The Atom

Pick up anything at all — this page's screen, your own hand, a mouthful of air — and imagine cutting it in half, then in half again, and again, and again. Keep going. Long before your scissors would give up, you reach a piece so small that cutting it once more would change what it is. That last, smallest piece of an element is an atom.

Everything you have ever seen, touched, breathed or tasted is built from these tiny bricks. And they are staggeringly small. A single atom has a radius of about 1\times10^{-10}\ \text{m} — that is 0.0000000001 of a metre, one ten-billionth. Lay ten million atoms in a row and the line is barely a millimetre long. You are made of roughly 7\times10^{27} of them.

So how is something this small put together? That is the whole story of this page: an atom is not a solid ball. It has a tiny, dense heart and a great deal of nearly-empty space.

Inside the atom: a nucleus and a cloud of electrons

Sitting right at the centre of every atom is a minuscule, unbelievably dense clump called the nucleus. It carries almost all of the atom's mass, yet it is fantastically small — around 10{,}000 times smaller than the whole atom. The nucleus is built from two kinds of particle packed together: protons and neutrons.

Whirling around that nucleus, far out at the edge of the atom, are the electrons. They do not fly about at random — they are arranged in layers called shells (or energy levels). The electrons are what give the atom its enormous size: the nucleus is a pinprick at the middle, and the electrons mark the distant outer boundary. Almost everything in between is empty space.

So an atom is three kinds of particle in two regions: protons and neutrons crammed into the central nucleus, and electrons spread out in shells around it.

Meet the three particles

Each of the three particles has a charge and a mass. Because the real numbers are awkwardly tiny, physicists compare them using simple relative values — the proton is the yardstick, set to 1.

Proton — relative charge +1, relative mass 1. Lives in the nucleus.
Neutron — relative charge 0 (neutral — hence the name), relative mass 1. Lives in the nucleus alongside the protons.
Electron — relative charge -1, relative mass \tfrac{1}{1836}\approx 0.0005 — so light it counts as almost nothing. Lives in the shells.

Two facts fall straight out of this table. First, since a proton is +1 and an electron is -1, an atom with equal numbers of protons and electrons has zero overall charge — it is a neutral atom, which is how atoms normally sit. Second, because the electron's mass is negligible, essentially all of the atom's mass is the protons and neutrons in the nucleus.

Two numbers name every atom

Every element is pinned down by counting the particles in its nucleus. Chemists write an atom of element \text{X} like this:

{}^{A}_{Z}\text{X}

The bottom number is the atomic number (or proton number) Z, and the top number is the mass number A.

The atomic number Z is the number of protons. It defines the element — every carbon atom has Z=6, every oxygen atom has Z=8. Change Z and you have a different element entirely.
The mass number A is the number of protons plus neutrons — the total count of particles in the nucleus: A = Z + N, where N is the number of neutrons.
So the number of neutrons is N = A - Z, and a neutral atom has as many electrons as protons: electrons = Z.

Worked examples: reading an atom

Example 1 — carbon. Carbon is written {}^{12}_{6}\text{C}. Read off Z=6 and A=12. Then:

Protons = Z = 6.
Neutrons = A - Z = 12 - 6 = 6.
Electrons (neutral atom) = Z = 6.

Example 2 — sodium. Sodium is {}^{23}_{11}\text{Na}, so Z=11 and A=23:

Protons = 11.
Neutrons = 23 - 11 = 12.
Electrons = 11 (a neutral sodium atom).

Example 3 — going the other way. Suppose someone tells you an atom has 8 protons and 8 neutrons. Which element is it, and what is its mass number?

The atomic number is the proton count: Z = 8. Element 8 is oxygen.
Mass number A = Z + N = 8 + 8 = 16.
So it is {}^{16}_{8}\text{O}, with 8 electrons.

Build your own atom

Time to make one. Drag the two sliders to choose how many protons and neutrons go into the nucleus. Watch the readout: the proton number Z picks the element, the mass number climbs as A = Z + N, and the atom stays neutral because the number of electrons in the shells always matches the number of protons. Notice how the electrons fill the inner shell (up to 2) before starting the next.

Reminder: the nucleus is drawn far too big here so you can count the particles. In a real atom the nucleus would be a speck you could never see at this scale — more on that below.

Electrons that jump between shells

The electron shells are not just parking spaces — they are energy levels. An electron sitting in an inner shell has less energy than one further out. And an electron can move between shells, but only by trading exactly the right packet of energy.

Absorb energy — for example, by soaking up a burst of electromagnetic radiation such as light — and an electron can jump up to a higher shell, further from the nucleus.
Emit energy — the electron falls back down to a lower shell and releases that energy as electromagnetic radiation (a flash of light).

This tiny bookkeeping of energy is why neon signs glow, why sodium street-lamps are orange, and why every element gives out its own private set of colours. The atom is not a dead lump — its electrons are constantly ready to catch and release light.

An atom is mostly empty space. If the nucleus were a marble on the centre spot of a football pitch, the nearest electrons would be out beyond the back row of the stands, with nothing at all in between. The solid-looking world around you is, up close, almost entirely gaps.
Electrons weigh almost nothing — yet they set the atom's size. It sounds backwards, but the featherweight electrons mark the outer edge of the atom, while the heavy nucleus is a tiny dot at the centre. Mass lives in the middle; size is set by the outside.
It is the proton number that names the element — not the mass number. Two atoms can share a mass number and be completely different elements, and one element can come with several different neutron counts (these are called isotopes). Always count the protons to know what element you have.
Neutral does not mean "no particles." A neutral atom is packed with charged protons and electrons — they simply balance (+1 for every -1), so the totals cancel to zero.

Let's blow one up. Take a single atom and magnify it until it is the size of a huge sports stadium. On that scale the entire nucleus — holding almost all of the atom's mass — would be no bigger than a pea resting on the centre spot. The electrons? A faint blur whizzing around the very top of the stands. Between the pea and the back row: nothing. That "solid" atom is about 99.9999999999999\% empty space.

The physicist Richard Feynman once said that if all scientific knowledge were destroyed and he could pass on just one sentence, it would be this: "All things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another." Almost everything you will ever learn about materials, chemistry and life grows from that one idea — that the world is made of these tiny, mostly-empty, restless bricks.