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.
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.