Atoms, elements and compounds
Everything around you, from the air you breathe to the screen you are reading, is built from tiny particles called atoms.
There are only about 100 different kinds of atom, one kind for each element.
Atoms join together in different combinations to make millions of different substances, in the same way that 26 letters make every word in a dictionary.
This unit is examined on Paper 1, and its ideas are also used on Paper 2.
Paper 2 questions on other topics have asked students to count the atoms in a formula, to complete or balance an equation (for example the burning of a fuel or the cracking of a hydrocarbon), to explain why filtering a solution leaves the dissolved substances in it, and to explain fractional distillation.
Everything in this unit is studied by all students, at Foundation and at Higher tier, apart from half equations and ionic equations in the next section, which are Higher only.
Atoms and elements
An element is a substance made of only one type of atom.
Gold is made only of gold atoms; oxygen is made only of oxygen atoms.
Some elements exist as single atoms (helium, neon, argon).
Others exist as small groups of identical atoms joined together, called molecules: oxygen gas is made of O₂ molecules, each two oxygen atoms bonded together.
Both are still elements, because only one type of atom is present.
A compound is different.
Its particles contain atoms of two or more different elements, held together by chemical bonds.
Water is a compound: every water molecule is two hydrogen atoms bonded to one oxygen atom.
The first box above holds an element made of separate atoms, the second an element made of molecules, and the third a compound.
In the compound, every particle is identical and contains two different kinds of atom joined together.
- Atom: All substances are made of atoms. An atom is the smallest part of an element that can exist.
- Chemical symbol: Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium.
- Element: a substance made of only one type of atom. There are about 100 different elements. Elements are shown in the periodic table.
- Compound: Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed.
- Chemical reaction: Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change.
Chemical symbols
Every element has a symbol of one or two letters.
The first letter is always a capital and the second is always lower case.
The case matters: Co is cobalt, an element, but CO is carbon monoxide, a compound of carbon and oxygen.
Some symbols come from the element's Latin name: Na for sodium (natrium), K for potassium (kalium), Fe for iron (ferrum), Cu for copper (cuprum), Ag for silver (argentum), Au for gold (aurum) and Pb for lead (plumbum).
You will be supplied with a periodic table in the exam, so you never have to memorise every symbol.
You do need to be able to use the names and symbols of the first 20 elements, the elements in Groups 1 and 7, and the other elements met in the course, quickly and without mixing them up.
Atomic number
Name
Symbol
Atomic number
Name
Symbol
1
hydrogen
H
11
sodium
Na
2
helium
He
12
magnesium
Mg
3
lithium
Li
13
aluminium
Al
4
beryllium
Be
14
silicon
Si
5
boron
B
15
phosphorus
P
6
carbon
C
16
sulfur
S
7
nitrogen
N
17
chlorine
Cl
8
oxygen
O
18
argon
Ar
9
fluorine
F
19
potassium
K
10
neon
Ne
20
calcium
Ca
The rest of Group 1 is rubidium (Rb), caesium (Cs) and francium (Fr).
The rest of Group 7 is bromine (Br), iodine (I) and astatine (At).
Other elements you will meet often are iron (Fe), copper (Cu), zinc (Zn), silver (Ag), gold (Au), lead (Pb), titanium (Ti), chromium (Cr), manganese (Mn) and nickel (Ni).
Compounds and chemical reactions
Compounds are formed from elements by chemical reactions.
When sodium, a soft shiny metal, reacts with chlorine, a poisonous green gas, the product is sodium chloride, the white solid we use as table salt.
The product has completely different properties from the elements that made it, because the atoms are now bonded in a new way.
That is the sign of a chemical reaction: at least one new substance forms.
Most reactions also give out or take in energy you can detect, as a temperature change, light or sound.
The photograph below shows the two elements and the compound they make.
Nothing about the white crystals looks like either a soft silvery metal or a green gas.
A compound always has the same elements in the same fixed proportions.
Every sample of pure water has two hydrogen atoms for every oxygen atom, wherever it comes from.
That is why a compound can be written as a formula: H₂O, CO₂, NaCl.
Because the atoms in a compound are held by chemical bonds, you cannot pull them apart by filtering, boiling or dissolving.
Compounds can only be separated into elements by chemical reactions.
Water can be split into hydrogen and oxygen by electrolysis, and iron is taken out of iron oxide by reacting it with carbon in a blast furnace.
