The cartoon picture (and why it's wrong)
Established You've probably seen the cartoon of an atom: a tiny solar system with a nucleus in the middle and electrons orbiting around it like planets. This picture is wrong, but it's usefully wrong. It captures three true things: atoms have a small, dense, positively-charged centre (the nucleus); there are negatively-charged electrons "around" the nucleus; and most of the atom is empty space.
Here's how empty. If a hydrogen atom were the size of a football stadium, the nucleus would be a marble at the centre, and the electron would be a speck of dust somewhere in the stands. Everything you think of as solid matter is, by volume, about 99.9999999999% empty space.
So why does your hand not pass through a table? Not because atoms are solid balls. Because of electromagnetic forces between electrons and the rules of quantum mechanics. Solidity is a force-field effect, not a material effect.
What's in the nucleus
The nucleus contains two kinds of particles:
Protons are positively charged and have a mass of about 1 atomic mass unit. Neutrons have no charge and the same mass. The number of protons defines what element an atom is. This is non-negotiable: 1 proton means hydrogen; 6 protons means carbon; 8 protons means oxygen; 79 protons means gold.
Change the proton count, change the element. This is what nuclear reactions do, and it is why chemical alchemy is impossible — chemistry rearranges electrons, never protons.
The number of neutrons can vary within an element. Different neutron counts of the same element are called isotopes. Carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons) are both carbon, but carbon-14 is radioactive — which is what makes carbon dating work.
Electrons, and the first big weirdness
Electrons are negatively charged. In a neutral atom, the number of electrons equals the number of protons. Here is where the cartoon breaks down: electrons do not orbit the nucleus like planets. They exist in probability clouds called orbitals.
Established An orbital is a 3D region of space where an electron is likely to be found. This is not a "we don't know where it is" situation. It is a "the electron genuinely doesn't have a precise location until you measure it" situation. This is one of the real weirdnesses of quantum mechanics, and it is not something we resolve in later modules — it is how reality works.
For now, hold this: electrons occupy fuzzy 3D regions around the nucleus, not pointlike orbits. The shape of those regions matters enormously, and we develop it in the next module.
If I added a proton to a carbon atom, what would I have? If I added a neutron? If I added an electron?
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With an extra proton, you'd have nitrogen — a different element. With an extra neutron, you'd have carbon-13, a heavier isotope of carbon. With an extra electron, you'd have a carbon ion with a −1 charge, written C⁻. Notice how only the proton change creates a different element.
What this module sets up
The fact that you've internalised: atoms have a fixed-identity nucleus surrounded by electrons in fuzzy 3D regions. The number of protons defines the element; the arrangement of electrons (next module) defines how the element behaves chemically.
What's still missing: why electrons arrange themselves the way they do, what determines the shapes of orbitals, and how any of this gives rise to chemical bonding. That's M-Chem-02. The "first big weirdness" we glanced at — that electrons don't have precise locations — will eventually become a module of its own (M-Phys, Quantum Mechanics for the Curious), but you can do an enormous amount of chemistry without resolving it.