The periodic table · Model
Groups and periods
Sodium explodes on water. Magnesium, the square next door, sits in it and barely fizzes. Potassium is two rows away and behaves exactly like sodium. So which tells you more about an element — its row or its column?
Start here
Sodium and magnesium sit next door to each other. Sodium and potassium are two rows apart.
Sodium explodes on water. Magnesium, its immediate neighbour, sits in water all day and barely fizzes. Potassium, which is nowhere near it on the row, behaves exactly like sodium only more so — and its compounds have the same formulae.
Which tells you more about an element: its row or its column?
The column. A column is a group, and a group is a family: its members react in the same way and form compounds with the same formulae. A row is a period, and going along one takes you from a violent metal at the left to an unreactive gas at the right. Neighbours along a row have almost nothing in common. Neighbours down a column have almost everything in common.
The groups are the vertical columns, numbered 1 to 7 and then 0. Elements in a group behave alike, and the group number is a fact about the atom itself: it is the number of electrons in the outer shell. That is why the family resemblance exists.
The periods are the horizontal rows. Going across a period the elements change steadily from metals on the left, through the staircase, to non-metals on the right — and then the row starts over.
Your turn · read the table
The first twenty elements. Tap any square.
| Period | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 0 |
|---|---|---|---|---|---|---|---|---|
| Period 1 | ||||||||
| Period 2 | ||||||||
| Period 3 | ||||||||
| Period 4 |
Tap any square to read its address and its family.
Group 3, period 3 · atomic number 13
Group 3 — where the staircase between metals and non- metals begins.
The most common metal in the Earth’s crust, yet one of the last everyday metals to be isolated — it is too reactive to be smelted with carbon.
Group 0, period 3 · atomic number 18
Group 0 — the noble gases. Full outer shells, so they react with nothing.
Nearly one per cent of the air, and completely unreactive — which is why it fills light bulbs and shields welding.
Group 3, period 2 · atomic number 5
Group 3 — where the staircase between metals and non- metals begins.
Sits right on the staircase between metals and non-metals, and behaves like both. Used in heat-resistant glass.
Group 2, period 2 · atomic number 4
Group 2 — the alkaline earth metals. Harder and less reactive than group 1.
A light, stiff, expensive metal used in aerospace. Group 2 metals are harder and less reactive than group 1.
Group 4, period 2 · atomic number 6
Group 4 — carbon at the top and silicon below it: the elements that build structures.
The element every living thing is built around. Comes as diamond, graphite and soot — same atoms, different arrangements.
Group 2, period 4 · atomic number 20
Group 2 — the alkaline earth metals. Harder and less reactive than group 1.
Below magnesium. Its carbonate is limestone, chalk, marble and your own bones.
Group 7, period 3 · atomic number 17
Group 7 — the halogens. Coloured, poisonous, and less reactive as you go down.
A green choking gas, directly below fluorine and reactive for the same reason. Used in tiny amounts to make water safe to drink.
Group 7, period 2 · atomic number 9
Group 7 — the halogens. Coloured, poisonous, and less reactive as you go down.
The most reactive element in the entire table — a pale yellow gas that attacks almost anything, including glass.
Group 1, period 1 · atomic number 1
Group 1 — the alkali metals. Soft, light, and more reactive as you go down.
A special case: it sits above group 1 but is a colourless gas, not a metal. It is the most abundant element in the universe.
Group 0, period 1 · atomic number 2
Group 0 — the noble gases. Full outer shells, so they react with nothing.
A noble gas with a full outer shell, so it reacts with nothing at all. Second element, and the second most abundant in the universe.
Group 1, period 4 · atomic number 19
Group 1 — the alkali metals. Soft, light, and more reactive as you go down.
Below sodium, and more violent again: it sets fire to the hydrogen it produces on water. The trend down group 1 is unmistakable.
Group 1, period 2 · atomic number 3
Group 1 — the alkali metals. Soft, light, and more reactive as you go down.
Soft enough to cut with a knife and light enough to float on water, which it also fizzes in. The least violent of the alkali metals.
Group 2, period 3 · atomic number 12
Group 2 — the alkaline earth metals. Harder and less reactive than group 1.
Burns with a blinding white flame. Next door to sodium and nothing like it — a period is not a family.
Group 5, period 2 · atomic number 7
Group 5 — nitrogen and phosphorus, both essential to life.
Four out of every five molecules in the air you are breathing. Almost unreactive as a gas, essential in every protein.
Group 1, period 3 · atomic number 11
Group 1 — the alkali metals. Soft, light, and more reactive as you go down.
