Radioactive Decay Alpha, Beta, and Gamma Decay

Radioactivity Emission of particles and energy from the nucleus of certain atoms This happens through a process called radioactive decay Occurs when a nucleus is unstable; usually too many neutrons in the nucleus of an atom

Terms to Recall Atomic number: number of protons in the nucleus; also identifies each element in the periodic table Isotope: atoms of an element with same number of protons and electrons, but with differing number of neutrons Mass number: total number of protons and neutrons in the nucleus of an atom

Various Isotopes of Carbon Isotope symbol Isotope Name Number of Protons Number of Neutrons Mass Number 12 6CCarbon CCarbon CCarbon

Carbon Isotopes Carbon-14 is radioactive Carbon-12 and carbon-13 are not Any isotope with an atomic number greater than 83 (bismuth) in the periodic table is radioactive Radioisotopes = isotopes that are radioactive; nucleus is unstable because of too many neutrons

Mass number →238 atomic number →92 U Mass number and atomic number for uranium

Three different types of radiation emitted by radioactive materials: Alpha particles Beta particles Gamma rays

Alpha Particle Radiation Consists of 2 protons and one neutron Same as the nucleus of a helium atom Denoted by Greek letter alpha (α) Low penetration power: a few centimetres of air or a piece of paper +2 charge on alpha particle interacts with air or paper molecules results in a larger, slower speed

Beta Particle Radiation High speed electrons with a -1 charge Denoted by Greek letter beta (β) A neutron from the nucleus breaks down into a proton and a high speed electron that is emitted by the radioactive nucleus Smaller than an alpha particle with higher speed Greater penetration into a metre of air and can be absorbed by tin foil

Gamma Radiation Energy rays emitted by the nucleus Carry no charge, but are highly energetic with a high frequency These are pure energy in the form of electromagnetic radiation Very penetrating and require about 7 cm of lead to be completely absorbed

Detecting Radioactive Decay Geiger Counter is used to detect alpha, beta, and gamma decay It ‘counts’ radioactive particles entering the detector tube See p.204 for a diagram of a Geiger counter

Writing Nuclear Equations Parent nucleus is the original, unstable nucleus Daughter nucleus is the resulting product; if it is not stable it will continue to decay Process of decay can be written like a chemical equation—a nuclear equation Each decay process has its own nuclear equation but follow similar pattern: Parent nucleus →daughter nucleus + radioactive decay products

Alpha Decay Common example is uranium decaying to thorium which releases an alpha particle: This reaction will continue because thorium is unstable.

Alpha Decay Mass number must be equal on both sides of equation Mass number of parent nucleus must equal the total of the mass number of the daughter nucleus plus decay products Atomic number of parent nucleus must equal the total of the atomic number of the daughter nucleus plus decay products

Beta Decay Common example is carbon decaying to nitrogen which releases a beta particle:

Gamma Ray Emission Nuclear equation shows no change in mass number or atomic number of the radioisotope An asterik (*) is used to indicate the parent nucleus is unstable and has an excess of energy; excess energy emitted as gamma rays:

Half-Life of Radioisotopes Radioactive substances decay at different rates The rate at which it decays is called its half-life; the time it takes for half (50%) of the nuclei in a sample of that radioisotope to decay Half-life of carbon-14 is about 5730 years; this means that 1 kg of carbon-14 would become just 500 g in about 5730 years; in about years if would be 250 g See p. 205 for more examples of radioisotope half-life

Decay Curve Half-life of any radioisotope can be graphed—a decay curve Graphs the mass of the radioisotope over time; you get a smooth curve showing half the radioisotope remaining at the end of each half- life Can only predict on average when half the nuclei will have decayed

Carbon Dating Used to determine the age of an object that was once living Our atmosphere contains constant ratio of stable carbon-12 to radioactive carbon-14 1 radioactive carbon-14 atom for every trillion carbon-12 atoms Living things maintain this ratio, but as soon as they die, they carbon-14 in them begins to decay

Carbon Dating Since half-life of carbon-14 is about 5730 years it is possible to estimate age of plants or animals Works for about 10 half-lives before amount of carbon-14 is too small to measure; this is about years For objects older than years you have to use other radioactive isotopes with longer half- lives Look at example on p. 208