The Nucleus

At the center of an atom lies its nucleus, consisting of one or more nucleons (protons or neutrons)
held together with considerably more energy than the energy needed to hold electrons in orbit
around the nucleus. The radius of the nucleus is about 100,000 times smaller than the radius of the
atom. Before we go on, let’s revisit some concepts introduced in chapter 3.

ATOMIC NUMBER (Z)

Z is always an integer, and is equal to the number of protons in the nucleus. As stated in chapter 3 ,
the number of protons is what defines an element: An atom or an ion or a nucleus is identified as
carbon, for example, if and only if it has 6 protons. Each element has a unique number of protons. Z
is used as a presubscript to the chemical symbol in isotopic notation, that is, it appears as a
subscript before the chemical symbol. The chemical symbols and the atomic numbers of all the
elements are given in the periodic table.

MASS NUMBER (A)

A is an integer equal to the total number of nucleons (neutrons and protons) in a nucleus. Let N
represent the number of neutrons in a nucleus. The equation relating A, N, and Z is simply:

A = N + Z

In isotopic notation, A appears as a presuperscript to the chemical symbol: It appears as a
superscript that comes before the chemical symbol. In general, then, a nucleus can be represented
as