You will meet both of these in later units (electrolysis, and the extraction of metals).
Naming compounds from formulae
You should be able to name compounds of these elements from given formulae or symbol equations.
A few rules cover almost every compound you will meet.
1. The metal (or hydrogen, or ammonium) comes first, with its name unchanged: sodium, copper, ammonium.
2. Two elements only: the second name ends in -ide. NaCl is sodium chloride, MgO is magnesium oxide, K₂S is potassium sulfide, LiBr is lithium bromide.
3. A group containing oxygen ends in -ate. SO₄ is sulfate, NO₃ is nitrate, CO₃ is carbonate. So Na₂SO₄ is sodium sulfate, not sodium sulfide.
4. OH is hydroxide and NH₄ is ammonium. NaOH is sodium hydroxide; NH₄Cl is ammonium chloride.
A few compounds keep their everyday names: H₂O is water, NH₃ is ammonia and CH₄ is methane.
Counting atoms in a formula
A formula tells you exactly how many atoms of each element are in one unit of the compound.
This skill is tested on both papers, and it goes wrong more often than you might expect.
- A subscript (small, low number) belongs to the symbol just before it. In CO₂ the 2 belongs to O: one carbon atom and two oxygen atoms.
- A symbol with no subscript means one atom.
- A subscript after a bracket multiplies everything inside the bracket. Mg(OH)₂ has one magnesium atom, two oxygen atoms and two hydrogen atoms.
- A large number in front of the whole formula multiplies all of it. 3CO₂ is three molecules: three carbon atoms and six oxygen atoms.
"How many atoms" and "how many elements" are different questions.
CO₂ contains three atoms but only two elements.
Recent papers have asked students to count the atoms of each element in a formula on both Paper 1 and Paper 2, sometimes in a table that also asks for each element's name or symbol.
Writing formulae from ions
Many compounds of a metal with a non-metal are made of ions: atoms that have lost or gained electrons and so carry a charge.
You will learn why ions form in the bonding unit.
For now you only need to know that the charges in a compound always add up to zero.
Common ions are listed below.
Positive ions
Negative ions
H⁺, Li⁺, Na⁺, K⁺, Ag⁺, NH₄⁺ (ammonium)
F⁻, Cl⁻, Br⁻, I⁻ (fluoride, chloride, bromide, iodide)
Mg²⁺, Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Fe²⁺
OH⁻ (hydroxide), NO₃⁻ (nitrate)
Al³⁺, Fe³⁺, Cr³⁺
O²⁻ (oxide), S²⁻ (sulfide), SO₄²⁻ (sulfate), CO₃²⁻ (carbonate)
To write the formula, use just enough of each ion for the charges to cancel.
Ca²⁺ and Cl⁻ need two chloride ions for every calcium ion, so calcium chloride is CaCl₂.
Ca²⁺ and O²⁻ cancel one to one, so calcium oxide is CaO.
A group such as NO₃ or SO₄ goes in brackets when you need more than one of it: Ca(NO₃)₂.
Recent papers have asked for the formula of a compound of a transition metal, worked out from the charges on its ions and chosen from a list of formulae.
The method is the same for every metal, whatever its charge.
Word equations and symbol equations
Chemical reactions can be represented by word equations or equations using symbols and formulae.
You should be able to write word equations for the reactions in this course, and write formulae and balanced chemical equations for them.
A word equation names every reactant on the left and every product on the right.
The arrow means "react to form" and the plus sign means "and".
magnesium + oxygen → magnesium oxide
A symbol equation uses formulae, and it must be balanced: there must be the same number of atoms of each element on each side, because atoms are not made or destroyed in a reaction.
2Mg + O₂ → 2MgO
To balance an equation:
1. Write the correct formula of every substance. Remember the elements that exist as diatomic molecules (two atoms each): H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Metals, carbon and the noble gases are written as single symbols (Mg, C, Ar).
2. Count the atoms of each element on each side.
3. Change only the large numbers in front of formulae (the coefficients). Never change a subscript. Changing H₂O to H₂O₂ would turn water into a different substance, hydrogen peroxide.
4. Check every element again once you have finished.
State symbols show the state of each substance: (s) solid, (l) liquid, (g) gas and (aq) aqueous, which means dissolved in water.
For example: 2Mg(s) + O₂(g) → 2MgO(s).
State symbols in an equation you write do not usually carry a mark unless the question says "Include state symbols", when they carry a mark of their own, and they have also been tested as separate tick items.
