Groups and Trends: Alkali Metals, Halogens and Noble Gases

The real power of the periodic table is that the elements in a group behave in predictable, related ways — and the changes in their behaviour as you go down a group follow clear trends. Crucially, every one of these trends can be explained by electronic structure: by how far the outer electrons are from the nucleus and how much they are shielded. This lesson, part of Topic C2 of OCR Gateway Science A, examines the three groups you must know in detail — Group 1 (the alkali metals), Group 7 (the halogens) and Group 0 (the noble gases) — and roots each trend in the electrons.

By the end of this lesson you should be able to describe the properties and reactions of the alkali metals and halogens, explain the reactivity trends in Groups 1 and 7 in terms of electronic structure, predict halogen displacement reactions, and explain why the noble gases are unreactive.

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Group 1: The Alkali Metals

Group 1 elements — lithium, sodium, potassium and below — are the alkali metals. They each have one electron in the outer shell, and they share a distinctive set of properties:

• they are soft (you can cut them with a knife) and have low densities (the first three float on water);
• they are very reactive, far more so than ordinary metals, so they are stored under oil to keep air and water away;
• they form +1 ions (by losing their single outer electron) and produce white ionic compounds that dissolve to give colourless solutions.

Reaction with water. The alkali metals react vigorously with water to produce a metal hydroxide (an alkali — hence the group's name) and hydrogen gas:

$\text{sodium} + \text{water} \rightarrow \text{sodium hydroxide} + \text{hydrogen}$sodium+water→sodium hydroxide+hydrogen $2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$2Na+2H2​O→2NaOH+H2​

The metal floats, fizzes and moves about on the surface as hydrogen is released. Lithium reacts steadily; sodium reacts faster and may melt into a ball; potassium reacts so vigorously that the hydrogen ignites with a lilac flame. This shows the trend clearly: reactivity increases down Group 1.

Explaining the trend. Going down the group, each element has one more electron shell, so the outer electron is further from the nucleus and is shielded from the nuclear charge by more inner shells. Both effects mean the outer electron is less strongly attracted to the nucleus, so it is lost more easily. Since reacting involves losing that outer electron, the metals get more reactive down the group.

Alkali metal	Reaction with water	Trend
Lithium	Steady fizzing, floats	least reactive
Sodium	Vigorous, melts into a ball, fizzes	more reactive
Potassium	Very vigorous, hydrogen ignites (lilac flame)	most reactive (of these)

Exam Tip: Reactivity increases down Group 1. The reason to write is: down the group the outer electron is further from the nucleus and more shielded, so it is lost more easily — and losing the outer electron is what reacting requires.

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Group 7: The Halogens

Group 7 elements — fluorine, chlorine, bromine and iodine — are the halogens. They are non-metals with seven outer electrons, and they exist as diatomic molecules ($\text{Cl}_2$Cl2​, $\text{Br}_2$Br2​, $\text{I}_2$I2​). They are coloured, and the colour gets darker down the group.

The trends down Group 7 run in the opposite direction to Group 1:

• melting and boiling points increase down the group — chlorine is a gas, bromine a liquid, iodine a solid at room temperature;
• the elements become darker and denser down the group;
• reactivity decreases down the group.

Explaining the reactivity trend. A halogen reacts by gaining one electron to complete its outer shell (forming a −1 ion). Going down the group, the outer shell is further from the nucleus and more shielded, so the atom attracts an incoming electron less strongly. Because gaining an electron becomes harder, the halogens get less reactive down the group — fluorine and chlorine are the most reactive, iodine the least.

Displacement reactions. Because reactivity falls down the group, a more reactive halogen will displace a less reactive halogen from a solution of its salt. For example, chlorine (more reactive) displaces bromine from sodium bromide solution:

$\text{chlorine} + \text{sodium bromide} \rightarrow \text{sodium chloride} + \text{bromine}$chlorine+sodium bromide→sodium chloride+bromine $\text{Cl}_2 + 2\text{NaBr} \rightarrow 2\text{NaCl} + \text{Br}_2$Cl2​+2NaBr→2NaCl+Br2​

Chlorine also displaces iodine from sodium iodide. But the reverse does not happen: iodine cannot displace chlorine, because iodine is less reactive. The displacement grid below summarises which reactions occur ("✓" = displacement happens).

Added → / In solution ↓	Chloride	Bromide	Iodide
Chlorine	—	✓	✓
Bromine	✗	—	✓
Iodine	✗	✗	—

Exam Tip: Reactivity decreases down Group 7 (the opposite of Group 1) — because the outer shell is further from the nucleus, so an electron is gained less easily. A more reactive halogen displaces a less reactive one: chlorine displaces bromine and iodine, but not vice versa.

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Group 0: The Noble Gases

Group 0 (also called Group 8) contains the noble gases — helium, neon, argon and below. They are extremely unreactive (inert) and exist as single atoms (monatomic), not molecules.

The reason for their inertness is the key point: the noble gases have a full (complete) outer shell of electrons — helium has 2, the others have 8 — which is a very stable arrangement. Because their outer shell is already full, they have no tendency to gain, lose or share electrons, so they hardly react at all. (This stable full-shell arrangement is exactly what the other elements are trying to achieve when they bond, as you will see in the bonding lessons.)

The noble gases also show a trend: their boiling points increase down the group, because the atoms get larger and the forces between them increase, so more energy is needed to separate them.

Their inertness makes them useful: helium (low density and non-flammable) is used in balloons and airships; argon (unreactive) provides an inert atmosphere in filament light bulbs and in welding to stop the hot metal reacting with air.

Property	Group 1 (alkali metals)	Group 7 (halogens)	Group 0 (noble gases)
Outer electrons	1	7	full (2 or 8)
Metal / non-metal	metal	non-metal	non-metal
Reactivity down group	increases	decreases	inert (unreactive)
Melting/boiling point down group	decreases	increases	increases

Exam Tip: The noble gases are inert because they have a full outer shell — a common error is to say "empty". A full outer shell is stable, so they do not need to gain, lose or share electrons.

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Worked Examples

Worked Example 1 — Predicting a Group 1 reaction

Rubidium is below potassium in Group 1. Predict how rubidium would react with water compared with potassium, and write a word equation for the reaction.