What are the atomic number and massof an element?
The atomic number is the number of protons in an atomic nucleus. The atomic mass
of an atom—usually measured in atomic mass units—is the total mass of the atom, or
the combined mass of its protons and neutrons (the mass of the electrons is negligi-
ble). The importance of atomic numbers and mass is simple: The atoms of each ele-
ment has a specific atomic number and mass—each determined by “adding” or “sub-
tracting” protons and neutrons within the atom.What is an ion?
Scientists know that atoms can gain or lose electrons, thus acquiring a negative or
positive electrical charge (determined by the number of protons minus the number of
electrons). For example, if there are 4 protons and 6 electrons, the net charge is 2
(often called the valence). An ion is an atom—or group of atoms—that takes on the
net electric charge, and can be positive (cation) or negative (anion). Based on the
“mathematics” of losing or gaining electrons, ions can be formed or destroyed. This is
one example of where math comes in handy in chemistry.What is an angstrom?
An angstrom (Å) is a unit of measurement often used in chemistry most often in refer-
ence to molecules; for example, the average molecule diameter is between 0.5 to 2.5
angstroms. An angstrom is equal to about 3.937 10 ^9 inch or one hundredth of a
millionth of a centimeter (10^8 centimeter). (For more information about measure-
ment, see “Mathematics in History.”)What is density?
Density (usually abbreviated as dor r) is a mathematical concept used to describe the
ratio between the mass of an object and its volume. The actual formula is the density
times the volume is equal to an object’s mass, or dvm. In the standard (Ameri-
can) measurement system, density is measured in pounds per cubic foot. But in the
sciences, the metric system is usually used and density is measured in grams per cubic
centimeter (or grams per milliliter). For example, the density of water is 1 gram per
cubic centimeter, lead is 11.3 grams per cubic centimeter, and gold is 19.32 grams per
cubic centimeter. (Note: In the majority of cases, the higher the density, the “heavier”
it feels to us on Earth.)What are formulasand equationsin chemistry?
Formulas and equations in chemistry don’t always mean the same as in mathematics.
Formulas in chemistry are representations of a chemical compound using symbols for
284 the elements and subscripts for the number of atoms present. For example, the chem-