IB Chemistry - Atomic Theory

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Syllabus ref: 2.1

The smallest building blocks of matter are the atoms. There are about 100 naturally ocurring types of atom. This chapter explains the ways in which atoms differ from one another.


Isotope definition

Isotopes are atoms of a specific element that have a definite number of neutrons and consequently a different mass. In effect all atoms are isotopes of one element or another.

Most elements have several isotopes, some of which are stable, and others that spontaneously break apart releasing radioactivity.

For example, the element hydrogen has three isotopes, 1H, 2H and 3H. 1H is the most common of the isotopes and makes up 99.99% of any sample of hydrogen. 2H is also called deuterium and comprises the other 0.01% of naturally occurring hydrogen. The third isotope is called tritium and is not very common.

Tritium is radioactive and breaks apart spontaneously releasing radioactivity, in this case, a fast moving electron.

Note that the product of this radioactive process is helium. Effectively, one of the neutrons in the tritium nucleus has emitted an electron (called a beta particle) and turned into a proton.

Summary of the hydrogen isotopes

deuterium, heavy hydrogen
tritium, super-heavy hydrogen


Relative abundance

The 'relative abundance' of an isotope means the percentage of that particular isotope that occurs in nature. Most elements are made up of a mixture of isotopes. Clearly the sum of the percentages of the specific isotopes must add up to 100%.

Example: Chlorine has two isotopes 35Cl and 37Cl, with relative abundance of 75% and 25% respectively.

This means that in any naturally occurring sample of chlorine 75% of the atoms are Cl-35 atoms and 25% of the atoms are chlorine-37 atoms.


Properties of isotopes

Isotopes differ only in their number of neutrons. This means that they have identical electronic configurations and identical chemical properties. The masses of the isotopes affects any characteristic that depends on mobility or mass of the particles.

Density is defined as mass/volume therefore isotopes have different densities.

Diffusion is a process that is dependent on the density of the diffusing species. According to Graham's law, the rate of diffusion is proportional to the square root of the reciprocal density and hence the reciprocal relative mass (of a gas):

Example: How does the rate of diffusion of hydrogen differ from the rate of diffusion of deuterium?

  • Hydrogen, H2, relative mass = 2
  • Deuterium, D2, relative mass = 4

Applying Graham's law, relative rates of diffusion = 1/√2 : 1/√4

Therefore 1/1.414 : 1/2 = 0.71 to 0.5

Hydrogen diffuses 1.4 x faster than deuterium

This principle is put to use in the purification of uranium 235 for use in the atomic energy industry. The uranium occurs naturally as two isotopes 235U and 238U with relative abundances of approximately 0.28% to 99.71% with the remainder due to other isotopes.

Only the uranium 235 can be used for atomic energy and so needs to be concentrated in the sample. This is done by reacting the uranium with hydrogen fluoride to form uranium hexafluoride UF6, which is a volatile solid that can be converted to a gas at easily attainable temperatures.

Once in the gaseous form, use is made of the different diffusion rates of the two compounds. The relative masses of the two hexafluorides are so similar, the gases must be diffused using a series of centrifuges, each one increasing the percentage of the required uranium isotope in the mixture.

The uranium hexafluoride is then turned into uranium dioxide for use in fuel pellets.


Uses of isotopes

Isotopes are used in medicine, industry, and in many other applications. The danger of radioisotopes revolves around their ability to cause cell damage by ionising the atoms that are involved in molecules and hence, breaking bonds. Radioisotopes may emit three different common types of radiation, alpha, beta and gamma radiation, depending on the specific atom.

14C - radiocarbon dating

Living organisms respire. Plants breathe in carbon dioxide and animals eat plants (and other animals!). The consequence is that all living things take up carbon throughout their lives. The percentage of the isotope carbon 14 remains fairly constant in our atmosphere, as it is produced in the upper atmosphere by cosmic bombardment of naturally occurring carbon-12 in the form of carbon dioxide. At the same time the carbon 14 nuclei are decaying. There is an equilibrium between these two processes:

carbon-12 carbon-14

This means that the proportion of carbon-14 compared to carbon-12 found in all living organisms is also constant. However, when a living organism dies it stops taking up both forms of carbon. The carbon -14 isotope decays naturally with a half life of about 5,600 years. So, a simple procedure involving counting the radioemissions due to carbon-14 from a sample of material that was once alive, can be used to estimate how long ago it died.

Therapeutic applications

Cobalt-60 is used in hospitals as a beta emission source in the treatment of cancer

Beta rays are fast moving electrons. They can be focussed onto cancerous tissue to destroy it using a cobalt 60 source. This form of treatment is known as radiotherapy.

Iodine-131 and Iodine-125 are used as medical tracers and for treating certain cancers.

In several conditions the body can be scanned for problems using iodine, which is easily taken up by the body and transported through the lymphatic system. The isotopes 131I and 125I are easy to detect and short lived in the body.

Use is made of the destructive effect on cellular tissue to destroy cancer cells in treatment with radioisotopes. Radioactive sources are used that have a short lifetime in the body, but which can be focussed in their effects on tissues.

Technetium-99, for example is used in gammagraphy, a technique where a sample of the radioisotope is injected into the body. After a few hours the technetium circulates around the body and binds to areas of bone damage. By detection of areas of unusually high concentration of radiation, it is possible to identify bone injuries that do not show up on X-rays.

The bone scans of radiation emissions are called gammagrams.

Industrial applications

Detection of leaks in gas pipes by injection of a radioisotope into the pipeline and detecting where the radiation emerges. Beta emitters are used in measurement of thickness in the paper industry.

Nuclear energy generation

Both uranium-235 and plutonium-239 are neutron emitting radioactive isotopes. The neutrons emitted cause further events in neighbouring nuclei leading to chain reactions, which release large amounts of energy as the nuclei break apart (fission). This energy is used to heat up water into steam to drive turbines for electricity production.

Nuclear energy remains controvertial and there are strong arguments both for and against its use.

Other applications

Americium-241 is a man-made isotope used in smoke detectors.