Unstable Nuclei and Radioactive Decay Section 4.4 Unstable Nuclei and Radioactive Decay

Objectives Explain the relationship between unstable nuclei and radioactive decay. Characterize alpha, beta, and gamma radiation in terms of mass and charge.

Radioactivity Chemical Reactions Atoms of one element do not change into atoms of another element during a chemical reaction. Why?

Radioactivity Chemical reactions involve the rearrangement of atoms – not their “destruction.” Chemical reactions involve only an atom’s electrons – the nucleus remains unchanged.

Radioactivity Nuclear Reactions These are reactions that involve an atom of one element changing into an atom of another element. These reactions involve the change of an atom’s nucleus.

Radioactivity In the late 1890’s, radioactivity was discovered. Radioactivity is a process in which some substances spontaneously emit radiation. Radiation is the name for the energy (“rays”) and particles emitted by radioactive material. Scientists discovered that by emitting radiation, atoms of one element could change into atoms of another element.

Radioactivity Why does radioactivity occur? Radioactive atoms emit radiation because their nuclei are unstable*. Unstable systems gain stability by losing energy. Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay. Radioactive decay continues until stable, non-radioactive atoms form. They are often of a different element.

Types of Radiation Research, begun in the late 1880’s, used electrically charged plates. Scientists were able to identify 3 types of radiation.

Types of Radiation Alpha Radiation Deflected to negatively charged plates. Made up of alpha particles: 2 protons and 2 neutrons with a +2 charge. Symbol for an alpha particle is 42α or 42He (the helium nucleus) Example of alpha decay

Alpha Radiation Note what happens to the atomic number of an element upon emission of an alpha particle. The atom giving up the alpha particle has its atomic number reduced by two. This results in the atom becoming a different element. In the example, Radium-226 undergoes alpha decay to form Radon-222.

Alpha Radiation 22688Ra --> 22286Rn + 42He This type of equation is known as a nuclear equation. The atomic numbers and mass numbers of all particles involved are shown. Note that both atomic number and mass number are conserved in a nuclear equation.

Practice Problems Complete the following nuclear equations for alpha decay: 24195Am ----> _____ ----> 42He + 23592U 23592U ----> _____ ----> 42He + 20684Po

Types of Radiation Beta radiation Deflected towards the positively charged plate Consists of fast-moving electrons called beta particles. They have a 1- charge. Symbol for a beta particle is 0-1ß or 0-1e. Example of beta decay

Beta Radiation The atom giving up the beta particle has its atomic number increased by one. This results in the atom becoming a different element. In the example, carbon-14 undergoes beta decay to form nitrogen-14, according to the equation: 146C ---> 0-1e + 147N. Note that, in the equation, atomic numbers and mass numbers are conserved.

Beta Radiation Since beta radiation is a type of nuclear radiation, the beta particle is emitted by the nucleus of an atom. How, you may ask, is it possible for an electron to come from a nucleus?? Scientists believe that neutrons are actually composed of 2 particles - an electron and a proton. In beta decay, the electron is emitted and the proton is left, increasing the atomic number.

Practice Problems Complete the following nuclear equations for beta decay. 1. 6027Co ----> 2. _____ ---> 0-1e + 23191Pa 3. 9740Zr ---> 4. _____ ---> 0-1e + 6630Zn

Types of Radiation Gamma Radiation Also called gamma rays It is high-energy electromagnetic radiation. It has no charge. It also has no mass so its emission does not change the element. However, gamma radiation often accompanies alpha and beta emission, which do change the element's identity. Symbol for a gamma ray is 00 ϒ or just ϒ Gamma rays account for most of the energy lost during the radioactive decay process.

Types of Radiation

Unstable* Nuclei Why are some atoms unstable? The primary factor in determining stability is the ratio of neutrons to protons. Atoms containing either too many or too few neutrons are unstable. Unstable nuclei lose enough energy and emit enough particles during radioactive decay to eventually form a nucleus with a stable neutron to proton ratio.