An atom is incredibly small, with its diameter needing to be increased 200 million times to equal the diameter of a penny. Atoms are made up of subatomic particles, including protons and neutrons in the central nucleus and electrons orbiting the nucleus. Most of an atom's mass comes from its nucleus. The number of protons determines the element and is called the atomic number. Isotopes are atoms of the same element that differ in their number of neutrons.

1. Atoms and Subatomic Particles

5. Atoms - Molecules - Ions Particle Mass (kg) Mass (amu) Charge Electron 9.10939 x 10 -31 0.00055 -1 Proton 1.67262 x 10 -27 1.00728 +1 Neutron 1.67262 x 10 -27 1.00728 0 Most of the mass in an atom is in the nucleus Subatomic Particles