Smaller than small
Every solid, liquid and gas is made of atoms, and atoms are astonishingly small. A single drop of water holds more of them than there are stars in the observable universe. Yet each one has the same simple architecture: a dense centre, the nucleus, wrapped in a much larger space where the electrons live.
Three kinds of particle
The nucleus contains protons, which carry a positive charge, and neutrons, which carry none. Almost all of the atom’s mass sits there. Around it move the electrons, negatively charged and nearly two thousand times lighter than a proton. In a neutral atom the number of electrons equals the number of protons, so the charges cancel.
The number of protons is the whole story of which element an atom is. Six protons is carbon, always. Add one and it is nitrogen; take one away and it is boron. Nothing else about the atom changes what it is called. That number is the element’s atomic number.
Shells, and why they matter
Electrons are not scattered at random. They occupy layers called shells, and each shell holds a limited number: two in the first, eight in the second, eight in the third for the elements shown here. Electrons fill the inner shells first.
Try it: choose He, then Ne. Both have a completely full outer shell. Now choose Na, which has exactly one electron beyond a full shell.
Chemistry is mostly the story of that outer shell. Atoms with a full one, like helium and neon, barely react with anything. Atoms with one electron too many, like sodium, or one too few, like chlorine, react eagerly to get to a full shell. That is why the elements in one column of the periodic table behave alike: they have the same number of outer electrons.
Clouds, not orbits
The rings in the scene are a picture, not a photograph. Electrons do not travel on tidy circular tracks. Each one is spread out in a region of space where it is likely to be found, and the shells are really nested regions of that kind. Switch Shells to Cloud to see the same atom drawn that way: a fuzzy layered haze whose brightness shows where the electrons spend their time, sprinkled with sparks that mark places an electron might turn up if you looked. The regions have shapes, called orbitals. An s orbital is a sphere; a p orbital, drawn lilac, is a dumbbell of two lobes. A shell with only some of its p orbitals filled, like carbon’s or oxygen’s, shows lobes; once all three are full, as in neon, they add up to a smooth sphere again. Which way the lobes point is arbitrary for a lone atom; the scene turns them to face you.
The cloud’s size follows real atoms. Each extra proton pulls the same shell in tighter, so carbon is smaller than lithium and neon smaller still; a new shell starts far out, so sodium is larger than neon. The layer sizes come from a standard estimate (Slater’s rules), all at one scale, so their ratios are true: neon is about half the width of carbon, and sodium more than four times neon. The rings draw in to match as the cloud appears, and the nucleus shrinks to a speck. Even that speck is thousands of times too big; at true scale it would be far too small to see.
Both pictures are useful. The shell picture counts electrons and predicts chemistry. The cloud picture is closer to how atoms actually behave.
Same element, different weight
Two atoms with the same number of protons can have different numbers of neutrons. They are the same element, with the same chemistry, but different masses. These variants are called isotopes. Most carbon has six neutrons; a small fraction has eight, and that heavier carbon slowly decays, which is what lets scientists date ancient wood and bone.