11.1. Radioactivity http://www.ck12.org
FIGURE 11.1
X-rays are a form of energy that can pass through skin and muscle but not
bone. That’s why bones show up clearly in an X-ray, while the rest of the
body is barely visible.Becquerel was a good scientist, so he wanted to repeat his experiment to confirm his results. He placed more uranium
on another photographic plate. However, the day had turned cloudy, so he tucked the plate and uranium in a drawer
to try again another day. He wasn’t expecting the uranium to leave an image on the plate without being exposed to
sunlight. To his surprise, there was an image on the plate in the drawer the next day. Becquerel had discovered that
uranium gives off rays without getting energy from light. He had discovered radioactivity, for which he received a
Nobel prize. To learn more about the importance of Becquerel’s research, go to this URL: http://nobelprize.org/no
bel_prizes/physics/laureates/1903/becquerel-bio.html.
Another scientist, who worked with Becquerel, actually came up with the term "radioactivity." The other scientist
was the French chemist Marie Curie. She went on to discover the radioactive elements polonium and radium. She
won two Nobel Prizes for her discoveries. You can learn more about Marie Curie at this URL: http://nobelprize.or
g/nobel_prizes/physics/laureates/1903/marie-curie-bio.html.
Radioisotopes
Isotopes are atoms of the same element that differ from each other because they have different numbers of neutrons.
Many elements have one or more isotopes that are radioactive. Radioactive isotopes are calledradioisotopes.
An example of a radioisotope is carbon-14. All carbon atoms have 6 protons, and most have 6 neutrons. These
carbon atoms are called carbon-12, where 12 is the mass number (6 protons + 6 neutrons). A tiny percentage of
carbon atoms have 8 neutrons instead of the usual 6. These atoms are called carbon-14 (6 protons + 8 neutrons). The
nuclei of carbon-14 are unstable because they have too many neutrons. To be stable, a small nucleus like carbon,
with just 6 protons, must have a 1:1 ratio of protons to neutrons. In other words, it must have the same number of
neutrons as protons. In a large nucleus, with many protons, the ratio must be 2:1 or even 3:1 protons to neutrons.
In elements with more than 83 protons, all the isotopes are radioactive (seeFigure11.2). The force of repulsion
among all those protons overcomes the strong force holding them together. This makes the nuclei unstable and
radioactive. Elements with more than 92 protons have such unstable nuclei that these elements do not even exist in
nature. They exist only if they are created in a lab.
Radiation in the World Around Us
A low level of radiation occurs naturally in the environment. This is called background radiation. It comes from
various sources. One source is rocks, which may contain small amounts of radioactive elements such as uranium.
Another source is cosmic rays. These are charged particles that arrive on Earth from outer space. Background
radiation is generally considered to be safe for living things.
A source of radiation that may be more dangerous is radon. Radon is a radioactive gas that forms in rocks
underground. It can seep into basements and get trapped inside buildings. Then it may build up and become harmful