WJEC Chemistry for AS: Study and Rev Guide

Half-life The nuclei in different radioactive substances decay at different rates. The time taken for one half of all nuclei in a radioisotope to decay is known as its half-life . The half-life depends only on which isotope is decaying, not on the quantity present. Calculations involving half-life can be set to find: ■ The time taken for the radioactivity of a sample to fall to a certain fraction of its initial value. ■ The mass of a radioactive isotope remaining after a certain length of time given the initial mass. ■ The half-life of a radioisotope given the time taken for a sample to fall to a certain fraction of its initial value. Example 1 Tritium has a half-life of 13 years. How many radioactive nuclei will be left after 52 years from an original sample which contained 16 million nuclei? 52 years = 4 half-lives 16 million 13 8 million 13 4 million 13 2 million 13 1 million Example 2 An isotope of phosphorus, 32 P, is radioactive and its radioactivity falls to 1/8th of its initial value in 42.9 days. Calculate the half-life of 32 P. 1 1/2 1/4 1/8 is 3 half-lives Half-life = 42.9 = 14.3 days 3 Consequences for living cells Radioactive emissions are potentially harmful. However, we all receive some radiation from the normal background radiation that occurs everywhere. Workers in industries where they are exposed to radiation from radioactive isotopes are carefully monitored to ensure that they do not receive more radiation than is allowed under internationally agreed limits. High energy radioactive emissions break chemical bonds in the cell molecules giving rise to changes in DNA which can cause mutations and the formation of cancerous cells at lower doses, or cell death at higher doses. When α -emitting isotopes are ingested they are far more dangerous than an equivalent activity of β emitting or γ emitting isotopes but fortunately α particles from outside cannot penetrate the skin. Half-life is the time taken for half the atoms in a radioisotope to decay or the time taken for the radioactivity of a radioisotope to fall to half its initial value. Key Term Pointer The greater the half-life of a radioactive isotope, the greater the concern, since the radioactivity of the isotope exists for a longer time. a. Outline why radioactivity may be a health hazard. b. Why is α radiation the most harmful if ingested but least harmful outside the body? QUICKFIRE QUICKFIRE QUICKFIRE QUICKFIRE QUICKFIRE QUICKFIRE  60 Co used in radiotherapy has a half-life of 5.3 years. Calculate how long it would take for the activity of the isotope to decay to 1/16th of its original value. 13 1.2 Basic ideas about atoms