Page 216 - Instant notes

G1
NUCLEAR STRUCTURE

Key Notes
Atomic nuclei are composed of protons and neutrons. A proton
has +1 atomic charge and a neutron has zero charge. The atomic
number, Z, of an atom is equal to the number of protons in the
nucleus and is unique to each element. The atomic mass number,
A, of an atom is equal to the sum of the number of protons and
neutrons in the nucleus, so the number of neutrons, N=A−Z. A
nucleus is represented by the chemical symbol for the element
with a preceding subscript and superscript equal to Z and A,
respectively; for example, .
Nuclei which contain the same number of protons but different
numbers of neutrons are called isotopes of that element. Isotopes
undergo identical chemical reaction.
The unit of nuclear mass is the atomic mass unit, or amu, and is
defined as exactly 1/12th the mass of a atom. The number of
atoms in exactly 12.000 g of is called the Avogadro number
23
and equals 6.0221×10 .
The total nuclear mass of an atom is always slightly less than the
sum of the individual masses of the constituent protons and
neutrons. The difference is called the binding energy of the
nucleus and arises because energy is released when neutrons and
protons combine to form the nucleus. The relationship between
2
binding energy and mass lost is E=mc .
The relative atomic mass of an individual atom is the atomic
mass relative to 1/12th the mass of an atom of carbon-12. The
relative atomic mass of an element is the weighted average of the
relative atomic masses of the naturally occurring isotopes.
One measure of nuclear stability is the binding energy per
nucleon, which increases rapidly with atomic mass number, A,
and reaches a maximum at A≈56. Nuclei with even numbers of
protons and neutrons are more stable than nuclei with odd
numbers of either or both. Nuclei with 2, 8, 20, 28, 50, 82 or 126
protons or neutrons are particularly stable.
Related topics Applications of nuclear Chemical and structural effects of
structure (G2) quantization (G7)