Chapter 11

Radioactive

Elements

Accidental Discovery of Nuclear Radiation

• In 1896, French scientist Henri Becquerel accidentally left some uranium samples on a photographic plate & found outlines of the substance on the film

• at first, he thought the uranium was giving off X-rays because the photographic paper had been exposed to sunlight

• but when it worked without the sun, he hypothesized that the uranium had given off some invisible energy that had never been detected before, later called radiation

Background Info•it was known that certain substances glowed when exposed to sunlight (fluorescent)

•but his experiment worked even when it wasn’t sunny

•he determined that the element uranium was the source of nuclear radiation

Other Discoveries of

Nuclear Radiation•Later, in 1898, Marie Curie & her husband, Pierre, discovered two other radioactive elements they named polonium & radium

An element that gives off nuclear radiation is said to

be radioactive.Properties include:1.nuclear radiation from radioactive

elements will alter photographic film

2.they produce fluorescence3.electric charge can be found in the

air surrounding radioactive elements

4.nuclear radiation damages cells in most organisms

Fluorescence

The Nucleus• contains protons & neutrons held

together by strong forces• strong forces are short-range forces

that only work over a very short distance; therefore, the bigger the nucleus, the farther apart the protons & neutrons

• as long as the protons & neutrons remain together due to the strong force, the nucleus remains stable

• the bigger the nucleus, the more unstable it becomes

Radioactivity•when the strong force is not large enough to hold the nucleus together tightly, the nucleus can decay & give off matter & energy

•this process where the nucleus breaks apart or decays is called radioactivity

•all elements with 83 or more protons are radioactive

Binding Energy

•the energy required to break up the nucleus

•if the binding energy within the nucleus is weak, the nucleus is unstable & will break apart or decay

Unstable•synthetic elements are unstable & decay quickly after being created in the laboratory

•many isotopes of elements (no matter the nucleus size) are also radioactive

•they are sometimes called radioisotopes

Isotopes

•an atom of an element that has the same number of protons, but a different number of neutrons

•many elements can have both radioactive & nonradioactive isotopes (see pg. 271 for examples)

3 Types of Nuclear Radiation

•alpha radiation (particles)

•beta radiation (particles)

•gamma radiation (waves of energy)

3 Types of Radiation

Alpha Particles•made of 2 protons + 2 neutrons•has an electric charge of +2•had an atomic mass of 4•more massive & most electric charge• lose energy more quickly when

interacting with matter• least penetrating (cannot pass

through a piece of paper)•very damaging to biological

molecules within the body

Alpha Decay•when an atom loses an alpha

particle, it is no longer the same element because the number of protons is different

• the new element formed has an atomic number two less than the original radioactive element, and a mass number that is four less (it lost two neutrons too)

Transmutation

•the process of changing one element to another through nuclear decay

•the atomic mass at the beginning of the equation must equal the atomic mass at the end

How Smoke Detectors Work

•when alpha radiation passes through matter, they exert an electrical force on the electrons

• this force pulls the electrons away fro the atoms, giving the atom a positive charge (cation)

How Smoke Detectors Work•some smoke detectors will give off

alpha particles to ionized the surrounding air

•normally, an electrical current can pass through the ionized air to form a circuit

•when smoke particles enter the ionized air, they absorbs the ions & electrons, thus breaking the circuit causing the alarm to go off

Beta Particles•move much faster than alpha

particles & are more penetrating• these particles are stopped by Al

foil•when a neutron in an unstable

nucleus decays into a proton, it emits an electron called the beta particle

•process is known as beta decay•caused by weak forces

Beta Decay/Transmutation

•now that an extra proton was produced in the beta decay, the atom changes to another element

•although the atom of this new element has a different atomic number, it still has the same atomic mass because the atom lost a neutron & gained a proton

Beta Decay

Gamma Rays•most penetrating form of radiation•not made of protons, electrons, or

neutrons•carry electromagnetic waves of

energy•have no mass, no charge•move at the speed of light•stopped by thick, dense materials

such as lead or concrete

Gamma Decay•alpha & beta decay are

accompanied by gamma decay, the release of a gamma ray from the nucleus

•since there is no mass or charge, the nucleus does not change into a different nucleus

•since energy leaves, the nucleus moves to a lower energy state

Electromagnetic Spectrum

Decay Series•the series of steps by which a radioactive nucleus decays into a nonradioactive nucleus

•spontaneous breakdown continues until a stable nucleus is formed

Artificial Transmutation

•bombard atomic nuclei with neutrons, alpha particles, or other nuclear “bullets”

•done in particle accelerators because of the need to hit the target nuclei with enough force with these high-energy particles

Particle Accelerator

Artificial Transmutation

• first done by Ernest Rutherford who discovered the nucleus of the atom

• Italian scientist Enrico Fermi was the first to use neutrons instead of charged particles

• neutrons cause the nucleus to disintegrate, get trapped inside or pass right through

• can be used to produce radioactive isotopes of natural elements (iodine)

Radioactive Half-Life

•a measure of the time required by the nuclei of an isotope to decay

• the half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay

• time varies from isotope to isotope

• the nucleus left after the isotope decays is called the daughter nucleus

Radioactive Dating•the ages of materials such

as rocks & fossils can be dated using radioactive isotopes & their half-lives

•different isotopes are useful in dating different types of materials

Half-Life Decay of Uranium

Radioactive Dating1. determine the amount of

radioactive isotope & its daughter nucleus in a sample

2. calculate the number of half-lives that would have passed to create that amount of daughter nucleus compared to remaining isotope

3. that gives you the amount of time that has passes since the object formed