The nucleus
Knowing which particles an atom holds is one thing. Working out how they are arranged inside it took a great deal longer.
In 1911, Ernest Rutherford put forward the nuclear model. At the centre of an atom, he said, is a very small region in which the mass is concentrated: the nucleus.
It was later found that the protons and neutrons are gathered in the nucleus together. Since protons are positive and neutrons carry no charge, that makes the nucleus positively charged.
How small is small?
If an atom were the size of a football pitch, the nucleus at its centre would be smaller than a gram seed lying in the middle of it.
Read that comparison slowly, because it says something surprising: almost all of an atom is empty space. Nearly all the mass sits in that speck at the centre, and the electrons occupy the enormous region around it.
So the picture to carry into the next section is this — a tiny, heavy, positive nucleus, with the electrons somewhere in the space around it. Where exactly, and moving how, is what section 3.1 answers.