How is the periodic table organised, and how do the properties of Group 1, Group 7 and Group 0 change down the group?
The structure of the periodic table, the work of Mendeleev, metals and non-metals, and the trends in reactivity and properties of Group 1 (alkali metals), Group 7 (halogens) and Group 0 (noble gases).
A focused answer to OCR Gateway GCSE Chemistry A topic C1.3 on the periodic table, covering Mendeleev's contribution, the arrangement by atomic number, metals and non-metals, and the trends in Group 1 alkali metals, Group 7 halogens and Group 0 noble gases.
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What this topic is asking
OCR wants you to describe how the periodic table is organised, recognise metals and non-metals and their positions, know the contribution of Mendeleev, and explain the trends in properties and reactivity down Group 1 (the alkali metals), Group 7 (the halogens) and Group 0 (the noble gases). These trends are explained using electron arrangement, which links straight back to electron configuration.
The structure of the periodic table
Metals are found on the left and centre of the table; non-metals are on the right (above a rough diagonal step). About three-quarters of the elements are metals. Metals tend to lose electrons to form positive ions, while non-metals tend to gain or share electrons.
Mendeleev and the development of the table
In 1869 Dmitri Mendeleev arranged the known elements in order of atomic mass and grouped together those with similar properties. His key insights were:
- He left gaps for elements he believed had not yet been discovered, and predicted their properties. When elements such as germanium were later found and matched his predictions, his table was accepted.
- He sometimes swapped the order of elements (placing them out of strict mass order) so that they fell into the correct group by properties.
The modern table is ordered by atomic number rather than mass, which removed the need for Mendeleev's swaps, because discovery of protons explained why the order should be by atomic number.
Group 1: the alkali metals
The Group 1 elements (lithium, sodium, potassium and below) are soft, reactive metals with 1 outer electron. They react with water to form a metal hydroxide (an alkali) and hydrogen gas, for example sodium + water produces sodium hydroxide + hydrogen.
Group 7: the halogens
The Group 7 elements (fluorine, chlorine, bromine, iodine) are reactive non-metals with 7 outer electrons. They exist as diatomic molecules (such as ) and react with metals to form salts. Going down the group, they get darker and change from gas to liquid to solid at room temperature.
Group 0: the noble gases
The Group 0 elements (helium, neon, argon and below) are unreactive (inert) non-metal gases. They are unreactive because they have a full outer shell of electrons (helium has 2, the rest have 8), which is a very stable arrangement, so they have no tendency to gain, lose or share electrons. Going down the group, their boiling points increase because the atoms get larger and the forces between them get stronger.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDescribe and explain the trend in reactivity of the Group 1 alkali metals as you go down the group, using ideas about electron arrangement.Show worked answer →
A Higher tier structured question. Reward: the Group 1 metals become more reactive as you go down the group (lithium is least reactive, then sodium, then potassium, and so on). The explanation: each Group 1 atom has 1 outer electron, which it loses when it reacts. As you go down the group, the outer electron is in a shell that is further from the nucleus and is shielded by more inner shells, so the attraction between the nucleus and the outer electron is weaker. The outer electron is therefore lost more easily, making the metal more reactive. Markers credit the trend (more reactive down the group), and the explanation in terms of the outer electron being further from the nucleus, more shielded, and so more easily lost.
OCR 20214 marksA more reactive halogen can displace a less reactive halogen from a solution of its salt. Predict what you would observe when chlorine water is added to potassium bromide solution, write a word equation for the reaction, and explain why the reaction happens.Show worked answer →
A C1.3 question on Group 7. Reward: chlorine is more reactive than bromine (reactivity decreases down Group 7), so chlorine displaces bromine. The observation is that the colourless solution turns orange or brown as bromine is formed. The word equation is chlorine + potassium bromide produces potassium chloride + bromine. The reaction happens because chlorine is more reactive than bromine: chlorine atoms gain electrons more readily, so chlorine takes the place of bromine in the compound and the bromide ions are turned into bromine. Markers credit the colour change to orange/brown, the correct word equation, and an explanation that chlorine is more reactive and so displaces the less reactive bromine.
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