Learn them and write them, but never spend time on them in place of getting the formulae right, because balancing numbers in front of a wrong formula earn nothing.
Ammonium sulfate, (NH₄)₂SO₄, is used as a fertiliser.
Give the name and number of atoms of each element in one unit of (NH₄)₂SO₄.
1. Nitrogen, N: 1 inside the bracket × 2 = 2 atoms.
2. Hydrogen, H: 4 inside the bracket × 2 = 8 atoms.
3. Sulfur, S: no subscript, so 1 atom.
4. Oxygen, O: subscript 4, so 4 atoms.
Answer: nitrogen 2, hydrogen 8, sulfur 1, oxygen 4: four elements and 15 atoms in all.
Only the N and the H are inside the bracket, so only they are doubled.
Name the compounds KBr, CaCO₃, Na₂S and Na₂SO₄.
1. KBr: two elements, potassium and bromine, so the ending is -ide: potassium bromide.
2. CaCO₃: calcium with the CO₃ group, which contains oxygen: calcium carbonate.
3. Na₂S: two elements only: sodium sulfide.
4. Na₂SO₄: sodium with the SO₄ group: sodium sulfate.
Answer: potassium bromide, calcium carbonate, sodium sulfide, sodium sulfate.
The last two show why the ending matters: one letter of formula (the O₄) changes the name and the substance.
Chromium(III) oxide is a green pigment used in paints.
It contains Cr³⁺ ions and O²⁻ ions.
Write its formula.
1. Find the smallest total charge both ions can make: 6, the lowest common multiple of 3 and 2.
2. Positive side: 2 chromium ions × (+3) = +6.
3. Negative side: 3 oxide ions × (−2) = −6.
4. Total charge: +6 − 6 = 0, so the formula balances.
Answer: Cr₂O₃.
The same method gives Fe₂(SO₄)₃ for iron(III) sulfate: two Fe³⁺ give +6, three SO₄²⁻ give −6.
Calcium burns in chlorine gas to form calcium chloride, CaCl₂.
Write the word equation and a balanced symbol equation.
1. Word equation: calcium + chlorine → calcium chloride.
2. Formulae: calcium is Ca; chlorine is diatomic, Cl₂; calcium chloride is CaCl₂.
3. Count: left, 1 Ca and 2 Cl; right, 1 Ca and 2 Cl. It is already balanced.
Answer: Ca + Cl₂ → CaCl₂, or with state symbols Ca(s) + Cl₂(g) → CaCl₂(s).
Tungsten, used in welding electrodes, is extracted by heating tungsten oxide in hydrogen.
Balance the equation. [2]
___ WO₃ + ___ H₂ → ___ W + ___ H₂O
1. Tungsten: 1 on each side, so WO₃ and W each keep 1.
2. Oxygen: 3 in WO₃, so put 3 in front of H₂O (1).
3. Hydrogen: 3H₂O has 6 hydrogen atoms, so put 3 in front of H₂ (1).
Answer: WO₃ + 3H₂ → W + 3H₂O. Check: 1 W, 3 O and 6 H on each side.
One mark is given for each correct coefficient; a common slip is the one in front of the diatomic gas.
Why it matters
Formulae and equations are the language of the rest of the course.
You will use them to work out masses in reactions, to explain bonding, and to describe how metals are extracted and how fuels burn.
A chemist reading "2Mg + O₂ → 2MgO" knows straight away which substances react, what forms and in what proportions, which is far more than a sentence could say as briefly.
Higher students take this further in the next section, with half equations and ionic equations.
When a question asks how many atoms are in a formula, write each element's name with its count, one per line, before you add anything up.
Common slips are swapping two counts, giving the number of elements for the number of atoms, and counting only the first atom inside a bracket.
Write subscripts visibly small and low, even when typing.
- Counting elements when atoms are asked for. CO₂ has two elements but three atoms. Read which one the question wants.
- Giving the subscript to the wrong symbol. In N₂O the 2 belongs to N: two nitrogen atoms and one oxygen atom. It is tempting to read the number as belonging to the symbol after it; it never does.
- Swapping charges into a formula without simplifying. Ca²⁺ and O²⁻ do not make Ca₂O₂. The charges already cancel one to one, so the formula is CaO.
- Writing O, Cl or H for the gas. These elements are diatomic: O₂, Cl₂, H₂. A lone symbol is one atom, not the gas.
