Key Stage 4 science · Chemistry – Atomic structure and the Periodic Table

GCSE Periodic Table and group trends: a parent’s guide

The Periodic Table is chemistry’s map. At GCSE, students learn that it is arranged by atomic number, that an element’s position tells you how its electrons are arranged, and that this in turn explains its properties and reactivity. They study the trends in Group 1, Group 7 and Group 0, and compare metals with non-metals, using the pattern to predict how elements will behave.

The Periodic Table and group trends, explained

In the modern Periodic Table the elements are arranged in order of atomic number, the number of protons in the nucleus. Rows are called periods and columns are called groups. Elements in the same group have the same number of electrons in their outer shell, which is why they react in similar ways. The group number tells you the number of outer electrons (apart from Group 0, the noble gases, which have a full outer shell) and the period number tells you the number of occupied shells. Students write electronic structures for the first 20 elements, such as sodium as 2,8,1 and chlorine as 2,8,7. Mendeleev’s nineteenth-century table was ordered mainly by atomic mass, and he left gaps for elements not yet discovered and predicted their properties, which later proved correct.

Metals are on the left and in the centre, and non-metals on the right. Metals are generally shiny, good conductors of heat and electricity, malleable, and tend to lose electrons to form positive ions. Non-metals are usually dull when solid, brittle, poor conductors and tend to gain electrons or share them when bonding. Many courses, especially separate chemistry, also compare the transition metals in the central block with Group 1: they have higher melting points and densities, are harder and stronger, react more slowly, and often form coloured compounds and act as catalysts.

Group 1, the alkali metals, are soft, with low melting points and densities, and react with water to produce hydrogen and an alkaline metal hydroxide. Going down the group they become more reactive, because the outer electron is further from the nucleus and is shielded by more full shells, so it is held less tightly and lost more easily. Group 7, the halogens, are non-metals that exist as molecules of two atoms. Going down, they become darker, with higher melting and boiling points, and less reactive, because the outer shell is further from the nucleus, making it harder to attract an extra electron. A more reactive halogen displaces a less reactive one from a solution of its salt. Group 0, the noble gases, are unreactive because their outer shells are full, and their boiling points increase going down the group.

The questions students meet ask them to predict and explain. They might be told how lithium, sodium and potassium behave and asked to predict rubidium, or given the properties of chlorine and bromine and asked about astatine. A full answer always links the position in the table to the electron structure, then to the behaviour. Students do not handle alkali metals themselves because the reactions can be dangerous: teachers demonstrate them with tiny pieces, or show recordings, and students work from their observations and data tables.

Words your child will hear

Group: A column of the Periodic Table. Elements in a group have the same number of outer electrons.
Period: A row of the Periodic Table. The period number is the number of electron shells being used.
Atomic number: The number of protons in an atom, used to order the elements in the Periodic Table.
Alkali metals: The elements in Group 1, which are soft, reactive metals that form alkaline solutions with water.
Halogens: The elements in Group 7, which are reactive non-metals with seven outer electrons.
Noble gases: The unreactive elements in Group 0, which have full outer shells.
Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive one in a compound.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the modern Periodic Table, showing elements arranged in order of atomic number
Students should be taught about the position of elements in the Periodic Table in relation to their atomic structure and arrangement of outer electrons
Students should be taught about properties and trends in properties of elements in the same group
Students should be taught about characteristic properties of metals and non-metals
Students should be taught about chemical reactivity of elements in relation to their position in the Periodic Table

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: a model answer on reactivity

Sodium and potassium are both in Group 1. Potassium reacts more vigorously with water than sodium. Explain why, using the electronic structures of the two atoms.

Write the electronic structures. Sodium has 11 electrons, arranged 2,8,1. Potassium has 19 electrons, arranged 2,8,8,1.
```
sodium     2,8,1   (3 shells)
potassium  2,8,8,1 (4 shells)
```
Say what they have in common. Both have one electron in the outer shell, which is why both are in Group 1 and why both react by losing that one electron to form a positive ion with a +1 charge.
State the difference. The outer electron in potassium is in the fourth shell, so it is further from the nucleus than sodium’s outer electron, and there is one more full shell between it and the nucleus.
Link to the attraction. Because the outer electron is further away and more shielded, the attraction between the nucleus and the outer electron is weaker in potassium.
Link to the reactivity. A weaker attraction means the electron is lost more easily, so potassium reacts more vigorously. The same argument predicts that rubidium is more reactive still.

Potassium’s outer electron is in a shell further from the nucleus, so it is held less strongly and lost more easily than sodium’s, making potassium more reactive.

Common mistakes and how to help

Saying that reactivity increases down every group

Group 1 is usually the first example, so students generalise. In Group 7 the trend is the opposite: reactivity decreases going down, because atoms gain an electron rather than losing one.

What helps: Ask whether the element’s reaction involves losing or gaining electrons, and how distance from the nucleus affects each. Draw both trends side by side.

Mixing up groups and periods, or reading the wrong number from the table

Both are labelled with numbers, and the table has columns and rows with similar names. The group number tells you the outer electrons, and the period number tells you the shells.

What helps: Pick any element and ask for its electron structure from its position, then check by counting. Chlorine is in Group 7, period 3, so 2,8,7.

Thinking all metals are hard, solid, grey and magnetic, or that non-metals never conduct

Everyday metals such as iron and steel set the picture. But sodium can be cut with a knife, mercury is liquid at room temperature, and graphite is a non-metal that conducts electricity.

What helps: Collect exceptions as a family game. Knowing that properties are typical, and not universal, helps students avoid wrong “always” statements in answers.

Saying noble gases are unreactive “because they are gases”, or that they want to be happy

Students often reach for reasons that sound plausible. In chemistry, the reason is the full outer shell, which means there is no tendency to gain, lose or share electrons.

What helps: Ask your child to complete the sentence “Noble gases are unreactive because…” using the electron structure of neon or argon.

How to help at home

Make a pattern map. Draw a blank outline of the first three periods and fill in the electron structures for each element. Use colour to show groups. Seeing the pattern repeat in each period is more convincing than being told about it.
Sort household materials. Collect a spoon, a coin, a pencil, a plastic cup, a piece of fruit and a cooking-foil sheet, and ask which are metals. Discuss which properties give it away, and which one (graphite in the pencil) is a surprise.
Spot the elements around you. Check labels and objects for elements: aluminium foil, copper pipes, helium balloons, the neon in some glowing signs. Ask where each sits in the table and what its position suggests about its properties.
Predict, then check. Give your teenager the facts for three elements in a group and ask for a prediction about the fourth, using the trend. Then check against a data table. Never try alkali metal reactions at home: leave them to the teacher’s demonstration.

Builds on

Key Stage 3: The Periodic Table