By Thread Academy · 17 September 2026 · Chemistry
Everything you see, touch and breathe is made of atoms. The chair you sit on, the water you drink, the air filling your lungs — all of it is built from tiny particles far too small to see, even with the most powerful school microscope. Chemistry begins with the atom because every reaction, every material and every living thing is an arrangement of atoms.
The word "atom" comes from the Greek word atomos, meaning "uncuttable". Ancient Greek thinkers guessed that if you kept cutting matter into smaller and smaller pieces, you would eventually reach pieces that could not be divided. They were right about the smallness, though not quite right about the uncuttability. As this article shows, atoms do have an internal structure — and understanding it unlocks the whole of chemistry.
The three subatomic particles
Atoms are made of three types of smaller particles, called subatomic particles: protons, neutrons and electrons.
- Protons sit in the centre of the atom. Each proton carries a positive charge of +1, and has a relative mass of 1. The number of protons decides which element the atom is.
- Neutrons also sit in the centre of the atom. Neutrons carry no charge — they are neutral — and also have a relative mass of 1.
- Electrons move around the centre in regions called shells. Each electron carries a negative charge of -1, and its mass is so tiny (about 1/1800 of a proton's mass) that we treat it as having a relative mass of almost zero.
The centre of the atom, containing the protons and neutrons packed tightly together, is called the nucleus. Because the protons are positively charged and the neutrons are neutral, the nucleus as a whole is positively charged. The electrons, being negative, are attracted to the nucleus and stay in their shells around it.
In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges cancel out and the atom has no overall charge.
Atomic number and mass number
Two numbers describe the composition of any atom, and they appear on the periodic table for every element.
The atomic number (sometimes called the proton number) is the number of protons in the nucleus. It is written as a small number to the bottom-left of the element symbol. Atomic numbers are unique: carbon is the element with atomic number 6, so every carbon atom has exactly 6 protons.
The mass number (sometimes called the nucleon number) is the total number of protons and neutrons in the nucleus. Since protons and neutrons each have a relative mass of 1, the mass number is effectively the mass of the atom in relative terms. Electrons are too light to matter here.
Because mass number counts protons plus neutrons, you can find the number of neutrons with a simple subtraction:
neutrons = mass number - atomic number
Worked example 1: Oxygen
An oxygen atom has atomic number 8 and mass number 16.
- Number of protons = atomic number = 8.
- Number of electrons = 8, because the atom is neutral.
- Number of neutrons = mass number minus atomic number = 16 - 8 = 8.
So an oxygen atom contains 8 protons, 8 neutrons and 8 electrons.
Worked example 2: Sodium
A sodium atom has atomic number 11 and mass number 23.
- Number of protons = 11.
- Number of electrons = 11 (neutral atom).
- Number of neutrons = 23 - 11 = 12.
So a sodium atom contains 11 protons, 12 neutrons and 11 electrons.
Try this one yourself before reading on: a calcium atom has atomic number 20 and mass number 40. How many neutrons does it have? (Answer: 40 - 20 = 20 neutrons.)
Electron shells and electronic configuration
Electrons do not wander randomly. They occupy shells around the nucleus, also called energy levels. The first shell, closest to the nucleus, can hold up to 2 electrons. The second shell can hold up to 8, and the third shell can hold up to 8 for the first 20 elements. Electrons fill the inner shells first.
The arrangement of electrons in shells is called the electronic configuration (or electron arrangement). We write it as numbers separated by full stops, shell by shell, starting from the innermost.
- Oxygen (atomic number 8): the first shell takes 2 electrons, leaving 6 for the second shell. Configuration: 2.6.
- Sodium (atomic number 11): the first shell takes 2, the second takes 8, and the last electron goes into the third shell. Configuration: 2.8.1.
- Chlorine (atomic number 17): configuration 2.8.7.
- Calcium (atomic number 20): configuration 2.8.8.2.
The electrons in the outermost shell are called valence electrons, and they control how an atom behaves in chemical reactions. Sodium has 1 valence electron and tends to lose it; chlorine has 7 and tends to gain one. This is why the arrangement of electrons is the key to the whole periodic table: elements in the same column have the same number of valence electrons and behave similarly.
A short history: from Dalton to Bohr
Our picture of the atom was not built in a day. It grew through a series of experiments, each one improving the model before it.
John Dalton (early 1800s) proposed that each element is made of tiny, solid, indivisible spheres, like miniature billiard balls. Different elements have spheres of different mass. His model explained why elements always combine in fixed mass ratios, and it put atoms at the centre of chemistry for the first time.
J. J. Thomson (1897) discovered the electron while experimenting with cathode ray tubes. Because electrons are negatively charged and atoms are neutral, Thomson proposed the "plum pudding" model: a ball of positive charge with electrons scattered through it, like plums in a pudding. The atom was no longer indivisible.
Ernest Rutherford (1911) fired tiny positive particles at thin gold foil. Most passed straight through, but a few bounced back sharply. Rutherford concluded that almost all the atom's mass and all its positive charge are concentrated in a tiny central nucleus, with electrons orbiting far away in mostly empty space. His nuclear model explained the experiment, but it had a problem: circling electrons should have spiralled into the nucleus.
Niels Bohr (1913) solved the problem by proposing that electrons orbit the nucleus only in certain fixed shells at set distances. Electrons could jump between shells by gaining or losing energy, but could not exist between them. Bohr's model explains the shell arrangements we use today, and it is still the model taught at IGCSE.
Notice the pattern: each scientist kept what worked from the previous model and changed only what new evidence demanded. That is how scientific models improve.
Why atomic structure matters
Atomic structure is not just a topic to memorise; it is the foundation everything else in chemistry stands on.
- The number of protons defines the element, so atomic structure explains what the periodic table really lists.
- The arrangement of electrons explains why elements in the same group react similarly.
- The gain and loss of electrons explains ionic bonding, while the sharing of electrons explains covalent bonding.
- The masses of protons and neutrons are the basis of all mole and quantitative chemistry calculations.
When a later topic feels difficult, come back to this one. Nearly every hard idea in chemistry becomes easier when you picture the atoms involved: their protons, their neutrons, and above all their electrons.
Key takeaways
- Atoms are built from protons (charge +1, mass 1), neutrons (charge 0, mass 1) and electrons (charge -1, almost no mass).
- Protons and neutrons sit in the nucleus; electrons occupy shells around it. A neutral atom has equal numbers of protons and electrons.
- The atomic number is the number of protons and identifies the element. The mass number is protons plus neutrons.
- Neutrons = mass number - atomic number. This subtraction is one of the most-used calculations in IGCSE chemistry.
- Electrons fill shells from the inside out: up to 2 in the first shell, then up to 8, then up to 8.
- The electrons in the outermost shell (valence electrons) control how an element reacts.
- The model of the atom improved in stages: Dalton's solid spheres, Thomson's plum pudding, Rutherford's nucleus, Bohr's shells — each driven by new experimental evidence.