CHEMISTRY
Jim Hollister
Definition of Terms
NOTE: You will not usually be tested on definitions, but on homework problems, as calculations
and nomenclature problems. Still, you must understand the basic terms to understand chemistry.
Atom — The basic particle from which matter is constructed. (The smallest part of an element
which can enter into a reaction (combine with other atoms) to form chemical compounds.)
mixture — Pure substances which exist together without chemically combining; the relative
amounts of each substance can vary.
pure substance — A single chemical or one kind of matter having its own unique set of chemical
and physical properties; either element or compound.
element — A pure substance that can not be decomposed into other pure substances by ordinary
chemical means. A pure substance that consists only of atoms with the same nuclear
charge (see atomic number below).
compound — A pure substance that can be decomposed into two or more other pure substances
by a chemical exchange. Example: CaS => Ca + S . The relative amounts of the
combined substances never varies.
molecules — Two or more atoms joined by a chemical bond. (The smallest unit particle of a
substance (compound or polyatomic element) which can exist independently and possess
the identity of the substance.)
diatomic molecules — A molecule of just two atoms. Can be heteronuclear or homonuclear.
Learn " I Brought Clay From Our New Home." as a mnemonic device to memorize those
elements which, when not combined with other elements, normally occur in nature as
homonuclear molecules.
metal — Has luster, is a good conductor of electricity and heat, and is malleable and ductile. The
metals compose about two-thirds of the elements in nature.
non-metal — Has no luster, is not a good conductor and is not malleable or ductile.
chemical symbols — Abbreviations for elements.
ion — A species that has either a deficiency of electrons ( in which case the ion is positively
charged) or an excess of electrons ( in which case the ion is negatively charged).
periodic table — An arrangement of the elements according to increasing atomic number, such
that elements with similar properties are in the same columns (groups) of the table.
period — A horizontal row of the Periodic Table.
group —All elements in the same column of the Periodic Table which have similar properties.
Also called a family. Newer Periodic Tables use all Arabic numbers for all groups,
groups 1 through 18. (In older Periodic Tables, groups are usually indicated with Roman
numerals.)
Definition of Terms continued
transition metals — elements belonging to groups between groups 2 and 13. (In older texts,
group 13 is called group 3 or (III).)
oxidation state — A common charge assigned to an atom or ion.
atomic mass unit — A unit of mass that is exactly 1/12 of the mass of an atom of carbon-12.
----------------------------------Numbers Associated with Elements--------------------------------atomic mass (atomic "weight") — The average mass of the atoms of an element expressed in
atomic mass units (compared to mass of atom of Carbon 12 at a value of exactly 12.)
(See atomic mass unit above).
atomic number(Z) — Equals the number of protons in each atom of an element. It does not
change, thus, each element is characterized by its atomic number.
mass number (A =Z+N) — Atomic number (Z) + number of neutrons (N). It is a whole number
(integer). (You must round off the atomic mass from the periodic table to a whole
number to get the mass number.)
isotope — Two or more atoms of the same element which have different atomic masses because of
different numbers of neutrons.
molecular mass (molec. "weight") — The sum of the atomic masses of the constituent elements of
a molecule or compound. (The average mass of molecules of a compound in a.m.u.s.)
-----------------------------------------------------------------------------------------------------------------mole — 1) One mole of any substance has a mass in grams which equals the numerical
value of its relative atomic or molecular mass.
2) One mole equals the amount of a substance that contains 6.022 x 1023 units,
which is the same number of units as the number of atoms in exactly 12g of
Carbon-12. (Units could be atoms, molecules, ions, etc.) (6.022 x 1023 is
called Avogadro's Number and its algebraic symbol is N0 ).
3) One mole of any gas occupies 22.4 liters at Standard Temperature and
Pressure (=STP: 0O C and one atmosphere).