The periodic table · Model
Group 0 and why groups exist
Argon is one per cent of the air and has been going in and out of human lungs for as long as there have been lungs. Chemists analysed air for a century and missed it. How do you miss an element that common?
Start here
Mendeleev's table had no room for argon, because nobody knew argon existed.
It makes up nearly one per cent of the air. It had been going in and out of every pair of human lungs for as long as there had been lungs. Chemists had been analysing air for a century and missed it entirely.
How do you miss an element that common?
Because the way you found an element in 1869 was to look at its compounds — and argon has none. It reacts with nothing, so it leaves no trace in any reaction, no colour in any flame test, no residue in any analysis. It was finally caught in 1894 by weighing: nitrogen taken from air was consistently a fraction heavier than nitrogen made in the lab, and the difference was argon hiding inside the sample.
Group 0 — helium, neon, argon, krypton, xenon — are the noble gases. Colourless, invisible, and so unreactive that for decades they were thought to form no compounds at all.
The reason is the same one behind every group in the table. An atom's outer shell of electrons is what does the reacting, and the group number tells you how many electrons are in it. Group 0 atoms have a full outer shell. There is nothing to lose and no room to gain, so there is no reaction to have.
Reference · the one idea behind all of it
Group number is outer electrons
0 of 4 opened
Reacting means losing that single electron, and one is an easy number to lose. Going down the group the atoms get bigger, the outer electron sits further from the nucleus, and it goes more easily still — which is why potassium is more violent than lithium.
Losing two takes more energy than losing one, so group 2 metals are less reactive than their neighbours in group 1. Magnesium sits quietly in cold water; sodium, one square to the left, does not.
Reacting means grabbing one more electron to complete the shell. A small atom pulls that incoming electron in hard, so fluorine at the top of the group is ferocious — and the bigger atoms lower down pull less, so reactivity falls going down. Same size argument as group 1, opposite conclusion.
There is no electron the atom needs to shed and no gap for one to fill, so there is no reaction available. This is not shyness or slowness: it is the absence of anything to react about. Helium manages it with two electrons, the rest with eight.
Key fact
The group number is the number of outer electrons. Group 0 has a full outer shell, so it reacts with nothing — and that one fact is why every group has the family resemblance it does.
Your turn · four unknown elements
You are told the group and nothing else. Predict anyway.
0 of 4 predicted
Element X · group 1, period 5
A colleague drops a piece into water while you are out of the room. What should you expect to have happened?
A violent reaction, and worse than potassium's — period 5 puts it below potassium in the column, and group 1 gets more reactive downwards. The element is rubidium. Expect hydrogen, a burst of flame and an alkaline solution.
Element Y · group 0, period 4
What will Y react with?
Essentially nothing. Group 0 means a full outer shell, and a full shell means no reaction to have. The element is krypton, and you can leave it in a jar with anything on the shelf for a century.
Element Z · group 7, period 5
Z is added to a solution of potassium chloride. What happens?
Nothing. Period 5 puts Z below chlorine in group 7, and reactivity falls going down — so it cannot displace something above it. The element is iodine, and this is the tube you already ran in the last lesson.
Element W · group 2, period 3
W is burned in air. What is the product, and what would it do to universal indicator once dissolved?
An oxide; the solution is alkaline. Metals form oxides that are bases; non-metals form oxides that are acidic. Group 2 period 3 is magnesium, which burns with a brilliant white flame to give magnesium oxide — a base you have already used to make a salt.
You have just described four elements you were told almost nothing about.
One number — the group — was enough to say how each would react, what it would react with, and roughly how hard. That is what the periodic table is for. It is not a list of the elements; it is a machine for predicting elements you have never met.
Three uses · unreactive is the point
Why anyone pays for a gas that does nothing
0 of 3 decided
Airships and weather balloons are filled with helium. Hydrogen is lighter and cheaper. Why is helium used?
Because hydrogen burns and helium cannot. Hydrogen does give slightly more lift, and it is far cheaper — but a hydrogen airship is a bomb waiting for a spark, as the Hindenburg demonstrated in 1937. Paying more for the gas that will not react is the entire point.
The inside of a filament light bulb is filled with argon rather than air. Why?
Argon stops the filament burning away. The filament runs white hot, and in air it would react with oxygen and burn through in seconds. Argon surrounds it with something that will not react at any temperature the bulb reaches. The same trick shields a weld from the air.
Neon signs glow orange-red. Is the neon reacting?
No — the atoms are excited by electricity and give out light. Passing a current through the gas pushes its electrons into higher energy levels; when they drop back they release the energy as light. Nothing is used up and nothing new is made, which is why a neon tube can run for decades. The colour is a physical process, not a chemical one.
Five 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
“The noble gases are unreactive because they are gases.”
They are all gases, and they all fail to react. Commit before you read on.
Fluorine is a gas and it is the most reactive element there is. Chlorine is a gas and it attacks almost every metal. Oxygen is a gas and nothing burns without it. Being a gas has no bearing on whether an element reacts.
The noble gases are unreactive because their outer shells are full — there is no electron they need to lose and no space for one to arrive. And it works the other way round too: they are gases because they are unreactive. Their atoms have so little attraction for each other that nothing holds them together as a liquid or a solid except at extreme cold.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Why are the group 0 elements unreactive?
Rung 2 · The one that catches people
What does an element's group number tell you about its atoms?
Rung 3 · Explain
Explain why elements in the same group react in similar ways, using group 1 and group 0 as your two examples.
Rung 4 · Take it somewhere new
A new element is made in a laboratory and placed in group 1, period 8. Predict three things about it — and then explain why the prediction is harder to trust than the one you made for rubidium.
Key note
Group 0, the noble gases, have full outer shells of electrons and so are almost completely unreactive. The group number of any element tells you how many electrons are in its outer shell, and that number is the reason elements in the same column react in the same way. Being unreactive is useful: it is why argon fills light bulbs and helium fills airships.
Going further
“Reacts with nothing” turned out to be slightly too strong. In 1962 Neil Bartlett persuaded xenon — the biggest of the ordinary noble gases, whose outer electrons are furthest from the nucleus and least tightly held — to form a compound with a platinum fluoride, and a shelf of xenon compounds followed. Nothing has ever been made from helium or neon. The rule survives with a footnote, which is what usually happens to good rules.
Helium is the only element on Earth that genuinely gets away. It is made underground by the slow radioactive decay of heavy elements and collected as a by-product of natural gas, and once it is released it is light enough to leave the atmosphere altogether — there is no getting it back. It is also the only coolant cold enough for the superconducting magnets in an MRI scanner. An element that cannot be manufactured, cannot be recovered and cannot be substituted is a genuinely finite thing, and helium is the clearest example of one in the whole table.
Before this lesson
Next in this unit
At GCSE this becomes
- Electron configurations written out shell by shell, ionic bonding as the transfer of outer electrons, and the noble gas structure as the target both sides are aiming at.
Where to next
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