Elements are made of atoms, which are particles that represent the most basic form of an element. An atom is the smallest particle of an element and cannot be further divided without becoming a less complex particle; in fact, the word 'atom' comes from the Greek word "atomos," meaning 'indivisible'.
An atom consists of a nucleus containing protons and neutrons surrounded by a cloud of electrons. The atom is mostly empty space because most of the mass is contained in the nucleus. Protons have a positive electrical charge and electrons have an equal-intensity negative charge, so they attract one another. Neutrons carry no charge but add extra mass to the nucleus. Almost all atoms except for hydrogen have neutrons in their nuclei. Each atom has the same number of protons and electrons to preserve electrical neutrality.
In the early 20th century, the atom was thought to be a miniature version of the solar system with electrons orbiting the nucleus like planets orbit the sun. Electron motion is far more chaotic, however; no electron can be definitely tracked or located. Instead, they exist in clouds and shells.
Atoms can bond with one another via ionic or covalent bonds, in which they gain, lose or share electrons to create molecules. The simplest bond is two hydrogen atoms bonding with an oxygen atom to make a water molecule.