Here's a simple video version of this text (~13min): Khan Academy "Elements and Atoms" on YouTube
Here's a more fun, more complicated video (10min): CrashCourse Chemistry 1: The Nucleus on YouTube
Atoms of an element are not all identical and indivisible as Dalton said. They are made of three mains types of particles.
| Particle | mass (amu) | charge |
|---|---|---|
| Electrons | 5.5 x 10–4 | negative (1–) |
| Protons | 1.0073 | positive (1+) |
| Neutrons | 1.0087 | neutral |
The protons and neutrons together form a very small dense center of the atom, called a nucleus. The nucleus contains all the mass of the atom except the small mass of electrons. The electrons move around the nucleus, and occupy a much bigger space than the nucleus, so that most of the atom is empty space.
An element is defined by the number of protons, or atomic number, which is equal to the number of electrons in the neutral element. Atoms of an element can have different numbers of neutrons, resulting in different masses. Isotopes are atoms that have the same number of protons, but different numbers of neutrons. Chemical properties depend mostly on the atomic number, so isotopes are nearly the same chemically. The atomic mass is the average mass of the atom, including all the different isotopes that are likely to be present. If you want to show what isotope of an element you can use special notation, like this: 12C. This is read as carbon-12. The 12 is the mass number, or the number of protons + neutrons. Since carbon always has 6 protons, 12C must have 6 neutrons also.
We use some very small units to describe atoms. For instance, the charge of a proton or electron (which we will use as the unit of charge) is 1.602 x 10–19 coulombs (C). The atomic mass unit, or amu, is 1.661 x 10–24 g. Sizes of atoms are usually measured in angstroms or Å, which is 1 x 10–10 m. The diameter of most atoms is 1-5 Å. The diameter of nuclei is roughly 10–4 Å. Electrons are smaller than nuclei, so most of the atom is empty.