d. The Nucleus. The basic particles of the nucleus are protons and neutrons.
Collectively, they are called nucleons.
(1) Protons. Protons are positively charged particles in the nuclei of atoms.
A proton has a mass about 1,836 times that of an electron. While protons are identical
for all elements, each element has its own characteristic number of protons. The
number of protons contained in the atomic nucleus is the atomic number, represented
by the symbol Z. The atomic number is also equal to the number of electrons in the
neutral atom. It is what identifies a particular element and determines its place in the
periodic table. It is usually designated at the lower left corner of a chemical symbol for
an element (for example, 92U).
(2) Neutrons. Neutrons, as the name implies, are electrically neutral
particles. A neutron has about the same mass as a proton. The number of neutrons in
the atomic nucleus of a given element varies. Nuclear stability depends crucially on the
number of neutrons associated with protons in the nucleus. The total number of protons
and neutrons in the nucleus make up the mass number of an element that is
represented by the symbol A. It identifies the isotopes of a particular element. It is
usually designated at the upper left corner of a chemical symbol (for example, 238U).
e. Extra-nuclear Structure
(1) The orbital electrons that whirl around the nucleus in continual motion
are arranged in shells or energy levels around the nucleus. The number of electrons
that each shell can hold is limited. The innermost shell (identified as K) holds only two
electrons, but the next shell (L) may contain up to eight electrons. For elements with
more than 10 electrons in their atomic structure, a third shell (M), which may hold up to
18 electrons, is present.
(2) The maximum number of electrons that may fill any Bohr orbital shell
may be represented by 2n2, where n designates the number of the shell. For example,
calculations for the first, second, and third shells, K, L, and M would be 2 X 12 = 2 (K-
shell), 2 X 22 = 8 (L-shell), and 2 X 32 = 18 (M-shell).
(3) However, in the atoms of elements of higher atomic number, additional
N, O, P, and Q shells are added. These shells may carry no more than 32 electrons
(2 X 42 = 32 for the N-shell), though they often carry fewer than 32. In many of these
heavier elements, electrons are found in an outer shell when a shell closer to the
nucleus is not completely filled. The extra-nuclear structure of uranium is an example of
this. Two electrons are found in the seventh shell even though the fifth and sixth shells
are not completely filled.
(4) In the neutral state, the number of orbital electrons is equal to the
number of positive charges (the number of protons) in the nucleus. However, in some
atoms the number of orbiting electrons may be less than the number possible for each
orbit. In figure 1-3, note that the hydrogen atom (the nucleus of which contains one