It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons. The relative mass of a nucleus is ...

Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom. Bohr's 'solar system' model of the atom is the way that most people think ...

He discovered the atomic nucleus and developed a model of the atom that was similar to the solar system. Like planets, electrons orbited a central, sun-like nucleus. Acceptance of this model grew ...