Structure and bonding · GCSE Chemistry
The periodic table
Groups and periods, Mendeleev, Group 1, Group 7, Group 0, and how electronic structure explains reactivity trends.
Group is the outer electrons. Period is the number of shells. Reactivity in Group 1 increases down the group because the outer electron is easier to lose; in Group 7 it decreases because an incoming electron is harder to attract.
The important bits
What you need to know
- 1
Elements are arranged in order of atomic number. Columns are groups (same number of outer electrons, similar properties). Rows are periods (same number of shells).
- 2
Mendeleev arranged by atomic weight but left gaps for undiscovered elements and swapped pairs to keep properties aligned. Later, atomic number explained why his order worked.
- 3
Group 1 alkali metals: one outer electron, stored in oil, react with water to give metal hydroxide and hydrogen. Reactivity increases down the group as the outer electron is further from the nucleus and more shielded.
- 4
Group 7 halogens: seven outer electrons, diatomic molecules. Reactivity decreases down the group. A more reactive halogen displaces a less reactive one from a solution of its salt.
- 5
Melting and boiling points of the halogens increase down the group as the molecules get larger and intermolecular forces get stronger. Fluorine and chlorine are gases, bromine a liquid, iodine a solid.
- 6
Group 0 noble gases: full outer shells, very unreactive, monatomic, used in lighting and as inert atmospheres. Boiling points increase down the group.
- 7
Metals are on the left and towards the bottom; non-metals on the right. A metal oxide is basic; many non-metal oxides are acidic.
Go deeper
Trends are shielding stories
Do not write “it gets more reactive because it is lower down”. Give the particle reason. Down Group 1, there are more shells, so the outer electron is further from the nucleus and the inner shells shield it. Attraction decreases, so the electron is lost more easily and reactivity rises. Down Group 7 the atom must gain an electron. Extra shells and shielding weaken the attraction for that extra electron, so reactivity falls. The nuclear charge actually increases down a group, but the distance and shielding dominate at GCSE. Lithium is less reactive than potassium; fluorine is more reactive than iodine. Displacement reactions are the practical proof for the halogens: chlorine will displace bromine and iodine; bromine displaces iodine; iodine displaces neither.
Go deeper
Mendeleev is an evidence question
He left gaps, predicted properties of missing elements, and those predictions were later confirmed. That is why his table was accepted. He also placed tellurium before iodine against atomic weight so that the group properties lined up — atomic number later justified the swap. A modern table is in atomic number order and includes noble gases, which Mendeleev did not have. When a question asks why scientists accepted the periodic table, give a prediction that came true, not “he was famous”. Group 0 sitting on the right with full outer shells is the electronic explanation that the nineteenth-century table did not yet have.
See the idea in action
Chlorine water is added to potassium bromide solution. The mixture turns orange-brown because chlorine displaces bromine: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine is higher in Group 7, so it is more reactive: its atoms have fewer shells and attract an extra electron more strongly. Iodine solution added to potassium chloride would not react, because iodine is less reactive than chlorine.
Exam technique
Turn knowledge into marks
For Group 1, write the water equation and mention hydrogen plus an alkaline solution. For Group 7, use displacement and colour changes. Always explain trends with distance of the outer electron from the nucleus and shielding, not with “it wants to”.
Common mistakes
Do not give these marks away
- 01
Saying Group 1 becomes more reactive down the group because atoms want to lose electrons more, without shielding and distance.
- 02
Reversing the halogen trend, or forgetting that bromine is a liquid at room temperature.
- 03
Writing that noble gases are unreactive because they are gases, rather than because of a full outer shell.
Why does potassium react more vigorously with water than lithium?
APotassium has fewer protons
BPotassium’s outer electron is further from the nucleus and more shielded, so it is lost more easily
CPotassium has a full outer shell
DLithium atoms are larger than potassium atoms
Show the answer
Potassium’s outer electron is further from the nucleus and more shielded, so it is lost more easily. Down Group 1 there are more electron shells. The outer electron is less strongly attracted and is lost more readily, so reactivity increases.
Quick questions
If this is the bit you searched
Why are elements in the same group similar?
They have the same number of electrons in the outer shell, so they form similar ions or bonds and show similar reactions.
What is a halogen displacement reaction?
A more reactive halogen takes the place of a less reactive halogen in a salt solution, for example chlorine displacing bromine from potassium bromide.