Directly below lithium, and it behaves like lithium only more violently. Its compound with chlorine is on every table in the country.
Group 0, period 2 · atomic number 10
Group 0 — the noble gases. Full outer shells, so they react with nothing.
A noble gas that glows orange-red when a current is passed through it. Reacts with nothing.
Group 6, period 2 · atomic number 8
Group 6 — oxygen and sulfur, both of which react hungrily with metals.
A fifth of the air, and the other half of water. Nothing burns without it.
Group 5, period 3 · atomic number 15
Group 5 — nitrogen and phosphorus, both essential to life.
One form catches fire in air on its own. Stored under water, and essential to every cell in your body.
Group 6, period 3 · atomic number 16
Group 6 — oxygen and sulfur, both of which react hungrily with metals.
Yellow, brittle and found pure around volcanoes. Burns with a blue flame to make the gas behind acid rain.
Group 4, period 3 · atomic number 14
Group 4 — carbon at the top and silicon below it: the elements that build structures.
Directly below carbon, and it forms the same kinds of compounds — SiO2 is sand. Also the element every microchip is cut from.
The group number is not how many electrons the atom has.
Every square prints two numbers and they are not the same thing. The atomic number is how many electrons the atom has altogether — chlorine has seventeen. The group number is how many are in the OUTER shell, and chlorine is in group 7. It is the outer ones that do the chemistry, which is why a column is a family and a row is not.
Key fact
Groups are the columns and hold families that behave alike. Periods are the rows and run from metal to non-metal. Same group means same chemistry; same period means almost nothing.
Four questions · use the table above
Read it, do not remember it
0 of 4 answered
Every one of these can be answered from the grid. Commit before you read the explanation.
Which element is in group 2 of period 3?
Magnesium. Count two columns across on the third row. Sodium is group 1 of period 3; calcium is group 2 but period 4 — one row too far down.
Lithium reacts with water to give lithium hydroxide and hydrogen. What would you expect potassium to do?
The same reaction, giving potassium hydroxide and hydrogen — because it is in the same group. That is what a group is for: knowing one member tells you about the rest. Potassium does it far more violently, which is the trend down the column.
Which two of these behave most alike: chlorine and argon, or chlorine and fluorine?
Chlorine and fluorine — both group 7. Chlorine and argon are neighbours in period 3, and could hardly be less alike: one is a violently reactive green gas, the other reacts with nothing at all.
Silicon forms SiO2. Carbon is directly above it. What would you expect the formula of carbon's oxide to be?
CO2 — same group, same combining ratio. This is the most powerful thing a group tells you, and it is exactly how Mendeleev predicted the formulae of compounds of elements nobody had found yet.
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
“Elements next to each other in the table are similar.”
Neighbouring squares do share a row. Commit before you read on.
Sodium and chlorine are in the same period, six squares apart. One is a metal soft enough to cut and violent in water; the other is a choking green gas. They are so unalike that they react together — and what they make is table salt.
Now compare sodium with potassium, two rows down the same column. Same appearance, same reaction with water only faster, same formula for every compound. Similarity runs down the columns, not along the rows. A period is a journey; a group is a family.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is a group in the periodic table?
Rung 2 · The one that catches people
Sodium and chlorine are in the same period, six squares apart. What does that tell you about their properties?
Rung 3 · Explain
Explain the difference between a group and a period, and why knowing an element's group is more useful than knowing its period.
Rung 4 · Take it somewhere new
An element you have never heard of is in group 1, period 5. Describe what you would expect it to be like and how it would react with water, and explain how you know.
Key note
Columns are groups and rows are periods. Elements in the same group have the same number of outer electrons, react in similar ways and form compounds with the same formulae. Going across a period the elements change from metals to non-metals, so elements side by side in a row are usually nothing alike.
Going further
The block of metals sitting between groups 2 and 3 from period 4 onwards — iron, copper, nickel, zinc and the rest — are the transition metals, and they break the neat pattern on purpose. They are harder, denser and much less reactive than the metals on the far left, their compounds are coloured, and many are catalysts. They are also, almost without exception, the metals anyone has ever built anything out of.
The group number is not a label somebody chose. Group 1 elements have one electron in their outer shell, group 2 have two, group 7 have seven and group 0 have a full shell. Everything a group does chemically follows from that number — which is why the same column reacts the same way, and why the table would have to be redrawn if it were not true.
Before this lesson
Next in this unit
At GCSE this becomes
- Electron configuration, why group number equals outer electrons, and the transition metals as a block with their own rules.
Where to next
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