Atomic Number, Neutron Number, and Mass Number
You’re probably somewhat familiar with the periodic table and know that there are over
100 different chemical elements. An element is defined by the number of protons in the
atomic nucleus. For instance, a nucleus with just one proton is hydrogen, a nucleus with
two protons is helium, and a nucleus with 92 protons is uranium, the heaviest naturally
occurring element. The number of protons in an atomic nucleus determines the atomic
number, Z. In an electrically neutral atom of atomic number Z, there will be Z protons
and Z electrons.
The number of neutrons in an atomic nucleus determines the neutron number, N.
Different nuclei of the same atomic number—that is, atoms of the same element—may
have different numbers of neutrons. For instance, the nuclei of most carbon atoms have
six protons and six neutrons, but some have six protons and eight neutrons. Atoms of the
same element but with different numbers of neutrons are called isotopes.
As we saw above, electrons weigh very little in comparison to protons and neutrons,
which have almost identical masses. The sum of the atomic number and the neutron
number, Z + N, gives us an atom’s mass number, A.
Chemical Notation
The standard form for writing the chemical symbol of an element, X, is:
The element’s mass number is written in superscript, and the atomic number is written in
subscript. You can infer the neutron number by subtracting A – Z. For instance, we would
write the chemical symbol for the two carbon isotopes, called carbon-12 and carbon-14, as
follows:
The same sort of system can be used to represent protons, neutrons, and electrons
individually. Because a proton is the same thing as a hydrogen atom without an electron,
we can represent protons by writing:
where the + sign shows that the hydrogen ion has a positive charge due to the absence of
the electron. Neutrons are represented by the letter “n” as follows:
Electrons and positrons, which are positively charged electrons, are represented,
respectively, as follows:
The number in subscript gives the charge of the particle— 0 in the case of the neutron and
–1 in the case of the electron. The number in superscript gives the mass. Though