What is the difference between an atom's mass number and its atomic number?
Atom's Two ID Cards
What is the difference between an atom's mass number and its atomic number?
Every atom carries two little ID cards. One is its "atomic number," and the other is its "mass number." They sound almost the same, and that's exactly why people mix them up. But they answer two completely different questions — so let's give each one its moment.
First, peek inside an atom. In the middle sits a clump called the nucleus, packed with two kinds of particles: protons and neutrons. Buzzing around the outside are tiny electrons. For today, the nucleus is the star of the show.
