Think of burnt toast. The black crumbs you can scrape off the bread are actually clumps of carbon atoms.
All carbon atoms are alike. They have the same number of protons. In the same way, all oxygen atoms are alike. All iron atoms are alike, and so on. Each kind of atom is an element. Carbon, oxygen, and iron are three elements. There are more than 100 elements.
The atoms of different elements differ in the number of protons and electrons they have. Atoms with more electrons tend to be larger. For example, uranium has 92 electrons. It’s larger than hydrogen, which has one electron.
Atoms also differ in mass. Mass is the amount of stuff or matter in something. For example, a ping-pong ball is about the same size as a golf ball. But a golf ball has greater mass. An atom’s mass is tied mostly to how many protons and neutrons it has. Mass is measured in atomic mass units. Each proton and neutron has a mass of one atomic unit or amu.
