1. Atom: is the smallest part of an element that can take part in a chemical change.
2. Ion: is an atom or radical carrying an electrical charge. (A cation is positively changed, an anion is negatively charged.)
3. Proton: is the electrically positive unit of the atom.
4. Electron: is the electrically negative unit of the atom. (It’s mass is 1/1840 of a proton.)
5. Neutron: is the electrically neutral unit of the atom, equal in mass to the proton.
6. Molecule: is the smallest particle of an element or compound that can exist by itself. (It never breaks up except to chemically react)
7. Electronic configuration: is the arrangement of electrons in the orbits of an atom.
8. Nucleus: in the center of the atom is a heavy, positively-charged body that consists of all the protons and neutrons of the atom.
9. Atomic weight: is the sum of the protons and neutrons in the nucleus of the atom of an element.
10. Relative atomic weight: is the atomic weight of an element as compared to an atom of hydrogen (1), or as compared to an atom of carbon (12).
11. Atomic number: of an element is the number of protons in its nucleus.
12. Isotopes: are atoms of the same element, having the same atomic number but different atomic weights.
13. Electrovalency: of an atom or radical; it it is the number of electrons which it transfers or gains in chemical reactions.
14. Covalent, or molecular compound: the compound formed by sharing electron pairs between their atoms.
15. Electrovalent, or ionic compound: the compound formed by the electrostatic attraction due to oppositely charged ions. Ions are formed by the transfer of electrons from one atom to another.