Protons, neutrons and nuclides

The proton carries one unit of positive charge and is stable. Quantum arguments rule out electrons inside the nucleus; all Z electrons of a neutral atom are outside it. So the nucleus has charge +Ze and contains exactly Z protons.

Deuterium and tritium each have one proton, yet the nuclear masses of hydrogen, deuterium and tritium are in the ratio 1 : 2 : 3. The extra mass must be neutral matter, about one and two proton masses respectively.

In 1932 Chadwick bombarded beryllium with alpha-particles and found a neutral radiation that could knock protons out of helium, carbon and nitrogen. If it were photons, energy and momentum conservation would demand far more energy than the reaction could supply.

Chadwick resolved this by proposing a new neutral particle, the neutron, whose mass he found to be very nearly the proton's. Today mn = 1.00866 u = 1.6749 × 10⁻²⁷ kg (Eq. 13.3). He received the Nobel Prize in Physics in 1935.

Left on its own, a neutron is unstable and turns into a proton, an electron and an antineutrino; its mean life is about 1000 s. Bound in a nucleus, it does not decay.

Z (atomic number) is the number of protons, N (neutron number) the number of neutrons, and A = Z + N (mass number) the total number of nucleons, a nucleon being a proton or a neutron.

A nuclide is written ᴬ_ZX, with X the chemical symbol. Gold, ¹⁹⁷₇₉Au, has 197 nucleons: 79 protons and 118 neutrons.

Isotopes have equal Z but different N: deuterium ²₁H has one proton and one neutron, tritium ³₁H one proton and two neutrons. Gold has 32 isotopes, from A = 173 to A = 204. Their electron structure is the same, so their chemistry is too.

Isobars share the mass number A, for example ³₁H and ³₂He. Isotones share the neutron number N but differ in Z, for example ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au.