Fundamental Chemical Definitions

Atoms and Their Components
Atom	The smallest particle that can exist and still be an element. An atom is composed of three types of subatomic particles: electrons, protons and neutrons. An atom has an equal number of electrons and protons and so is electrically neutral (i.e., has no electrical charge).
Nucleus	The heavy, dense central core of an atom that houses the protons and neutrons. The nucleus contains the vast majority of the mass of the atom.
Proton	A positively charged subatomic particle that is housed within the nucleus.
Neutron	A chargeless subatomic particle that has approximately the same mass as a proton and that is housed within the nucleus.
Electron	A negatively charged subatomic particle that has negligible mass and moves around in a cloud around the nucleus.
Ion	An atom or molecule that has an electrical charge due to it having either extra electrons (negative ion or anion) or having missing electrons (positive ion or cation).
Isotope	A variety of an element that has a specific number of neutrons. All isotopes of an element contain the same number of protons (i.e., have the same atomic number) but have different numbers of neutrons (i.e., have different atomic mass).
Atomic Number	The number of protons in an atom's nucleus. Every element has a unique atomic number.
Atomic Weight	The weight of an atom which is the sum of the weights of the atom's protons and neutrons. An element can have numerous isotopes, so an element's atomic mass is expressed as the average of the atomic weights of all of an element's isotopes. Note that the average is weighted for the abundance of each isotope.

Materials Composed of Atoms
Element	A substance that cannot be broken down into simpler substances by ordinary chemical or physical means. Examples of elements are gold, iron, silicon and carbon.
Compound	A substance formed by the chemical combination of two or more elements in specific proportions. Examples of simple compounds include water (hydrogen and oxygen), carbon dioxide (carbon and oxygen) and table salt or halite (sodium and chlorine).
Molecule	The smallest possible particle of an element or compound. A molecule can consist of only one element (e.g., oxygen = O₂) or more than one element (e.g., carbon dioxide = CO₂).

Atomic Bonds
Covalent Bond	A chemical bond between atoms that is produced by the sharing of electrons. Covalent bonds tend to be strong.
Ionic Bond	A chemical bond that is produced by the mutual attraction of two ions of opposite charge.
van der Waals Bond	A very weak chemical bond between electrically neutral materials due to instantaneous and fleeting imbalances in electrical charge as electrons move around their nuclei.
Metallic Bond	A chemical bond characteristic of metals, in which electrons move freely from atom to atom. This free movement of electrons results in the high electrical conductivity and malleable nature of metals.