Atoms: Structure of the Atom with a Live 3D Model

Could you cut forever?

Imagine cutting a pebble in half, then in half again, and again and again. Would you ever reach a piece that cannot be cut? Around 400 BC the Greek thinker Democritus said yes. He called the last piece atomos, which means uncuttable, and he pictured atoms of many shapes moving through empty space. He had no instruments, so this was an idea, not a measurement. Other thinkers, Aristotle among them, preferred to think matter was smooth all the way down, and for about two thousand years that view mostly won. Watch the picture: a block divides and divides until one speck is left.

Dalton and the atoms of an element

In 1808 the English teacher John Dalton turned the old idea into science. Elements, he proposed, are made of atoms, and all atoms of one element are alike, while atoms of different elements differ, above all in mass. Compounds form when atoms of different elements join in fixed whole-number ratios. That explained why water always holds the same share of hydrogen and oxygen. We now know that atoms of one element can differ slightly in mass, and that atoms can be split. Still, chemistry as counting and rearranging atoms is exactly how every equation in this book is written.

Finding the electron

In 1897 J. J. Thomson studied rays in a glass tube and found particles roughly two thousand times lighter than the lightest atom. They carried negative charge. He had found the electron, the first piece inside the atom. His model pictured a soft positive ball dotted with electrons, like raisins in a pudding, and it lasted fourteen years. Notice what it predicts: if the positive charge is spread out evenly, then a fast, heavy particle fired at a thin sheet should plough straight through with only a small nudge. Keep that prediction in mind for the next block.

Rutherford's gold foil

In 1909 Hans Geiger and Ernest Marsden, working in Ernest Rutherford's laboratory in Manchester, fired alpha particles at an extremely thin gold foil. Nearly all of them passed straight through. But about one in eight thousand bounced back, which Rutherford later called almost as incredible as a large shell bouncing off tissue paper. In 1911 he explained it: nearly all the mass and all the positive charge sit in a tiny nucleus, and the rest of the atom is mostly empty space holding light electrons. The nucleus is about 100 000 times narrower than the atom. Scaled up, if the atom were a stadium, its nucleus would be a grain of sand at the centre.

Bohr's shells

In 1913 the Danish physicist Niels Bohr placed electrons on fixed levels around the nucleus, like shelves, and showed that this explains the colours atoms give out when heated. Each shell holds a limited number of electrons. The first takes 2, the second takes 8, and for the first eighteen elements the third is filled to 8. Take sodium, with 11 electrons: 2 on the first shell, 8 on the second and one left alone on the third. That lone outer electron is why sodium is so eager to react. In the picture, watch an electron jump from shell to shell as it takes in or gives out light.

Clouds, not orbits

Bohr's circles worked well for hydrogen but failed for bigger atoms. In 1925 and 1926 Werner Heisenberg and Erwin Schrödinger replaced orbits with orbitals: regions where an electron is likely to be found. An electron is not a tiny planet. It behaves more like a wave of probability, so the atom looks like a fuzzy cloud around the nucleus. Shells survive as a handy way of keeping count. The picture now shows the cloud as thousands of faint dots that thicken where the electron is most likely to be, around a nucleus so small that you must zoom in a hundred thousand times to see it.

Build an atom

Now build your own. Under the picture, add or remove protons, neutrons and electrons and watch the label change. The number of protons, the atomic number Z, decides the element: 6 protons is always carbon and 8 is always oxygen. The mass number A is protons plus neutrons, and the charge is protons minus electrons. Worked example: 11 protons, 12 neutrons and 10 electrons make sodium-23, with A = 11 + 12 = 23 and charge 11 − 10 = +1, written Na⁺. Change only the neutrons and you get another isotope of the same element. A lopsided mix of neutrons and protons is a hint that the nucleus is unstable and may decay.

Atoms: the smallest pieces