Radioactive Substances and Nuclear Reactions

Radioactive Substances and Nuclear Reactions

In ordinary chemical reactions, only the outer electrons of the atoms are disturbed. The nuclei of the atoms are not affected. In nuclear reactions, however, the nuclear changes that occur are independent of the chemical environment of the atom. For example, the nuclear changes in a radioactive 3^ 1H atom are the same if the atom is part of an H*2 molecule or incorporated into H2O. 

Considering the two types of nuclear reactions. One type is radioactive decay:this involves a nucleus spontaneously disintegrating, giving off radiation. The radiation consists of one or more of the following, depending on the nucleus: electrons, nuclear particles (such as neutrons), smaller nuclei (usually helium-4 nuclei), and electromagnetic radiation.

The second type of nuclear reaction is a nuclear bombardment reaction, this is typically a nuclear reaction in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle. If there is sufficient energy in this collision, the nuclear particles of the reactants rearrange to give a product nucleus or nuclei. First, in this lecture article on radioactive substances and nuclear reactions, we will look at radioactive decay.

What is Radioactivity?

The phenomenon of radioactivity was discovered by Antoine Henri Becquerel in 1896. He discovered  that photographic plates develop bright spots when exposed to uranium minerals, and he concluded that the minerals give off some sort of radiation.

Radioactivity is simply the spontaneous disintegration of  a radio-active nuclei resulting in radiactive particles

The radiation from uranium minerals was later shown to be separable by electric (and magnetic) fields into three types, alpha (α ), beta ( β), and gamma (ϓ ) rays. Alpha rays bend away from a positive plate and toward a negative plate, indicating that they have a positive charge; they are now known to consist of helium-4 nuclei (nuclei with two protons and two neutrons). Beta rays bend in the opposite direction, indicating that they have a negative charge; they are now known to consist of high-speed electrons. Gamma rays are unaffected by electric and magnetic fields: they have been shown to be a form of electromagnetic radiation that is similar to x rays, except they are higher in energy with shorter wavelengths (about 1 pm, or 1 10 12 m). Uranium minerals contain a number of radioactive elements, each emitting one or more of these radiations. Uranium-238, the main uranium isotope in uranium minerals, emits alpha rays and thereby decays, or disintegrates, to thorium-234 nuclei.

A sample of uranium-238 decays, or disintegrates, spontaneously over a period
of billions of years. After about 30 billion years, the sample would be nearly
gone. Strontium-90, formed by nuclear reactions that occur in nuclear weapons testing
and nuclear power reactors, decays more rapidly.

 A sample of strontium-90 would be nearly gone after several hundred years. In either case, it is impossible to know when a particular nucleus will decay, although, as you will see in Section 20.4, precise information can be given about the rate of decay of any radioactive sample.

Nuclear  Reactions/Equations

You can write an equation for the nuclear reaction corresponding to the decay of uranium-238 much as you would write an equation for a chemical reaction. You represent the uranium-238 nucleus by the nuclide symbol 238-92U. < The radioactive decay of 238-92U by alpha-particle emission (loss of a 4-2He nucleus) is written

Radioactive Substances and Nuclear Reactions

The product, in addition to helium-4, is thorium-234. This is an example of a nuclear equation, which is a symbolic representation of a nuclear reaction. Normally, only the nuclei are represented. It is not necessary to indicate the chemical compound or the electron charges for any ions involved, because the chemical environment has no effect on nuclear processes.

The decay of a nucleus with the emission of an electron, 0-1e ( i.e the 0 is up( molar mass and the -1 is down, as in atomic number), is usually called beta emission, and the emitted electron is sometimes labeled 0-1β . A positron is a particle similar to an electron, having the same mass but a positive charge. A gamma photon is a particle of electromagnetic radiation of short wavelength (about 1 pm, or 10 ^-12 m) and high energy.

Nuclear Stability

At first glance, the existence of several protons in the small space of a nucleus is puzzling.

Why wouldn’t the protons be strongly repelled by their like electric charges? The existence of stable nuclei with more than one proton is due to the nuclear force. The nuclear force is a strong force of attraction between nucleons that acts only at very short distances (about 10 15 m). Beyond nuclear distances, these nuclear forces become negligible. 

Therefore, two protons that are much farther apart than 10 15 m repel one another by their like electric charges. Inside the nucleus, however, two protons are close enough together for the nuclear force between them to be effective. This force in a nucleus can more than compensate for the repulsion of electric charges and thereby give a stable nucleus.

The protons and neutrons in a nucleus appear to have energy levels much as the electrons in an atom have energy levels. The shell model of the nucleus is a nuclear model in which protons and neutrons exist in levels, or shells, analogous to the shell structure that exists for electrons in an atom. Recall that in an atom, filled shells of electrons are associated with the special stability of the noble gases. The total numbers of electrons for these stable atoms are 2 (for He), 10 (for Ne), 18 (for Ar), and so forth.

Experimentally, note that nuclei with certain numbers of protons or neutrons appear to be very stable. These numbers, called magic numbers and associated with specially stable nuclei, were later explained by the shell model. According to this theory, a magic number is the number of nuclear particles in a completed shell of protons or neutrons.

Because nuclear forces differ from electrical forces, these numbers are not the same as those for electrons in atoms. For protons, the magic numbers are 2, 8, 20, 28, 50, and 82.
Neutrons have these same magic numbers, as well as the magic number 126. For protons,n calculations show that 114 should also be a magic number.

Recommended lectures on Radioactivity-  Types of Radioactive Decay