NucleiAtomic Masses and Composition of Nucleus
Atoms are far too light to weigh in kilograms comfortably: one atom of carbon-12 has a mass of 1.992647 × 10⁻²⁶ kg. So atomic masses are measured in a unit of their own, the atomic mass unit (u), defined as one-twelfth of the mass of one carbon-12 atom:
Work it out: 1 u =
More precisely, 1 u = 1.660539 × 10⁻²⁷ kg. On this scale, the atomic masses of most elements come out close to whole-number multiples of the mass of a hydrogen atom, but there are striking exceptions. Chlorine, for example, has an atomic mass of 35.46 u. Why not 35 or 36?
Isotopes on a See-saw
Atomic masses are measured very accurately with a mass spectrometer. It reveals that one element can have atoms of different masses, with exactly the same chemical properties. Such atoms of one element that differ in mass are called isotopes (Greek for "same place": they sit in the same place in the periodic table). Practically every element is a mixture of several isotopes, and the mixture differs from element to element.
Chlorine has two isotopes, of masses 34.98 u and 36.98 u, each close to a whole number of hydrogen masses. The atomic mass in the periodic table is their average, weighted by how common each isotope is. In the mass spectrum below each bar is one isotope, with its height showing its abundance; the average mass is the point where the bars would balance.
Set the abundances so that each element balances at its periodic-table value.
Abundance of the lighter isotope:
The purple wedge marks the weighted average, and it lands on the dashed amber line of the periodic-table value only for the natural mixture. For chlorine, 75.4 % of the atoms are chlorine-35 and 24.6 % chlorine-37:
which agrees with the measured atomic mass of chlorine.
The Lightest Element
Even hydrogen has three isotopes, with masses 1.0078 u, 2.0141 u and 3.0160 u. The nucleus of the lightest one, which makes up 99.985 % of natural hydrogen, is the proton:
That is the mass of the hydrogen atom, 1.00783 u, minus the mass of its one electron, mₑ = 0.00055 u. The other two isotopes are deuterium and tritium. Tritium nuclei are unstable; they do not occur naturally and are made in laboratories.
A proton carries one unit of the fundamental charge, +e, and is stable. It was once thought that the nucleus might also contain electrons, but quantum theory ruled that out: all the electrons of an atom are outside its nucleus. An atom of atomic number Z has Z electrons, of total charge −Ze. The atom is neutral, so what is the charge of a uranium nucleus (Z = 92)?
The charge of a nucleus is +Ze, so it contains exactly Z protons: the atomic number counts the protons in the nucleus.
Discovery of Neutron
Deuterium and tritium are isotopes of hydrogen, so each of their nuclei has just one proton. Yet the masses of the hydrogen, deuterium and tritium nuclei are in the ratio 1 : 2 : 3. The heavier two must also hold some neutral matter: about one proton's mass of it in deuterium and two in tritium. Nuclei seemed to contain, besides protons, neutral matter in multiples of a basic unit.
In 1932 James Chadwick proved it. Put the steps of his argument in order, first step first.
The only neutral radiation known at the time was the photon, but conservation of energy and momentum showed that photons could not do it. Chadwick's new particle, the neutron, has a mass now known very precisely:
Chadwick received the 1935 Nobel Prize in Physics for the discovery. Unlike a free proton, a free neutron is unstable: it decays into a proton, an electron and an antineutrino, with a mean life of about 1000 s. Inside a nucleus it is stable.
Z, N and A
The make-up of a nucleus is described by three numbers:
- Z, the atomic number = the number of protons [13.4(a)]
- N, the neutron number = the number of neutrons [13.4(b)]
- A = Z + N, the mass number = the total number of protons and neutrons [13.4(c)]
A proton or a neutron is called a nucleon, so A is the number of nucleons. A nuclear species, or nuclide, is written , where X is the chemical symbol. The nucleus of gold, , has 197 nucleons, of which 79 are protons and
Now the isotopes make sense: the nuclei of the isotopes of an element have the same number of protons but different numbers of neutrons. Deuterium, , has one proton and one neutron; tritium, , has one proton and two neutrons. Gold has 32 isotopes, from A = 173 to A = 204. The atoms of isotopes have the same electrons, so they behave the same way chemically and share one place in the periodic table.
Build a Nucleus
Two more families of nuclides have names. Nuclides with the same mass number A are isobars, such as and . Nuclides with the same neutron number N but different Z are isotones, such as and .
On the chart below every square is one nucleus that has been observed: protons Z across, neutrons N up. Teal squares are stable; outlined ones exist but decay. The thick outline is nitrogen-14. Add protons and neutrons (or tap a square) to build three relatives of nitrogen-14.
On a chart of nuclides the families are lines. The blue column holds the isotopes of nitrogen (same Z), the green row its isotones (same N), and the amber diagonal its isobars, where Z + N = 14 stays the same.
Speed Round: Isotopes, Isobars, Isotones
Same Z: isotopes. Same A: isobars. Same N: isotones. Get five right to move on.