- Changing a subscript to balance. It feels quicker, but it changes the substance. Balance with coefficients only.
- Sulfide for sulfate. Na₂S is sodium sulfide; Na₂SO₄ is sodium sulfate. A wrong ending gives a different compound, and a name that does not match the formula loses the mark.
- Giving two answers to be safe. A right formula with a wrong name beside it (S²⁻ called a "sulfur ion"), or two lines drawn from one box, cancels the mark. Commit to one answer.
- Swapping counts when writing a formula from a model. Count each kind of atom in the drawn molecule separately and write the formula of that molecule (C₂H₆ for two carbon and six hydrogen atoms), never a general formula such as CₙH₂ₙ₊₂.
Check your understanding
1. This question is about substances and formulae.
1.1 Which formula represents a compound?
Tick (✓) one box. [1]
- Co ☐
- CO ☐
- O₂ ☐
1.2 Calcium hydrogencarbonate has the formula Ca(HCO₃)₂.
How many atoms of each element are in one unit of Ca(HCO₃)₂? [2]
Calcium ________
Hydrogen ________
Carbon ________
Oxygen ________
1.3 A compound has the formula Li₂SO₄.
Which name is correct for this compound?
Tick (✓) one box. [1]
- Lithium oxide ☐
- Lithium sulfate ☐
- Lithium sulfide ☐
2. Complete the sentences about compounds. Choose the answers from the box. [2]
chemically
different
fixed
physically
similar
Compounds contain two or more elements ________ combined.
The elements in a compound are always in ________ proportions.
3. Draw one line from each formula to the name of the compound. [2]
Formula
Name
KNO₃
potassium nitride
K₂O
potassium nitrate
potassium oxide
potassium hydroxide
4. This question is about reactions that make salts.
4.1 Zinc reacts with dilute sulfuric acid to produce zinc sulfate and hydrogen.
Write the word equation for this reaction. [1]
4.2 Iron(III) chloride contains Fe³⁺ ions and Cl⁻ ions.
What is the formula of iron(III) chloride?
Tick (✓) one box. [1]
- FeCl ☐
- FeCl₃ ☐
- Fe₃Cl ☐
4.3 Balance the equation for the reaction of lithium with water. [2]
___ Li + ___ H₂O → ___ LiOH + H₂
5. A student writes:
"Water is a mixture of hydrogen and oxygen, because it contains both of them."
Explain why the student is wrong. [2]
6. Lithium reacts with chlorine to form lithium chloride, LiCl.
Write a balanced symbol equation for this reaction. [2]
7. Magnesium nitrate contains Mg²⁺ ions and NO₃⁻ ions.
Give the formula of magnesium nitrate. [1]
Answers
1. 1.1 CO (1). Co is the element cobalt (a capital then a lower-case letter is one symbol); O₂ is an element made of molecules. CO has two different elements combined.
1.2 Calcium 1, hydrogen 2, carbon 2, oxygen 6.
All four correct (2); two or three correct (1).
The bracket doubles H, C and O₃.
1.3 Lithium sulfate (1).
Lithium sulfide would be Li₂S; lithium oxide would be Li₂O.
2. chemically (1); fixed (1). Must be in this order.
3. KNO₃ to potassium nitrate (1); K₂O to potassium oxide (1). A box with more than one line from it scores nothing.
4. 4.1 zinc + sulfuric acid → zinc sulfate + hydrogen (1). Allow "zinc + sulphuric acid". Formulae in place of names are not a word equation.
4.2 FeCl₃ (1).
One Fe³⁺ needs three Cl⁻ for the charges to cancel.
Fe₃Cl comes from putting the 3 on the wrong ion.
4.3 2Li + 2H₂O → 2LiOH + H₂: 2 in front of LiOH (and Li) (1); 2 in front of H₂O (1).
Allow multiples.
5. Water is a compound, not a mixture (1). The hydrogen and oxygen atoms are chemically combined (bonded) in fixed proportions, two hydrogen atoms to one oxygen atom, so water cannot be separated into hydrogen and oxygen by a physical process (1).
6. 2Li + Cl₂ → 2LiCl: correct formulae, including Cl₂ (1); balanced (1). Allow multiples. Ignore state symbols.
7. Mg(NO₃)₂ (1). One Mg²⁺ needs two NO₃⁻; the bracket is needed so the 2 applies to the whole nitrate group. Do not accept MgNO₃₂ or MgNO₃.