The number of protons and atom possesses is what gives the atom its identity—all atoms of a particular element have the same number of protons in their nuclei. For example, all of the sulfur atoms in the pile of sulfur we looked at above have 16 protons in their nucleus. If they had one more proton in their nucleus, they would have a different identity—they’d be chlorine (Cl) atoms, and with one less, they’d be phosphorus (P) atoms.
Atoms of a given element can, however, differ in the number of neutrons
they contain, and atoms of the same element that have different
numbers of neutrons are known as isotopes. Most elements have
at least two isotopes that occur naturally, although a few have just
one. Now take a look at how atoms are usually symbolized:
This represents a carbon atom that has 6 protons and 6 neutrons. In
this notation, the atomic number (A),
which is the number of protons the atom contains, is indicated by the
subscript, and the mass number (Z?), which
is
the number of the atom’s protons plus the number of its neutrons, is
indicated by the superscript. Some relatively common isotopes of
carbon can contain 5, 7, or 8 neutrons, so although their atomic
numbers would all be 6, their mass numbers, respectively, would be 11
(6 + 5), 13 (6 + 7), and 14 (6 + 8). Isotopes can also be written as
carbon-14, carbon-15, carbon-16, etc., or C-14, C-15, C-16, where the
number represents the mass number of the atom.
The last thing you should know about the basic structure of an atom is that atoms have the same number of protons and electrons, and since protons are positively charged and electrons are negatively charged, neutral atoms have no net electrical charge.