Stable and unstable nuclei

The chemical properties of any element are governed by the number of protons, the proton number, which is given the code Z.  The stability of the nucleus depends on a combination of the proton number and the neutron number.  A nuclide is any isotope of any element. 

Remember that the nucleus is very tiny compared to the size of an atom; its diameter is about 10^-14 m as opposed to 10^-10 m for an atom.  If the atom were the size of the school canteen, the nucleus would be the the size of a pea (or sweet-corn) dropped in the middle.

In this tiny space there are lots of protons, all positively charged.  Why does the nucleus not fly apart?  The answer is provided by the strong nuclear force which holds the nucleons together.  Its attractive force balances the repulsive forces of the protons.

At very short ranges, below 0.5 femtometres (0.5 × 10^-15 m) the strong nuclear force is repulsive.  It is attractive up to its maximum range of 3 fm (3 × 10^-15 m).  If we plot a graph of force against distance, we would see:

The repulsive force between the two protons is in the order of 200 N (yes, 200 N).  So it is a very strong force.

Possible modes of decay for unstable nuclei

Alpha radiation (a) mostly comes from heavy nuclides with proton numbers greater than 82, but smaller nuclides with too few neutrons can also be alpha emitters.  The term Q stands for the energy.  The animation shows the idea:

The general decay equation is summarised below.

  We should note the following:

• The alpha particle is a helium nucleus (NOT atom).

• Energy is released in the decay. The energy is kinetic, with the majority going to the alpha particle and a little going to the decayed nucleus. 

• The velocity of the alpha particle is much greater than that of the nucleus.

• The nucleon number goes down by 4, the proton number by 2.

• A typical alpha decay is:

 

Question 1

Is this equation balanced?  Explain your answer.

ANSWER

Alpha particles are intensely ionising.  They smash through air molecules, knocking off electrons as they go.  However this reduces the kinetic energy, so that in the end they stop.  Then they pick up a couple of free electrons to become helium atoms.  To collect an appreciable sample of helium from an alpha emitter would take a very long time.

Neutron rich nuclei tend to decay by beta minus (b-) emission.  The beta particle is a high-speed electron ejected from the nucleus, NOT the electron clouds.  It is formed by the decay of neutrons, which are slightly more energetic than a proton.  Isolated protons are stable; isolated neutrons last about 10 minutes.

The neutron, having emitted an electron, is converted to a proton, and this results in the proton number of the nuclide going up by 1.  A new element is formed.  The reaction at the nucleon level is:

Notice that as well as the neutron (n) and the proton (p), the beta particle is represented as an electron (e).  The strange symbol n_e (‘nu-bar e’) is a strange little particle called an electron antineutrino.  The general equation for b- decay is:

A typical decay is:

Notice that:

• The nucleon number remains the same ;

• The proton number goes up by 1.

• The beta particle is created at the instant of the decay.

• The antineutrino is very highly penetrating and has a tiny mass.  It is very hard to detect.

• A precise amount of energy is released, according to the nuclide.

• That energy is shared among the nucleus, the electron and the antineutrino. 

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