions with a −1 charge
- halogens further down the group are less reactive (or vice versa)
- halogens further down the group have more shells / energy levels (or vice versa)
Linked statements / explanations:
- they have 7 outer electrons, so they need to gain one electron to reach the electronic structure of a noble gas
- further down the group there are more shells / energy levels, so the outer shell is further from the nucleus
- more shells also means more shielding of the incoming electron from the nucleus
- so there is less attractive force on the incoming electron and an electron is less easily gained — the halogen is less reactive (or vice versa going up the group)
Allow the whole argument in reverse (going up the group: fewer shells → outer shell closer to the nucleus → electron gained more easily → more reactive).
Source: AQA GCSE Chemistry (levels-of-response question).
Noble Gases (Group 0)
The noble gases occupy Group 0 — the rightmost column of the periodic table. They are all colourless, odourless gases at room temperature.
Why Noble Gases Are Unreactive
Noble gases are chemically inert (very unreactive). Each noble gas atom already has a full outer electron shell, so it has no tendency to gain, lose, or share electrons. There is no chemical "drive" to react.
| Noble gas | Symbol | Electronic structure |
|---|---|---|
| Helium | He | 2 (first shell is full — holds maximum 2 electrons) |
| Neon | Ne | 2, 8 (second shell full) |
| Argon | Ar | 2, 8, 8 (third shell full) |
Trend in Boiling Points
The boiling points of the noble gases increase with increasing relative atomic mass going down Group 0. This is because larger atoms have stronger intermolecular forces, requiring more energy to overcome.
| Noble gas | Relative atomic mass | Boiling point (°C) |
|---|---|---|
| Helium (He) | 4 | −269 |
| Neon (Ne) | 20 | −246 |
| Argon (Ar) | 40 | −186 |
| Krypton (Kr) | 84 | −153 |
| Xenon (Xe) | 131 | −108 |
Uses of Noble Gases
Their inertness makes noble gases valuable where reactions with surrounding materials must be avoided:
| Noble gas | Uses | Why suitable |
|---|---|---|
| Helium (He) | Balloons and airships | Less dense than air, so it floats. Unlike hydrogen it is non-flammable — much safer. |
| Neon (Ne) | Advertising signs (neon lights) | Glows with a characteristic red-orange colour when an electric current passes through it. |
| Argon (Ar) | Filling of filament light bulbs; inert atmosphere for welding; inert atmosphere when making reactive metals | Prevents the hot filament or molten metal from reacting with oxygen in the air. Argon is used rather than helium or neon because it is cheaper — it is the most abundant noble gas in the atmosphere. |
🧪 Exam-style questions
Explain why the noble gases are very unreactive.
Show answer
- Each noble gas atom has a full outer shell of electrons 1 mark
- So it has no tendency to gain, lose or share electrons / it is already stable 1 mark
Note: helium is full with just 2 electrons; the others have 8 in their outer shell.
Predict how the boiling points of the noble gases change going down Group 0, and explain why.
Show answer
- Boiling point increases going down the group 1 mark
- Because the atoms get bigger (larger relative atomic mass), so the intermolecular forces are stronger and more energy is needed to overcome them 1 mark