AQA Chemistry 5.1.2.1
The periodic table
An element's place in the table comes straight from its electrons. Build atoms shell by shell, watch where they land, and use the position to predict how they react.
Combined Science · Higher tier
Start here
Shelves in a supermarket.
Shops put things that are alike on the same shelf: all the cereals together, all the soaps together. The periodic table stacks elements in columns in a similar way.
If you had to guess, would elements in the same column behave alike, or differently?
Build an atom · find its square
Shells give the period. Outer electrons give the group.
The elements are arranged in order of atomic number: the number of protons. A neutral atom has the same number of electrons. Add electrons one at a time and watch the square move.
Li · 3 protons, 3 electrons · 2.1
PERIOD = SHELLS
Period 2 · 2 shells
GROUP = OUTER ELECTRONS
Group 1 · 1 outer
Group 1: one outer electron, like the other alkali metals below it.
Same group, similar reactions · and the trends
Down Group 1, more reactive. Down Group 7, less.
Elements in the same group have the same number of outer electrons, so they have similar chemical properties. Metals are on the left and in the middle; non-metals are on the right.
Group 1 · lose one electron
Going down, each atom has another shell. The outer electron is further from the nucleus, so it is less strongly attracted and is lost more easily. Reactivity increases down the group.
Group 7 · gain one electron
Going down, the outer shell is further from the nucleus, so an incoming electron is less strongly attracted and is gained less easily. Reactivity decreases down the group.
Group 0 · already full
The noble gases have a full outer shell: 8 electrons, or 2 for helium. They are unreactive, all the way down the group.
Think again · down a group
“Everything gets more reactive as you go down a group.”
Your call
PredictChlorine is above bromine in Group 7. Which is more reactive?
Sort
Same group or same period?
Place each pair of elements. Use the builder above if you need it.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Predict
- Use the element's position and the group trend.
- Explain
- Shells, distance from the nucleus, attraction, then lost or gained more or less easily.
- Give
- A short answer: the group, the period, or the electronic structure.
Key fact
Elements are arranged in order of atomic number. The period number is the number of occupied shells; the group number is the number of outer electrons. Elements in the same group have similar chemical properties.
Key note · AQA 5.1.2.1
The periodic table
- Elements are arranged in order of atomic (proton) number.
- Period number = number of occupied shells. Group number = number of outer electrons.
- Same group: same number of outer electrons, so similar chemical properties.
- Group 0: full outer shell (8, or 2 for helium); unreactive.
- Down Group 1: outer electron further from the nucleus, lost more easily, more reactive.
- Down Group 7: outer shell further from the nucleus, electron gained less easily, less reactive.
Photograph this card. Tomorrow, cover it and say each line first.
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Mixing up groups and periods?
The builder covers the first 20 elements. Hydrogen is drawn on its own above the table, as on the AQA periodic table.