Nuclear Structure

Contents

Structure of atoms

Atoms are the smallest units of matter that retain the chemical properties of an element, and elements are lots of atoms of the same type.

The planetary structure of the atom has the nucleus as the sun and electrons like planets.

In the planetary model of the atom, which is based on the solar system, the nucleus is at the centre like the sun, with the electrons (e−) orbiting like planets. This model is conceptually useful, but somewhat lacking, as we will see later.

Inside the nucleus, we find two principal subatomic particles, together termed nucleons. These are the protons (p+) and neutrons (no). Protons are positively charged, neutrons have no electric charge, and both have similar masses (approximately 1 atomic mass unit). The number of protons equals the number of electrons in a neutral atom, but the neutron number is variable and gives rise to the various isotopes of an element. Isotopes are labelled with the atomic mass at top left of the symbol and the atomic number at bottom left.

12

C

6

The atomic number (Z) of an atom is the number of protons in its nucleus, or equivalently the charge (in units of e, the electron charge) on its nucleus. The atomic mass (A) is the number of nucleons in total (neutrons + protons). Since X (the chemical symbol of the atom) implies the atomic number (X = "C" implies that Z = 6), it is often omitted, hence you'll usually see 12C representing the isotope carbon-12. Isotopes have the same atomic number, but different atomic masses. For example, carbon's three isotopes are 12C (the vast majority), 13C (which is useful in NMR, nuclear magnetic resonance) and 14C (used in radiocarbon dating).

Electrons are negatively charged (having an equal but opposite charge to protons) and are tiny, weighing only about 1 ⁄ 1850 as much as a proton. Consequently, they can be largely ignored for the purposes of mass and molar calculations. However, all the chemistry of an atom is determined by its electrons.

The nucleus attracts electrons until the overall charge on the atom is zero, and in a neutral atom, the number of electrons therefore equals the number of protons in the nucleus. However, ions are charged atoms that have lost or gained electrons. The process of gaining electrons is called reduction, and that of losing electrons, oxidation (OILRIG: oxidation is loss, reduction is gain).

Nuclear physics and radioactivity

A nucleus is not a solid, random ball of neutrons and protons. Like the electrons, neutrons and protons reside in orbitals, each having a different energy level. Think of two stacks of bricks. One represents the neutrons piled up in a tower, with the energy required to keep a neutron on the top of the pile getting bigger as you go up it. The second pile is the protons, whose electric charge causes them to repel each other. Consequently, they are stacked on top of a few centimetres of thin air, representing the energy required to squash such positively charged particles together as closely as they are in the nucleus.

Protons are positively charged, so how on earth do you crush so many particles of the same charge into such a tiny space? The answer is the strong interaction. There are four fundamental physical interactions (forces): gravity, electromagnetism, and the strong and weak forces. The strong interaction is very much more powerful than the electromagnetic one that repels like charges, and it is this that provides the glue that keeps the protons (and neutrons) together.

Radioactivity is what happens when a nucleus becomes unstable. How does a nucleus become unstable though? Strange as it may seem, neutrons themselves are inherently unstable: an isolated neutron will fall apart in about 11 minutes if isolated from a nucleus. The forth force of nature, the weak interaction, has a penchant for neutrons, destroying them, and using the energy to create an electron, a proton, and an anti-electron-neutrino. It can do this because the mass of a neutron is slightly higher than that of the three things it decays into. However, when you create a nucleus by squashing together nucleons, energy is released and therefore by E = m c2 mass is destroyed (the mass deficit mentioned earlier). This means the neutrons no longer have enough energy to create a proton, electron and antineutrino from.

However, if the pile of neutrons is much higher than the pile of protons, the neutron will have sufficient energy to decay, because it can decay into a bound proton, which itself has a reduced mass. So although a bound neutron cannot decay into a free proton, under some circumstances, it can decay into a bound proton. The decay of neutron-rich nuclei is called beta decay: the beta particle is a very high energy electron expelled from the nucleus. In excessively proton-rich nuclei, a similar process occurs, whereby a proton decays by the weak force into a neutron, position (antielectron) and electron-neutrino.

Nuclei can also decay in two other ways.

Gamma decay is simply the loss of energy as a photon, and is quite akin to loss of energy by heat, only on a rather more dangerous scale. It's not very exciting.

Alpha decay on the other hand involves the ejection of a 4He2+ (helium nucleus) from very large nuclei. Again the way to understand this is to think of the energy of the nucleus. The amount of energy released by binding two lumps of nucleons together depends on the size of the lumps. For small lumps, like deuterium (2H), the energy released is very large. This is why nuclear fusion of small nuclei to form bigger nuclei generates energy. However, the strong force that binds the nucleons together is also very short-ranged, and for very large lumps of nucleons, the protons on either side of the nucleus are only slightly attracted by the strong force, and their electromagnetic repulsion starts to win against their strong-force attraction. Very large nuclei are therefore unstable. If you were to try and fuse two large nuclei together, the energy you'd need to overcome the repulsion of the protons would be less than the energy released when the strong force finally managed to bind them together. Hence this is a non-starter for energy generation: for small nuclei, fusion releases more energy than you put in, but for large nuclei, fission into two smaller nuclei is energetically favoured. This is the source of alpha radiation: very large nuclei will lose a small chunk of their nucleons, and in-so-doing will become more energetically stable. It is somewhat more complicated than this, as alpha particles shouldn't be able to leave the nucleus, because there is too much of an energy barrier (think of it as a river that would like to flow to the sea, but finds a very big mountain in the way). Because of the bizarreness of quantum physics, and Heisenburg's uncertainty principle in particular, it can 'borrow' the energy required for a tiny amount of time, and 'quantum tunnels' its way out.

The cross over between fusion or fission being the energetically useful option occurs at 56Fe, iron, a fact that stars learn to their cost. Stars fuse hydrogen to helium, helium to lithium, then to carbon, oxygen, and eventually to iron. When burning hydrogen to helium, a lot of energy is released, and the star burns brightly. Later, when it resorts to burning heavier elements to yet heavier elements, the energy released is less, and the star cools and swells to become a red giant. At first, it doesn't notice the energy drain that occurs when it finally resorts to trying to fuse iron. However, this blissful ignorance doesn't last very long, and the star rapidly collapses, crushing its core to a superdense white dwarf, pulsar, or even black hole, and losing its outer layers as a nebula, or more spectacularly, as a supernova.

Test yourself

  1. Hydrogen exists as three isotopes, with 0, 1 and 2 neutrons. Deuterium is used as a nuclear fusion fuel and in H-bombs, and tritium is a radioactive isotope useful in biochemical tracing. Fill in the following table to describe their nuclei.

    Name

    Neutrons

    Atomic number (Z)

    Atomic mass (A)

    Symbol

    Protium (H)

    0

    1

    1

    1H

    Deuterium (D)

    1

     

     

     

    Tritium (T)

    2

     

     

     
  2. Why does 238U decay by α emission, whereas 14C decays by β emission?.

Answers

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