Elements, compounds & atomic theory

What you will learn

the main ideas of atomic theory and how it developed,
how to tell elements, compounds and mixtures apart,
how atoms combine to form molecules and compounds,
how to read chemical symbols and simple chemical formulas,
the basic layout of the periodic table.

Worked example 0 Real-world example: sea salt and fresh water

You boil away a pan of sea water and are left with a white solid (salt, $\text{NaCl}$ ). Was the sea water a compound, a mixture, or an element?

Sea water is water ( $\text{H}_2\text{O}$ ) with salt ( $\text{NaCl}$ ) dissolved in it.
Both parts keep their own properties, and you could recover them by physical means (boiling).
So sea water is a mixture — specifically a solution.
Water and salt are each compounds (two or more different elements chemically joined).

Key idea: if a physical method (filtering, boiling, magnetism) can separate the parts, the substance is a mixture, not a compound.

1. Atomic theory

John Dalton (1803) proposed the first modern atomic theory. Updated for today, its main ideas are:

All matter is made of tiny particles called atoms.
Atoms of one element are identical (in chemical behaviour); atoms of different elements are different.
Atoms cannot be created, destroyed, or subdivided by ordinary chemical reactions.
Atoms combine in simple whole-number ratios to form compounds.
A chemical reaction is the rearrangement of atoms — never a change in the atoms themselves.

2. Elements, compounds, mixtures

An element is a pure substance made of only one kind of atom. There are about 118 known elements. Examples: oxygen (O), iron (Fe), gold (Au), carbon (C).
A compound is a pure substance made of two or more different elements chemically joined in a fixed ratio. Examples: water ( $\text{H}_2\text{O}$ ), carbon dioxide ( $\text{CO}_2$ ), sodium chloride ( $\text{NaCl}$ ).
A mixture is two or more substances physically mixed but not chemically joined. Each substance keeps its own properties. Examples: air, sea water, sand in water, trail mix.

Elements (single type of atom), compounds (fixed ratio of two or more atom types chemically bonded), and mixtures (two or more substances physically combined).

3. Chemical symbols and formulas

Each element has a one- or two-letter symbol. The first letter is always capitalised; the second (if any) is lowercase.

Element	Symbol	Element	Symbol
Hydrogen	H	Sodium	Na
Oxygen	O	Iron	Fe
Carbon	C	Copper	Cu
Nitrogen	N	Gold	Au
Chlorine	Cl	Silver	Ag

A chemical formula tells you which atoms are present and how many of each. Subscripts (“little down numbers”) count the atoms.

$\text{H}_2\text{O}$ — two hydrogen atoms, one oxygen atom per water molecule.
$\text{CO}_2$ — one carbon atom, two oxygen atoms.
$\text{NaCl}$ — one sodium, one chlorine.
$\text{C}_6\text{H}_{12}\text{O}_6$ — six carbon, twelve hydrogen, six oxygen (glucose).

Worked example 1 Counting atoms in a formula

How many atoms of each element are in one molecule of sulfuric acid, $\text{H}_2\text{SO}_4$ ?

H: subscript 2 $\to$ 2 hydrogen atoms.
S: no subscript $\to$ 1 sulfur atom.
O: subscript 4 $\to$ 4 oxygen atoms.
Total atoms in the molecule: $2 + 1 + 4 = 7$ .

Worked example 2 Reading a coefficient

How many oxygen atoms are in $3\text{CO}_2$ ?

The coefficient $3$ means “three molecules of $\text{CO}_2$ ”.
Each $\text{CO}_2$ has $2$ oxygen atoms.
Total oxygen atoms: $3 \times 2 = 6$ .

Key idea: the coefficient multiplies everything in the formula; the subscript only multiplies the atom it follows.

4. Molecules and why atoms bond

Most elements are happier combined with other atoms than alone. When atoms bond they share or transfer electrons to form stable arrangements called molecules (for non-metals) or ionic compounds (for metal + non-metal).

Some elements exist naturally as diatomic molecules: $\text{H}_2$ , $\text{O}_2$ , $\text{N}_2$ , $\text{Cl}_2$ .
Water ( $\text{H}_2\text{O}$ ) has very different properties from hydrogen gas or oxygen gas because the atoms are now bonded into a new substance.

5. The periodic table (introduction)

The periodic table arranges elements by their atomic number (number of protons per atom). Elements with similar chemical behaviour fall in the same vertical column, called a group. Horizontal rows are called periods.

Main features to know at Year 8:

Group 1 (far left, not counting hydrogen): alkali metals — very reactive metals such as sodium and potassium.
Group 17: halogens — reactive non-metals such as chlorine.
Group 18 (far right): noble gases — extremely unreactive (helium, neon, argon).
A rough zig-zag line divides metals (left) from non-metals (right).

Worked example 3 Predicting from the group

Sodium (Na) is a very reactive metal. Potassium (K) is in the same group. What would you predict about potassium?

Same group $\to$ similar chemical behaviour.
So potassium is also a reactive metal, and probably reacts with water in a similar way to sodium.
In reality, potassium is even more reactive than sodium.

Key idea: the periodic table is organised so you can predict an unfamiliar element’s behaviour from its group mates.

Practice: Year 8

Fluency

Elements, compounds, mixtures

Classify each as element, compound or mixture: (a) oxygen gas, $\text{O}_2$ ; (b) carbon dioxide, $\text{CO}_2$ ; (c) air; (d) pure gold; (e) sea water; (f) table salt, $\text{NaCl}$ .
Give the chemical symbol for: hydrogen, oxygen, carbon, sodium, chlorine, iron.
Name the element with symbol (a) Au, (b) Cu, (c) Ag, (d) K, (e) Fe.
How many atoms in total are in one molecule of (a) $\text{H}_2\text{O}$ , (b) $\text{CO}_2$ , (c) $\text{NH}_3$ ?
How many hydrogen atoms are in $\text{CH}_4$ ?

Fluency

Formulas and coefficients

How many oxygen atoms are in $2\text{H}_2\text{O}$ ?
How many atoms of each element in $\text{Ca(OH)}_2$ ?
Which has more oxygen: $3\text{CO}_2$ or $2\text{SO}_3$ ?
Write the formula for a compound with 2 hydrogen atoms and 1 sulfur atom.
What does the subscript $3$ tell you in $\text{NH}_3$ ?

Reasoning

Explain and classify

Explain the difference between a compound and a mixture using water and sea water as examples.
Why are diamond and graphite both considered elements even though they look completely different?
A substance can be separated by filtering. Is it an element, a compound or a mixture? Justify.
Explain why $\text{Co}$ and $\text{CO}$ mean completely different things.

Problem solving

Applied contexts

Baking soda is $\text{NaHCO}_3$ . Count the atoms of each element in one formula unit.
A scientist analyses a gas and finds it contains only one type of atom. What type of substance is this (element, compound, or mixture)?
Using the idea of atomic theory, explain why burning wood does not violate “atoms cannot be created or destroyed” — even though the wood seems to disappear.
Predict whether argon (Ar, Group 18) reacts easily with other elements, and give a reason.

Challenge

Reasoning

Harder reasoning

A compound has the formula $\text{X}_2\text{Y}_3$ . In $4$ formula units, how many atoms are there of X and Y?
A sample contains $\text{N}_2$ , $\text{O}_2$ and $\text{Ar}$ all mixed together. Classify this sample and name a real-world example.
Explain why the discovery that the same element could have atoms of different masses (isotopes) did not overturn atomic theory.
Mendeleev left gaps in his first periodic table where no known element fitted. Why was this a scientific strength rather than a weakness?