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Last week the club learned to read a position straight off an arrangement. Today a jar comes out of the store cupboard labelled K, with the metal sitting under a layer of oil.
Potassium is 2,8,8,1. Which group does that put it in?
Group 1, Group 7 and Group 0: Trends You Can Predict
Use outer-shell electrons to predict how alkali metals, halogens and noble gases behave, including why group 1 gets more reactive down the group while group 7 gets less.
What a learner can do afterwards
- Predicts the products when a named group 1 metal meets water and writes the equation
- Explains the reactivity trend down group 1 using distance from the nucleus and inner-shell shielding
- Explains why the trend in group 7 runs the other way
- Predicts whether one halogen will displace another from a solution of its salt
1 · Read
Last week ended with a rule: where an atom sits tells you whether it loses or gains electrons. Three columns on the chart above the bench make that vivid. Group 1 on the far left has one outer electron to hand over. Group 7 needs one more to finish its shell. Group 0 at the edge needs nothing, and that stays true the whole way down, so it is the only one with no reactivity trend to explain. Noble gases even drift as single atoms, while gases like oxygen and chlorine travel in pairs. Within the other two columns, which member reacts hardest?
Every group 1 metal meets water the same way: metal + water gives a metal hydroxide and hydrogen. Drop in lithium and it fizzes steadily. Sodium melts into a ball and skates across the surface. Potassium sets its own hydrogen alight with a lilac flame. In symbols the pattern never changes, only the metal: 2Na + 2H₂O → 2NaOH + H₂. A number in front of a formula says how many of that whole unit there are, so 3H₂O would mean three whole water molecules, which is 6 hydrogen atoms and 3 oxygen. Swap Na for K or Li and the pattern holds.
So why does it get fiercer further down? Every group 1 atom has one outer electron and reacts by losing it. Lithium is 2,1. Sodium is 2,8,1. Potassium is 2,8,8,1. Step down and that outer electron sits in a shell further from the nucleus, with more full shells packed in between. Those inner shells shield it from the pull of the protons. Further away and better shielded means held more loosely, so it comes off more easily. Easier to lose is what more reactive means here.
Group 7 reacts by gaining an electron rather than losing one, so the same two effects push the other way. Chlorine is 2,8,7 and needs one more. Bromine's outer shell sits further out and behind more shielding, so an incoming electron feels a weaker pull. Iodine's is further still. Down group 7 the atoms grab electrons less strongly, so reactivity falls instead of rising. Their looks change down the column too: chlorine is a green gas, bromine an orange-brown liquid, iodine a dark grey solid that colours a solution brown. That is a separate pattern, but it makes them easy to tell apart.
Try it together
Displacement puts the trend to work. A salt here means the compound a halogen makes with a metal, dissolved in water. On the bench, add chlorine water to colourless potassium bromide solution. Chlorine sits above bromine, so it is the stronger grabber: it takes the electrons and turns the bromide back into bromine. Cl₂ + 2KBr → 2KCl + Br₂, and the free bromine colours the solution. Run it the other way and nothing new forms. Iodine sits below bromine, so it cannot push bromine out of potassium bromide.
A group 1 metal and water give a metal hydroxide and hydrogen, and the reaction gets fiercer down the column because the outer electron is further out and better shielded, so it is easier to lose. Group 7 gains an electron instead, so the same two causes make it harder and reactivity falls. A halogen displaces any halogen below it. Group 0 starts full, so it has no reactivity trend at all.
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3 · Play
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.