Chemistry 101: Nuclear Chemistry

The nuclear force holds protons and neutrons together in the nucleus.

In very large molecules, or ones with an over-abundance of neutrons, the nuclei can become unstable and decay.

An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons (without the electrons).

Alpha decay is a nuclear decay and occurs when a nucleus emits an alpha particle. The daughter nucleus of alpha decay will have an atomic number 2 less and mass number 4 less than the parent nucleus.

23490Th

In single alpha decay, an alpha particle is emitted. To identify the daughter nucleus, therefore, simply subtract 2 from the parent nucleus’ atomic number, and 4 from its mass. Subtracting 2 protons from uranium (92) shifts it to thorium (90).

Alpha decay is typical only in large nuclei (atomic number 60 or greater).

Many of the most famous radioactive elements, such as radium (88), uranium (92), and plutonium (94), decay primarily via alpha decay.

A beta particle is simply an electron, a massless negatively-charged particle.

Beta decay is a nuclear decay and occurs when a nucleus emits a beta particle. The daughter nucleus of beta decay will have an atomic number 1 more* and mass number identical to the parent nucleus.

*When the electron is ejected, total charge must remain constant, so a neutron is converted to a new proton.

147N

In single beta decay, a beta particle (electron) is emitted. To identify the daughter nucleus, therefore, simply add 1 to the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one larger than carbon is nitrogen.

Beta decay is typical in smaller nuclei, when the isotope’s mass number is greater than the element’s atomic weight.

Ex: The carbon 16 isotope, which is heavier than carbon’s atomic weight of 12.011, decays via beta decay.

A positron is an anti-electron, a massless positively-charged particle.

Positron decay is a nuclear decay and occurs when a nucleus emits a positron. The daughter nucleus of positron decay will have an atomic number 1 less* and mass number identical to the parent nucleus.

*When the positron is ejected, total charge must remain constant, so a proton is converted to a new neutron.

115B

In positron decay, a positron is emitted. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one less than carbon is boron.

Electron capture is a nuclear decay and occurs when a nucleus captures one of its own electrons. The electron merges with a proton to form a neutron. The daughter nucleus of electron capture will have an atomic number 1 less and mass number identical to the parent nucleus.

73Li

In electron capture, an electron merges with a proton, forming a neutron. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with atomic number one less than beryllium is lithium.

Both positron emission and electron capture are typical when the isotope’s mass number is less than the element’s atomic weight or when there is a superabundance of protons.

Ex: The carbon 11 isotope, which is lighter than carbon’s atomic weight of 12.011, decays via positron emission.

A gamma ray is a high energy photon, a massless and uncharged particle.

Gamma emission is a nuclear decay and occurs when a nucleus emits a gamma ray. The daughter nucleus of gamma emission is simply lower energy, hence will have identical atomic and mass numbers to the parent nucleus.

A radioactive isotope’s half-life is the time it takes for exactly one-half of the individual atoms in a population of the isotope to undergo radioactive decay.

After one half-life elapses, exactly one half of the original amount of the parent nuclei will still be present; the remainder will have decayed into the daughter nuclei.

One-eighth of the original amount will remain after 3 hours.

For each half-life that elapses, one half of the remaining isotopes will decay. So after one hour, 1/2 the original population remains. During the second hour, 1/2 of the remaining half (1/4 more) decays, leaving behind one quarter of the original population. In the third hour, 1/2 of the remaining quarter (1/8 more) decays, leaving behind one eighth of the original.

6 days

For each half-life that elapses, one half of the remaining isotopes will decay. After 3 days, 60 g of Z will decay, and 60 g will remain. After 3 more days, half of the 60 g will decay, leaving behind 30 g.

An atom’s mass defect is the difference between the sum of the individual proton and neutron masses in the atom’s nucleus compared to the overall mass of the nucleus.

The mass defect exists because some of the neutrons’ and protons’ mass is converted to energy, which is used to hold the nucleus together, so the nucleus weighs less than the protons and neutrons that make it up.

A nucleus’ binding energy is the energy needed to cause the nucleus to decompose into protons and neutrons.

The binding energy is exactly the same magnitude as the mass defect; the energy released when a nucleus is formed is exactly the same as the energy needed to break it apart.

E = mc2

where:
E = the nucleus’ binding energy
m = the nucleus’ mass defect
c = the speed of light, 3x108 m/s

Since the value of c2 is so large, a very small mass defect accounts for a very large binding energy.