Building blocks of matter
Everything around you is made of tiny particles called atoms. Your body contains about 7 billion billion billion atoms (7 × 10²⁷). A single atom is about one ten-millionth of a millimetre wide, far too small to see with an ordinary microscope.
The idea of atoms is very old. Around 400 BCE, the Greek thinker Democritus suggested that matter was made of tiny pieces that couldn’t be cut, which he called atomos, meaning "uncuttable." We now know atoms are made of even smaller parts.
Inside an atom
| Particle | Charge | Where it is | Relative mass |
|---|---|---|---|
| Proton | Positive (+1) | Nucleus | 1 |
| Neutron | None (0) | Nucleus | 1 |
| Electron | Negative (−1) | Around the nucleus | About 1/1836 |
- The nucleus is tiny but holds almost all the atom’s mass.
- Electrons move around the nucleus in regions called energy levels or shells.
- In a neutral atom, the number of electrons equals the number of protons.
How empty is an atom?
If an atom were the size of a football stadium, the nucleus would be about the size of a marble at the centre. The rest is mostly empty space where electrons move.
What makes elements different
The number of protons, called the atomic number, decides which element an atom is.
| Element | Atomic number (protons) | Neutrons (most common) | Electrons |
|---|---|---|---|
| Hydrogen | 1 | 0 | 1 |
| Carbon | 6 | 6 | 6 |
| Oxygen | 8 | 8 | 8 |
| Iron | 26 | 30 | 26 |
| Gold | 79 | 118 | 79 |
Change the number of protons and you change the element. That’s why alchemists could never turn lead (82 protons) into gold (79 protons) with chemistry.
Mass number = protons + neutrons. Carbon-12 has 6 protons + 6 neutrons = 12.
Isotopes
Isotopes are atoms of the same element with different numbers of neutrons.
- Carbon-12: 6 neutrons (most common)
- Carbon-14: 8 neutrons (radioactive; used to date ancient objects)
Isotopes behave the same chemically but have different masses. Some are radioactive and are used in medicine, energy and dating fossils.
Ions
If an atom gains or loses electrons, it becomes an ion with an electric charge.
- Losing electrons → positive ion (Na⁺)
- Gaining electrons → negative ion (Cl⁻)
Table salt is made of sodium ions and chloride ions held together.
Electron shells
Electrons fill shells around the nucleus:
| Shell | Maximum electrons (for the first 20 elements) |
|---|---|
| 1st | 2 |
| 2nd | 8 |
| 3rd | 8 |
The electrons in the outer shell, called valence electrons, control how an atom reacts with others. Atoms with full outer shells, like neon, rarely react.
How we discovered the parts
| Year | Scientist | Discovery |
|---|---|---|
| 1803 | John Dalton | Proposed that elements are made of atoms |
| 1897 | J. J. Thomson | Discovered the electron |
| 1911 | Ernest Rutherford | Discovered the nucleus with his gold foil experiment |
| 1913 | Niels Bohr | Proposed electron energy levels |
| 1932 | James Chadwick | Discovered the neutron |
Rutherford fired tiny particles at thin gold foil. Most passed straight through, but a few bounced back. He concluded that atoms have a tiny, dense, positive nucleus. Before this work in England, he had won a Nobel Prize for research done at McGill University in Montreal.
Even smaller
Protons and neutrons are made of even smaller particles called quarks. Electrons, as far as we know, are not made of anything smaller.
Common mistakes
- Thinking electrons orbit like planets. They move in fuzzy clouds of probability.
- Thinking the number of neutrons decides the element. Protons do.
- Thinking atoms are solid balls. They’re mostly empty space.
Try it yourself
- Build a model of a carbon atom with clay or candy: 6 protons and 6 neutrons in the centre, 6 electrons around (2 in the first shell, 4 in the second).
- Use a periodic table to find the number of protons, electrons and neutrons in sodium (mass number 23).
- How many neutrons does oxygen-18 have?
(Answers: 11 protons, 11 electrons, 12 neutrons; 10 neutrons)
Quick recap
- Atoms are the building blocks of all matter.
- Protons and neutrons are in the nucleus; electrons surround it.
- The atomic number (protons) identifies the element.
- Isotopes have different numbers of neutrons.
- Atoms gain or lose electrons to become ions.