About 300 thousand years after the Big Bang , the Universe had cooled enough for electron s to be captured by protons and alpha particles to form atoms. An electron is pulled towards a proton because their opposite electric charges attract each other. They stick together to form a totally new kind of object called an atom of hydrogen . In the same way two electrons were attracted to each alpha particle, which contained two protons, and were held close to it. The atom they made is called a helium atom .
Atoms are fantastic things.
Around the outside of the atom the electron forms a large thin shell. Inside the atom is empty space, except for the tiny heavy proton at the center. An atom is like a football. The electron in an atoms is like the skin of the football. Under this skin the atom is almost empty.