SingaporeChemistrySyllabus dot point

How do the properties of the alkali metals and the halogens change down their groups, and why?

Describe the properties and trends of Group I (the alkali metals) and Group VII (the halogens), including reactivity trends down each group and displacement reactions of the halogens

A focused answer to the O-Level Chemistry outcome on Groups I and VII. Properties and reactivity trends of the alkali metals and the halogens, why reactivity increases down Group I and decreases down Group VII, and halogen displacement reactions.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe the characteristic properties of Group I (the alkali metals) and Group VII (the halogens), explain the trends in reactivity down each group in terms of electronic structure, and describe the displacement reactions of the halogens. These two groups are the standard examples of group trends, so they are examined often and reward a clear cause-and-effect explanation.

The answer

Group I: the alkali metals

The alkali metals (lithium, sodium, potassium and below) are soft, reactive metals with low densities. Each has one electron in its outer shell, which it loses to form a $+1$ ion. Their characteristic properties:

They are soft enough to cut with a knife and have low melting points for metals.
They react vigorously with water to give a metal hydroxide (an alkali) and hydrogen. For example, sodium + water gives sodium hydroxide + hydrogen.
They are stored under oil because they react with air and water.

Reactivity increases down the group. Going down, atoms have more electron shells, so the single outer electron is further from the nucleus and held less strongly. Because these metals react by losing that outer electron, it being lost more easily makes them more reactive: potassium reacts more violently with water than sodium, which reacts more than lithium.

Group VII: the halogens

The halogens (fluorine, chlorine, bromine, iodine) are reactive non-metals that exist as diatomic molecules (such as $\text{Cl}_2$ ). Each has seven electrons in its outer shell and gains one electron to form a $-1$ ion (a halide). Their properties:

They are coloured and become darker and denser down the group: chlorine is a pale green gas, bromine a red-brown liquid, iodine a grey-black solid.
They react with metals to form salts (such as sodium chloride).

Reactivity decreases down the group. Going down, the outer shell is further from the nucleus, so the atom attracts an incoming electron less strongly. Because halogens react by gaining an electron, gaining it less easily makes them less reactive: chlorine is more reactive than bromine, which is more reactive than iodine.

Halogen displacement reactions

Because reactivity decreases down Group VII, a more reactive halogen displaces a less reactive halogen from a solution of its salt (a halide). For example, chlorine displaces bromine from potassium bromide:

\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2

The solution changes colour as the displaced halogen forms (orange for bromine, brown for iodine). These reactions are used to compare the reactivities of the halogens, and the colour change is the visible evidence.

Worked example

Bromine water is added to separate solutions of sodium chloride and sodium iodide. Predict and explain what happens in each, and write an equation for any reaction.

Step 1: Order the halogens by reactivity

Reactivity decreases down Group VII: chlorine is more reactive than bromine, which is more reactive than iodine.

Step 2: Bromine with sodium chloride

Bromine is less reactive than chlorine, so bromine cannot displace chlorine from chloride. No reaction occurs; there is no colour change beyond the bromine's own colour.

Step 3: Bromine with sodium iodide

Bromine is more reactive than iodine, so bromine displaces iodine from iodide. The solution turns brown as iodine forms.

Step 4: Write the equation for the reaction

\text{Br}_2 + 2\text{NaI} \rightarrow 2\text{NaBr} + \text{I}_2

So bromine reacts only with the iodide (displacing the less reactive iodine) and not with the chloride (it cannot displace the more reactive chlorine).

Examples in context

Example 1. Storing and handling sodium. Sodium is kept under oil and cut behind a screen because it reacts so vigorously with moisture and air. Its high reactivity, explained by the easily lost outer electron, is exactly what makes it both useful for demonstrations and hazardous to handle.

Example 2. Using chlorine to treat water. Chlorine, the most reactive common halogen, is added to drinking water and swimming pools to kill bacteria. Its strong tendency to gain an electron makes it a powerful disinfectant, an everyday use that reflects its position at the top of Group VII.

Try this

Q1. State the charge of the ion formed by a Group I metal and by a Group VII element. [1 mark]

Cue. Group I forms $+1$ ions; Group VII forms $-1$ ions.

Q2. Explain why potassium is more reactive than lithium. [2 marks]

Cue. Potassium is lower in Group I, so its outer electron is further from the nucleus and lost more easily, making it more reactive.

Q3. State what is observed when chlorine water is added to potassium iodide solution, and name the reaction type. [2 marks]

Cue. The solution turns brown as iodine is formed; this is a displacement reaction (chlorine displaces the less reactive iodine).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marks(a) Describe and explain the trend in reactivity down Group I (the alkali metals). (b) Sodium reacts with water. Write a word equation and state two observations.

Show worked answer →

(a) Reactivity increases down Group I. Going down the group, the atoms have more electron shells, so the outer electron is further from the nucleus and more easily lost. Since these metals react by losing their single outer electron, losing it more easily makes them more reactive.

(b) Sodium + water $\rightarrow$ sodium hydroxide + hydrogen. Observations (any two): the sodium floats and moves around on the surface; it melts into a ball; it fizzes as hydrogen is given off; it eventually disappears; the resulting solution turns universal indicator blue or purple (alkaline).

Markers reward reactivity increasing down the group explained by the outer electron being further from the nucleus and lost more easily, a correct word equation, and two valid observations.

Original4 marksChlorine water is added to a solution of potassium bromide and to a solution of potassium iodide. (a) State what is observed in each case. (b) Explain these results in terms of the reactivity of the halogens.

Show worked answer →

(a) With potassium bromide: the solution turns orange (or brown), as bromine is displaced. With potassium iodide: the solution turns brown (a dark brown or black with iodine forming).

(b) Chlorine is more reactive than both bromine and iodine (reactivity decreases down Group VII). A more reactive halogen displaces a less reactive halogen from a solution of its salt, so chlorine displaces bromine from bromide and iodine from iodide.

Markers reward the colour changes showing the displaced halogen, and the explanation that the more reactive chlorine displaces the less reactive bromine and iodine.