The periodic table · Model
Group 1 — the alkali metals
Cut a lump of sodium and the fresh surface is a mirror for about four seconds, then dulls while you watch. Nothing touched it. What is attacking it?
Start here
A lump of sodium is kept in a jar of oil. Lift it out and it is dull grey. Cut it and the cut surface is a mirror — for about four seconds.
The shine dulls while you watch. Nothing touched it, nothing was added, and the room is at ordinary temperature.
What is happening to the fresh surface?
It is reacting with the oxygen in the room. Sodium is reactive enough that plain air is an attack: the shiny metal is turning into a dull layer of sodium oxide within seconds of being exposed. That is also why it lives under oil — the oil keeps air and water off it. The whole of group 1 behaves like this, and it gets worse as you go down.
Group 1 is the first column: lithium, sodium, potassium and three more below them. They are all soft, all light enough to float on water, and all far too reactive to exist as the metal anywhere in nature.
They are called the alkali metals because of what they leave behind. Drop one into water and it makes a hydroxide, which dissolves — and a dissolved hydroxide is an alkali: metal + water metal hydroxide + hydrogen.
Your turn · the water trough
One small piece, one trough of water, indicator already added.
0 of 3 run
Predict before you drop it in.
Choose a metal, predict, then drop it in.
A steady fizz that lasts nearly a minute.
melts at 180 °C · floats
- Floats on the surface and stays as a solid lump.
- Fizzes steadily, giving off hydrogen from all over its surface.
- Gradually gets smaller and disappears.
- The indicator turns purple: the solution left behind is alkaline.
Lithium + water lithium hydroxide + hydrogen
Melts into a ball and skates across the surface.
melts at 98 °C · floats
- Floats, then melts into a silver ball within a second or two.
- Whizzes around the surface, pushed by the hydrogen streaming off it.
- Fizzes hard and may finish with an orange flame.
- The indicator turns purple: sodium hydroxide has formed.
Sodium + water sodium hydroxide + hydrogen
Sets fire to its own hydrogen. Lilac flame, and sometimes a bang at the end.
melts at 63 °C · floats
- Melts instantly and moves faster than the sodium did.
- The hydrogen catches fire, burning with a lilac flame.
- Can crack or spit at the end of the reaction.
- The indicator turns purple: potassium hydroxide has formed.
Potassium + water potassium hydroxide + hydrogen
All three did the same thing. Only the violence changed.
Every one of them floated, fizzed, produced hydrogen and left an alkaline solution behind. That sameness is what a group is. What changes going down the column is how hard the reaction is pushed: lithium steady, sodium fast enough to melt itself, potassium hot enough to set its own hydrogen alight.
Reactivity increases going down group 1.
Key fact
Group 1 metals react with water to give a metal hydroxide and hydrogen, leaving an alkaline solution. Reactivity increases down the group.
Three predictions · rubidium
The next one down. You have never seen it and you can still describe it.
0 of 3 predicted
Rubidium sits directly below potassium. Commit to each prediction before you read what actually happens.
Will rubidium be harder or softer to cut than potassium?
Softer. The group gets softer going down — lithium is the firmest of them and each one below cuts more easily. Rubidium is soft enough to deform under its own weight in a warm room.
How will rubidium react with water?
More violently than potassium — the trend continues. Rubidium reacts so fast that the hydrogen ignites instantly and the reaction is usually described as explosive. Caesium, one further down, shatters the container.
What will the solution left behind do to universal indicator?
Turn it purple. It makes rubidium hydroxide, which is a strong alkali, exactly as the other three do. This is the part that does not change down the group — the products are the same family every time, and only the speed changes.
Six words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Sodium melted because the water was hot.”
The water was straight from the tap. Commit before you read on.
The heat came from the reaction, not from the water. Sodium reacting with water gives out energy — and sodium melts at only 98 °C, which is a low melting point for a metal. The reaction produces enough heat, fast enough, to melt the piece of sodium taking part in it.
Potassium goes one step further: it releases the same energy even faster, and the hydrogen being produced catches fire in it. The ball of molten metal is evidence about the reaction, not about the trough.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What are the products when a group 1 metal reacts with water?
Rung 2 · The one that catches people
Why is the trend in group 1 the opposite way round to what most students expect?
Rung 3 · Explain
A piece of sodium put on water melts into a ball within a second. Explain where the heat came from, and why the same thing does not happen to lithium.
Rung 4 · Take it somewhere new
Caesium is two places below potassium in group 1. Write a description of what you would expect to see if a small piece were dropped into water, and justify every part of it from the trend.
Key note
The group 1 metals are soft, shiny when cut and less dense than most metals — lithium, sodium and potassium all float on water. They react with water to give a metal hydroxide and hydrogen, and the hydroxide makes the water alkaline. Reactivity increases down the group, which is why they are stored under oil.
Going further
The reason for the trend is the outer electron. Every group 1 atom has exactly one, and reacting means losing it. Going down the group the atoms get bigger, so that outer electron sits further from the nucleus and is held on less tightly — easier to lose means more reactive. The same argument run in reverse explains why group 7 does the opposite, which is the next lesson.
No group 1 metal has ever been found as the metal in nature; they are all locked into compounds. Sodium and potassium were only isolated in 1807, when Humphry Davy passed electricity through their molten hydroxides — the first time anyone had taken an element apart with a battery. Both are now indispensable: potassium in every fertiliser, sodium in every cell of your body, and lithium in the battery of whatever you are reading this on.
Before this lesson
Next in this unit
At GCSE this becomes
- Explaining the trend with atomic radius and shielding, and writing balanced and ionic equations for the reactions with water.
Where to next
Ask Mr Badmus AI
Still not sure why reactivity increases downwards?
Every reaction on this page is a teacher demonstration, behind a safety screen, with the smallest piece that can be cut and eye protection worn by everyone in the room. Potassium can crack or spit at the end of the reaction, which is why the screen stays up until the fizzing has stopped. Nothing here is a class practical at any scale.
Lesson content © MrBadmusAI.