Atoms, ions and isotopes

Every atom of an element has the same number of protons. That number is the atomic number and it decides what element you have. Chlorine always has 17 protons. But atoms of chlorine are not identical: the count of neutrons can differ, and these versions are called isotopes. Chlorine-35 has 18 neutrons, while chlorine-37 has 20, and both are still chlorine because both hold 17 protons.

The mass number is the total of particles in the nucleus: protons plus neutrons. Electrons are not counted because each one weighs almost nothing next to a proton. Writing mass number = protons + neutrons keeps that straight. If you know an atom has 17 protons and 18 neutrons, the mass number is 35, never 18. The neutrons alone never tell the whole story.

Charge is a separate question from mass number. A neutral atom has exactly as many electrons as protons, so the positives and negatives cancel and the charge is 0. The common mistake is bringing neutrons into the charge, or guessing the charge from the mass number. Neutrons are neutral particles: they add mass, but they add no charge at all. To predict an ion's charge, count the electrons, count the protons, and take the difference.

Ions form when electrons are gained or lost. If this chlorine atom picks up one electron, it has 17 protons and 18 electrons, giving a net charge of -1. The nucleus never changed, and the element did not change either: it is still chlorine. Check protons for identity, protons plus neutrons for mass, and the difference between protons and electrons for charge.