Atomic number is the number of protons, the number of protons determines the type of atom. All carbon atom have 6 protons, all nitrogen atoms have 7 protons. If the number of protons changes, the element changes. Mass number is the total mass of the atom, the number of protons and the number of neutrons are added. Elements can have different numbers of neutrons, if the number of neutrons is different then the atoms are isotopes. For example: 14C has 6 protons and 8 neutrons,13C has 6 protons and 7 neutrons,12C has 6 protons and 6 neutrons. 14C, 13C, and 14C are all isotopes of carbon.

Radioactivity is the spontaneous emission of radiation from unstable nuclei. In general, isotopes with more neutrons than protons tend to be unstable and will emit radioactivity.

Types of radiation:

Examples of nuclear reactions: Emission of an alpha particle:

For emission of a beta particle, a neutron is converted into a proton and an electron and the electron is emitted:

Emission of gamma rays do not change the element, or isotope, as gamma rays to not have mass or charge.

Class activities:

Radioactivity uses in medicine

Gamma rays are used for killing tumor cells. The gamma rays destroy DNA (the genetic information for the cells) and the cells can no longer divide and grow wildly.

For cancer information, and other fun web stuff, click here.

Radioactive compounds decay, and lose radioactivity.

This decay is exponential, and the life time of a radioactive compound is measured in "half-lives". A half life is the time it takes for a radioisotope to decay to one half of its original radioactivity. Click here to see a decay curve for the decay curve of 99mTc. Click here to see the decay curve for Tritium 3H. Note that after 5 half lives, the radioactivity is essentially gone.

Radioisotopes are also used for diagostic purposes.

Positron Emission Tomography

Three dimensional images are generated by computers by detecting positrons. A positron has the same mass as an electron, but has a positive charge. It is emitted from a nucleus, and then when it collides with a nearby electron, a great deal of energy is released, in the form of gamma rays (remember: gamma rays do not have mass.) This is an example of how mass is converted into energy.

The high energy gamma rays are then detected by a computer and an image is generated. This technique is useful for generating brain images and for studying dynamic processes such as blood flow.

Carbon 14 dating:

14C is continually produced in the atomosphere and it is incorporated into all living creatures. The amount of 14C in a living creature is constant. When the animal dies, the amount of 14C starts to decrease, because no new 14C from the atmosphere is being incorporated. By counting the remaining radioactivity, it is possible to tell how old an item is. The half life of 14C is 5730 years. This method is good for dating objects that are between 500 and 50,000 years old. Can you explain this range using the decay curve shown here.

Some sample questions:

1. Which one of the following is most penetrating?
a) alpha particle
b) beta particle
c) gamma ray
d) visible light

2. Which of the following particles has a mass of 4 amu and a charge of 2+?
a) alpha particle
b) electron
c) neutron
d) proton

3. Which type of radioactive emission is very similar to xrays?
a) transmutation
b) alpha particle
c) beta particle
d) gamma ray

4. After three half-lifes, what fraction of the original radioactive isotope remains in a sample?
a) none
b) 1/16
c) 1/8
d) 1/4

5. Radioactive alpha emitters pose the greatest potential health risk when they are
a) unshielded
b) on the skin
c) injested
d) all of the above

6. Which one of the following radioactive isotopes is useful in the investigation of thyroid problems?
a) Uranium 235
b) Barium 120
c) Iodine 131
d) Cesium 145

7. Positron emission tomography (PET) is a
a) therapy for cancer using positrons
b) diagnostic technique for monitoring dynamic processes in the body, such as brain activity
c) device for containing a nuclear fusion reaction
d) mechanism for transmutation of elements

8. Technitium 99m is a radioisotope used in a variety of diagnostic tests. It has a short half life (6 hr). The advantage of a short half-life for diagnostic purposes is
a) the radioactivity is easier to monitor
b) the radioactivity does not linger in the body
c) the radioactivity lasts for a long time
d) the chemical reactions induced by the technitium are more rapid

9. In carbon14 dating
a) the radioactivity of carbon is artificially induced
b) the radioactivity of carbon entered the sample when the sample was part of a living being
c) radioactive carbon is added to the sample to be dated
d) none of the above

10. A piece of fossilized wood has a carbon14 radioactivity that is 1/4 that of new wood. The halflife of carbon14 is 5730 years. How old is the fossilized wood?
a) 1 x 5730 = 5730 years
b) 2 x 5730 = 11,460 years
c) 3 x 5730 = 17,190 years
d) 0.25 x 5730 = 1432 years

If you have enjoyed today's Web experience, please send me an e-mail message by clicking here to send me a message at

Back to "Environmental Chemistry"
Back to "The Chemicals in Your Life"

Copyright 1995 Koni Stone