How do the properties of elements change down Group I, down Group VII and across the noble gases?
Describe the properties and trends of Group I metals, Group VII non-metals and the Group 0 noble gases, and link them to electron arrangement
A focused answer to the N(A) Chemistry outcome on group trends. The reactive Group I metals, the Group VII halogens and their displacement reactions, and the unreactive Group 0 noble gases, each linked to electron arrangement.
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What this dot point is asking
The syllabus wants you to describe the properties and trends of three parts of the table: the Group I alkali metals, the Group VII halogens, and the Group 0 noble gases, and to explain those trends using electron arrangement. The key idea is that elements in a group behave similarly because they have the same number of outer electrons, and the gradual change down a group comes from the outer shell being further from the nucleus.
The answer
Group I: the alkali metals
The alkali metals (lithium, sodium, potassium) each have one outer electron. They are:
- soft, easily cut metals with low densities,
- very reactive, stored under oil to keep them from air and water.
They react with water to give an alkali (a metal hydroxide) and hydrogen gas. Reactivity increases down the group: potassium reacts more violently than sodium, which is more violent than lithium. This is because the outer electron is in a shell further from the nucleus down the group, so it is lost more easily.
Group VII: the halogens
The halogens (chlorine, bromine, iodine) each have seven outer electrons. They are coloured non-metals that exist as molecules (, , ):
- chlorine is a green gas, bromine a red-brown liquid, iodine a dark grey solid,
- they react with metals to form salts, gaining one electron to make ions.
Reactivity decreases down the group: chlorine is more reactive than bromine, which is more reactive than iodine. A more reactive halogen will displace a less reactive one from its salt solution.
Group 0: the noble gases
The noble gases (helium, neon, argon) have a full outer shell. This makes them:
- unreactive (inert), because they do not need to gain, lose, or share electrons,
- colourless gases that exist as single atoms.
Their lack of reactivity makes them useful, for example argon in light bulbs and helium in balloons.
Examples in context
Example 1. Why sodium is stored under oil. Sodium is so reactive that it reacts with the moisture and oxygen in air. Storing it under oil keeps air and water away, which is a direct consequence of having one easily lost outer electron. The same care is needed even more for potassium.
Example 2. Chlorine in water treatment. Chlorine is added to drinking water and swimming pools because it is a reactive Group VII element that kills bacteria. Its high reactivity, which also lets it displace bromine and iodine from their salts, is what makes it an effective disinfectant.
Try this
Q1. State and explain how the reactivity of the Group I metals changes going down the group. [2 marks]
- Cue. Reactivity increases down the group, because the outer electron is in a shell further from the nucleus and is lost more easily.
Q2. Explain why the noble gases in Group 0 are unreactive. [2 marks]
- Cue. They have a full outer shell of electrons, so they do not need to gain, lose, or share electrons, and therefore do not react.
Q3. Bromine water is added to potassium iodide solution. State whether a reaction occurs and what you would see. [2 marks]
- Cue. Bromine is more reactive than iodine, so it displaces iodine; the solution turns brown as iodine is released.
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 marksLithium, sodium and potassium are all in Group I. (a) Describe what is seen when a small piece of sodium is added to water and name the gas produced. (b) State how the reactivity changes from lithium to potassium and explain why.Show worked answer →
(a) The sodium floats and moves about on the surface, fizzes, and melts into a ball as heat is given out; a gas is produced. The gas is hydrogen. The solution formed is alkaline.
(b) Reactivity increases down the group from lithium to potassium. Going down the group the outer electron is in a shell further from the nucleus, so it is lost more easily, making the metal more reactive.
What markers reward: clear observations (floats, moves, fizzes, melts), naming hydrogen, reactivity increasing down the group, and the outer electron being lost more easily further down.
Original4 marksChlorine water is added to a colourless solution of potassium bromide. (a) State what you would observe. (b) Name the type of reaction and explain why it happens.Show worked answer →
(a) The solution turns orange (or brown), because bromine is formed.
(b) The reaction is a displacement reaction. Chlorine is more reactive than bromine (it is higher in Group VII), so chlorine displaces bromine from the potassium bromide, forming potassium chloride and bromine.
What markers reward: the colour change to orange or brown for bromine, naming displacement, and chlorine being more reactive than bromine because it is higher in the group.
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