Atoms, elements and compounds · Classify
Chemical symbols
Chemists in Lagos, Osaka and São Paulo all need to write down salt. What do they write, and why?
Start here
A chemistry book in Japanese.
You cannot read a word of Japanese. Someone hands you their chemistry textbook, open at a page about water.
If you had to guess, when the book gives the formula for water, will it be H2O, or written in Japanese?
Good guess. The words around it are all Japanese, but the formula is H2O, exactly as you would write it.
Fair guess. Everything else on the page is in Japanese. But chemical symbols are shared by every country, so water is H2O there too.
That is what symbols are for: each one means exactly one element, in every country, with nothing to translate. Next: where nine symbols come from.
Every element has a symbol of one or two letters. The first is always a capital; the second, if there is one, is always lower case. That rule looks fussy until you meet the pair it exists for.
Your turn · nine symbols
Where does each symbol come from?
0 of 9 placed
Some symbols are the first letter of the English name. Some are the first two. And some look like a mistake until you know where they came from.
H
Hydrogen
First letter of the English name, and it got there first — no other element beginning with H could have it.
C
Carbon
First letter. Calcium arrived later and had to take two letters instead.
Ca
Calcium
C was already taken by carbon, so calcium takes the first two letters of its name.
Cl
Chlorine
Not the first two letters: Cl is the first and the third, exactly like magnesium below. The rule is looser than it looks.
Mg
Magnesium
Two letters, and not the first two: Ma would have clashed, so it uses the first and third.
Na
Sodium
From natrium, the Latin name. It is why its compounds have been called soda for centuries.
Fe
Iron
From ferrum, the Latin for iron — which is also where farrier and ferrous come from.
Pb
Lead
From plumbum, the Latin for lead, and the reason a person who fits water pipes is called a plumber.
Au
Gold
From aurum. Every element with a Latin symbol is one that people were already working with thousands of years ago.
The odd ones are not random. They are Latin, and they are the elements people had already been working with for thousands of years before English existed — gold, iron, lead, copper, sodium, potassium. A symbol that looks wrong is usually a fossil.
Key fact
One capital letter starts one element. CO is carbon and oxygen; Co is cobalt. The case of the second letter is not a style choice.
Four words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Read it back · four labels
How many elements are in this?
0 of 4 read
Count the capital letters. Each one starts a new element, and a lower-case letter always belongs to the capital in front of it.
NaCl
Table salt
Two elements. Two capitals: Na and Cl. Sodium and chlorine. Four letters, two elements.
CO
The gas from a faulty boiler
Two elements. Two capitals: C and O. Carbon and oxygen — carbon monoxide.
Co
A hard silvery metal
One element. One capital, one lower-case letter. Cobalt. The same two letters as the line above, and a completely different substance.
CaCO3
Chalk, limestone and marble
Three elements. Three capitals: Ca, C and O. Calcium, carbon and oxygen. The small 3 is a count, not an element.
Think again
“The symbol is just a short version of the name, so sodium should be So and it does not matter much if you write NA.”
Commit to what CO and Co mean before you read on.
CO is two capitals, so it is two elements: carbon and oxygen, joined — carbon monoxide, the gas that kills people in their sleep. Co is one capital with a lower-case letter after it, so it is one element: cobalt, a hard silvery metal used in magnets and batteries.
Writing NA instead of Na is not untidy handwriting; it says nitrogen-and-something. And sodium is Na because its old name is natrium, which is also why its compounds were called soda long before anyone knew what was in them.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
How many elements are in the formula MgO?
Rung 2 · The one that catches people
A student writes the formula for carbon monoxide as Co. What have they written?
Rung 3 · Explain
Explain why chemists agreed on symbols rather than names, using two things symbols can do that names cannot.
Rung 4 · Take it somewhere new
A new element is made in a laboratory and needs a symbol. Every one-letter and most two-letter options are taken. Say what rules the symbol must follow, suggest one, and say how you checked it was free.
Key note
Every element has a one- or two-letter symbol: capital first, lower case second. A capital letter always starts a new element, so counting capitals counts elements. Some symbols come from Latin names — Na, K, Fe, Pb, Au, Cu — and those are the elements people knew first.
Going further
Before symbols, alchemists used pictures — a crescent for silver, a circle with a dot for gold — and kept them deliberately obscure so that rivals could not read their notes. Chemistry only became a subject that could be shared when Berzelius proposed letters taken from the Latin names in 1813. The change from a private code to a public notation is not a footnote in the history of chemistry; it is most of the reason there is one.
Before this lesson
Next in this unit
At GCSE this becomes
- Balanced symbol equations, and reading a formula for what it says about the amounts.
Where to next
Ask MrBadmus
Not sure why Na is not So?
Lesson content © MrBadmus.