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Nuclear structure 203

Nuclear structure

The nucleus of an atom is composed of protons and neutrons bound together by short-
range nuclear forces. The protons and neutrons are collectively known as nucleons. The
charge on a proton is equal to +1 atomic charge unit and neutrons have zero charge. The
number of protons in the nucleus is called the atomic number, or atomic charge, of the
atom and has the symbol Z. The atomic number is unique to each element. In a neutral
atom the positive charge of the protons in the nucleus is balanced by an equal number of
negatively charged electrons in orbitals surrounding the nucleus.
The total number of protons and neutrons together in the nucleus is called the atomic
mass number of the nucleus and has the symbol, A. The number of neutrons in the
nucleus, N, is therefore (A−Z). To represent a particular atomic nucleus, the chemical
symbol is written with a preceding subscript equal to Z and a preceding superscript equal
to A. For example, an atom of carbon-12 (with 6 protons and 6 neutrons) is written as
. The Z subscript is not strictly necessary since the chemical symbol also uniquely defines
Z.
The nucleus constitutes only a very small fraction of the total volume of an atom. The
length of nuclear radii lie in the approximate range of (1–8)×10 −15 m.

Isotope

The atomic number, or number of protons, Z, is unique to each element, but different
atoms of a particular element may contain different numbers of neutrons and thus have
different atomic mass numbers, A. Atoms that have the same number of protons but
different numbers of neutrons are known as isotopes of that element. For example,
oxygen has three isotopes whose nuclei contain either 8, 9 or 10 neutrons, in addition to
the 8 protons. The isotopes are written as , and . Isotopes of an element
undergo identical chemical reaction since the number of protons and electrons which
define the bonding characteristics are the same for each isotope.
Most elements, like oxygen, have isotopes that are stable under normal conditions.
Such stable isotopes do not emit ionizing radiation and do not spontaneously transform
into atoms of a different element. Isotopes that are unstable and decompose naturally into
other elements, with the release of harmful subatomic particles and/or radiation, are
known as radioisotopes (Topic G2).

Nuclear mass

By definition, the atomic mass unit, or amu (also called Dalton, Da), is equal to 1/12th
the mass of an atom of the isotope of carbon, i.e. the mass of equals exactly
12.000 amu. In practice this means that 1 amu=1.660540×10 −27 kg. The number of atoms
in exactly 12.000 g of atoms is known as the Avogadro constant or Avogadro