Lecture 19 Unstable Nuclei and Radioactive Decay Ozgur Unal NIS – CHEMISTRY Lecture 19 Unstable Nuclei and Radioactive Decay Ozgur Unal

Radioactivity Is it possible to change the chemical properties of a substance by physical means? Is it possible to change the chemical properties of a substance by chemical means? How can we change the identity of an element? Is it possible to change one element into another? It is possible to turn an element into another using nuclear reactions that involve nuclei of atoms. Nuclear reactions involve a change in an atoms nucleus.

Radioactivity In the late 1890s, scientists noticed that some substances spontaneously emitted radiation in a process they named radioactivity. These substances were called radioactive atoms and emitted rays and particles called radiation. These radioactive atoms undergo nuclear reactions and change their identities. This is very different from chemical reactions where the atoms do not change their identities. Why do radioactive atoms emit radiation?

Radioactivity Radioactive atoms emit radiation because they are unstable. In order to gain stability, atoms need to release some of their energy. This energy is released in the form of light or other particles. Consider the picture below. The ball at the top of the hill is unstable, because with a little push it will fall down. As the ball rolls down the hill the ball loses its potential energy and it becomes stable.

Radioactive Decay Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay. Radioactive decay for a particle continues until the particle becomes stable. Here, particle simply means nucleus. There are three types of radiation in a radioactive decay. Alpha radiation Beta radiation Gamma radiation These 3 radiations have 3 different electric charges. Scientists were able to distinguish these three radiations by investigating the effect of electric fields on radiation.

Alpha Radiation The radiation that was deflected toward negatively charged plate was named alpha radiation. The particles emitted were named alpha particles denoted by α or He. An alpha particle contains two protons and two neutrons only. Therefore, its charge is +2. These particles are heavy and they cannot penetrate deep into an object.

Examples of Alpha Radiation: Note that a new element on the right is created after the nuclear reaction. The type of equation above is called nuclear equation.

Beta Radiation The radiation that was deflected toward the positively charged plate was named beta radiation. This radiation consists of fast moving beta particles. Beta particles are electrons with -1 charge and they are shown by the symbol β or e-.

Examples of Beta Radiation:

Gamma Radiation The third common type of radiation is called gamma radiation or gamma rays. Gamma rays are high-energy radiation that has no mass and charge and is denoted by the symbol γ. Gamma rays ususally accompany alpha and beta radiation and they account for most of the energy lost during radioactive decays.

Gamma Radiation

Nuclear Stability Nuclei gain stability by losing energy. The primary factor in determining an atom’s stability is its ratio of neutrons to protons. Atoms that contain either too many or too few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus.

Alpha, Beta and Gamma Radiations