1.1 What is Chemistry?

24.1. Nuclear Radiation http://www.ck12.org

1 amu and electrons are much smaller, the mass number is approximately equal to the mass of the atom in units of
amu.

Isotopes are atoms that have the same atomic number but different atomic weights. The difference in weight is due
to variations in the number of neutrons within the nucleus. Because they are uncharged, a slight difference in the
number of neutrons has very little effect on the chemical properties of a given atom. We can designate the mass
number of an isotope (and sometimes the atomic number as well) using numbers to the left of the chemical symbol.
The top number is the mass number, and the bottom number (if any) is the atomic number. For example, one isotope
of krypton is the following:

84

36 Kr

Because an atomic number of 36 is implied by the fact that this is an atom of krypton, we can also write this as just

(^84) Kr. Other ways to designate this isotope are krypton-84 or Kr-84. The number of neutrons in the nucleus of this
isotope can be determined by subtracting the atomic number (36) from the mass number (84), giving a value of 48
neutrons.
Remember that the atomic weights listed on the modern periodic table are weighted averages of all naturally
occurring isotopes. For example, the atomic weight of Cl is listed as 35.453, even though none of the isotopes
of chlorine actually have that atomic weight. Chlorine has many possible isotopes ranging from^28 Cl to^51 Cl, but
essentially all naturally occurring chlorine atoms are either^35 Cl or^37 Cl. Based on the ~3:1 ratio in which these
isotopes occur, the average atomic weight of chlorine is 35.453 amu. Even though no individual chlorine atom has
a mass of 35.453 amu, a mole of naturally occurring chlorine atoms will have a mass of 35.453 grams.
Discovery of Radioactivity
In 1896, a researcher named Henri Becquerel was studying the phosphorescent properties of uranium salts. He
believed these properties were related to X-rays, which had just been discovered the previous year by Wilhelm
Röntgen. However, he soon found that certain properties of the radiation emitted from uranium did not match those
of X-rays. For example, uranium could expose a photographic plate without any external input of energy.

FIGURE 24.1

By placing a metal cross between the plate and the radioactive uranium

salt, a shadow is left on the plate due to the effects of exposure to

radioactivity.

Pierre and Marie Curie further studied the properties of uranium salts in order to understand the details of this newly
discoveredradioactivity. The major contributions to their work came from Marie, who showed that the amount of
radioactivity present was due to the amount of a specific element in the sample and was not the result of a chemical
reaction. She discovered the element polonium and named it after her native Poland. Madame Curie shared the 1903
Nobel Prize in Physics with her husband Pierre along with Henri Becquerel. She won the Nobel Prize again in 1911
for her work in Chemistry.

Later on, Ernest Rutherford (whose name you may remember from the Gold Foil Experiment) showed that there
are three distinct types of radioactive emissions that differ in their mass, their charge, and their ability to penetrate