MPN - Atoms and Radioactive Decay

Atoms and Radioactive Decay 

In chemistry you learned about the structure of atoms and how that structure allowed atoms of different elements to interact. In physics we also look at atomic structure, but focus more on the operations of individual atoms, primarily in the nucleus. Let's first review a few important atomic facts you should remember from chemistry.

  • The nucleus of an atom is made up of two types of nucleons, the proton and the neutron. These particles are similar in size but have different electric charge. The proton has a positive charge while the neutron is electrically neutral.
  • Atoms of different elements are distinguished from each other by the number of protons in the nucleus. This is called the atomic number. The sum of protons and neutrons in an atom is referred to as the atomic mass number. A special symbol is used to represent an element with a specific atomic mass number: , where X is the chemical symbol (from the periodic table), A is the atomic mass number, and Z is the atomic number. On the periodic table atomic mass is measured in atomic mass units (amu). Converting to kilograms, 1 amu = 1.66054 x10-27 kg. If we consider the mass-energy equivalence given by E = mc2, 1 amu = 1.492 x10-10 J. This is a very small amount of energy so another unit of energy is often used when referring to small particles, the electron volt (eV). 1eV=1.602x10-19 J , so 1 amu = 931.49 x106 eV or 931.49 MeV.
  • Changing the number of neutrons in an atom results in an isotope of that element. Changing the number of protons results in an entirely different element (remember, elements are defined by the number of protons in their nuclei).
  • Protons and neutrons are made up of smaller particles called quarks. Quarks, like electrons, are considered fundamental particles because they cannot be broken down into anything more basic.

Nuclear Forces

We know that protons are positively charged and neutrons are neutral. We also know that like electric charges repel. So, how does the nucleus of an atom stay together with multiple protons? The answer is the strong nuclear force. One of the fundamental forces of the universe, the strong force is responsible for holding the nucleons together at the core of the atom. Clearly this force must be stronger than the electromagnetic force, otherwise we have the problem of protons flying apart. The strong nuclear force is only attractive, like the gravitational force. Unlike the gravitational force, the strong nuclear force only acts over incredibly small distances, like the size of a nucleus. For small atoms, A = 30 - 40, the strong nuclear force can sustain atoms with equal numbers of protons and neutrons. Nuclei larger than this require more neutrons than protons in order to maintain stability. Above atomic number 82 (lead), there are no more completely stable nuclei. Unstable nuclei tend to come apart resulting in radioactive decay.

The last of the fundamental forces shows itself in certain types of radioactive decay, like beta decay. It is called the weak nuclear force.

Radioactive Decay

Radioactive decay is the result of unstable nuclei changing their nuclear structure in an attempt to become more stable. It was discovered in the late 1800s that the process of radioactive decay resulted in emission of three different types of radiation.

  • Alpha Decay - The emission of a helium nucleus (alpha particle, ,) during decay from a parent nucleus. Emitting an alpha particle means losing two protons, therefore the parent nucleus undergoes transmutation into another element. For example, the unstable radium-226 nucleus will undergo alpha decay and leave behind the daughter nucleus radon-222 and an particle.. Notice the loss of four from the atomic mass and two from the atomic number. This type of decay is common in very large atoms where the strong nuclear force isnt strong enough to hold the massive nucleus together. Alpha particles are positively charged and not particularly energetic as a sheet of paper can easily stop them.
  • Beta Decay - The emission of an electron (beta particle, β) from the nucleus of an atom. The force allows for the quarks making up a neutron to change such that instead of a neutron we gain a proton, electron, and a tiny particle known as a neutrino. The symbol used to represent a neutrino is the greek letter nu, ν (pronounced noo). This type of decay doesn't depend on the size of the nucleus and is, instead, related entirely to the neutrons in an atom. For this reason you can see beta decay occur in smaller atoms, like carbon. Carbon 14 is a radioactive isotope of the more common carbon 12. Here is what it looks like when carbon 14 undergoes β decay:. Notice that the nucleon number stays the same since a neutron is swapped for a proton. However, this extra proton increases the atomic number by one, leaving us with a stable nitrogen atom. Beta particles are negatively charged and more energetic than alpha particles. Beta particles have been found to be able to pass through as much as 3mm of aluminum.
  • Gamma Decay - The emission of a high-energy photon (gamma ray, γ) when a nucleus falls from an excited state. Nuclei of atoms can occupy excited energy states much like their orbiting electrons. This can be the result of an atomic collision or a leftover from a previous radioactive decay. Gamma rays do not change the nuclear make-up of the atom. Gamma rays are so energetic it can take several centimeters of lead to stop them. Since the gamma rays are emitted very soon after a radioactive decay or other nuclear process (see the last lesson of this unit) they are often associated as a by-product of those decays.

In each of these types of radioactive decay all of the conservation laws you've learned about in physics apply - conservation of energy, linear momentum, angular momentum, and electric charge. A new conservation law is also observed, the law of conservation of nucleon number. This means that the total number of nucleons (protons and neutrons) remains the same in any of these processes, though one may change into another. This video sums up the information above about radioactive decay and gives additional examples of alpha and beta decay.

Half-Life

When you have a sample of a radioactive substance not all nuclei undergo radioactive decay at the same time. Each nucleus decays randomly, however every radioactive substance tends to decay at a set rate. This means we can determine, through probability, how much of the substance will have undergone decay after a given time. The primary way we measure this is through the substances half-life. We're not talking about an awesome, classic video game series (Seriously, try doing a web search for half-life. You'll learn more about the video game than physics on the first page of results).The half-life is the time it takes for half of the substance to undergo radioactive decay. This video gives you a nice, visual explanation of the concept of half-life.

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