Atoms, Elements, and Compounds
Protons, neutrons and electrons — how atoms combine into the substances around us.
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Inside the atom
An atom has a dense nucleus of positively charged protons and neutral neutrons, surrounded by negatively charged electrons. The number of protons — the atomic number — determines which element the atom is.
Elements and the periodic table
An element is a substance made of only one kind of atom. The periodic table arranges elements by atomic number; columns (groups) share similar chemical behavior because they have the same number of outer electrons.
Compounds and bonding
Atoms bond to reach a stable outer electron shell. In ionic bonding, electrons transfer between atoms, as in sodium chloride. In covalent bonding, atoms share electrons, as in water. A compound has properties completely different from the elements that formed it.