Proton to Neutron Ratios and the Stability of Nuclei – Beta Radiation

Adding more neutrons to the nucleus does not change the chemical properties of the atom. A hydrogen atom with one or even two neutrons has the same chemical properties as normal hydrogen. However the number of neutrons does affect the physical properties.

As neutrons are added to the nucleus their energy increases with respect to the protons present. At some stage this become unstable and the neutron converts to a proton.

To do this it must change from becoming neutral to become positive. The way that it does this is to emit an electron. This electron has quite a bit of energy (i.e. they are travelling very fast).

This is beta β radiation.

The ratio of the number of protons to neutrons that form stable nuclei varies from about 1 to1 for light atoms to about 1 to 1.5 for heavier atoms.

neutron proton ratio and stability

Neutron to Proton Ratios and Stability (showing line of stable nuclides and where number of protons = neutrons)

About Beta Radiation

As we have said before electrons have very little mass compared to protons and neutrons. However, they do have an electric charge and therefore interact quite strongly with matter. They can be stopped by a thin sheet of metal or thicker pieces of other materials such as concrete or water.

Since the number of protons in the nucleus has increased it has changed to a different element.

For example tritium which is the isotope of hydrogen with two neutrons and one protons changes to an isotope of helium when it decays via beta decay

Tritium dacay

Note that the atomic mass (3) does not change since this is the sum of the protons and neutrons.

Another example is the decay of Carbon – 14 to Nitrogen

carbon-14 decay

Sometimes there are nuclei with too many protons compared to neutrons. In this case a proton changes to a neutron and emits a positron. This is the anti-particle of the electron (yes anti-matter does exist outside of StarTrek) and has the same mass but a positive charge.

The two type of beta radiation are often called β (Beta minus) and β+ (Beta plus) radiation. Apart from having opposite charges they are very similar.

For example Carbon-11 decays to Boron-11

cardon - 11 decay




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