Atoms

Atoms are particles that make up everything in the world, even the air. They were discovered by J.J. Thompson.

They are so small you can't see them unless you look under a very powerful microscope (so they're called microscopic).

All atoms are made up of protons and electrons, and some of them have neutrons too. The protons and neutrons make up the center of the atom (called the nucleus) and the electrons revolve around the nucleus.

The protons have a positive charge and the electrons have a negative charge, while the neutrons are neutral; they have no charge at all. Usually, the atom as a whole has no charge because the number of electrons is the same as the number of protons, but if an electron is added or subtracted, the atom gets a charge. An atom with a charge is known as an ion. If it has more electrons than protons, it is a negative ion (called an "anion"), and if it has less electrons than protons, it is a positive ion (called a cation). A whole object can get a charge if it has lots of ions.

Opposite charges attract and like charges repel, so if two items with the same charge meet, they will move away from each other, and if two objects with different charges meet, they will attract. A charged object can even attract to a neutral object, because some of the particles within the neutral item will repel, leading to the surface of the item having an opposite charge and the item attracting. Static electricity happens when something gets a charge due to electrons rubbing off of or onto an object.

The atomic number of an atom is the number of protons, and that's what determines what element the atom is (for example, a hydrogen atom has only one electron).

The periodic table is a way of listing elements, in order of their atomic number.

The atom's outer shells are the levels at which the electrons are circling. The shell closest to the nucleus can hold two electrons, the second can hold eight, and the third can hold 18. To be stable (i.e. impossible to charge), the atom's outermost shell (called the valence shell) needs to be either full or empty (and therefore nonexistent). When two atoms share an electron, making one's valence shell empty and completing the other's, they mix together, creating a compound.