Section 1: What is Radioactivity? Preview Key Ideas Bellringer Nuclear Radiation Nuclear Decay Math Skills Radioactive Decay Rates

Key Ideas What happens when an element undergoes radioactive decay? How does radiation affect the nucleus of an unstable isotope? How do scientists predict when an atom will undergo radioactive decay?

Bellringer, continued 2. Complete the table below to indicate how many protons and neutrons are in the nuclei of each atom.

Nuclear Radiation What happens when an element undergoes radioactive decay? After radioactive decay, the element changes into a different isotope of the same element or into an entirely different element. radioactive decay: the disintegration of an unstable atomic nucleus into one or more different nuclides

Nuclear Radiation, continued nuclear radiation: the particles that are released from the nucleus during radioactive decay There are different types of nuclear radiation. nuclear radiation can contain alpha particles beta particles gamma rays neutrons

Types of Nuclear Radiation

Nuclear Radiation, continued Alpha particles consist of protons and neutrons. alpha particle: a positively charged particle that consists of two protons and two neutrons and that is emitted from the nucleus during radioactive decay Beta particles are electrons produced from neutron decay. beta particle: an electron or proton that is emitted from a nucleus during radioactive decay

Nuclear Radiation, continued Gamma rays are high-energy electromagnetic radiation. gamma ray: a high-energy photon emitted by a nucleus during fission and radioactive decay When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays Neutron radioactivity occurs in an unstable nucleus. Neutron emission consists of matter that is emitted from an unstable nucleus. Neutrons have no charge. Neutrons are able to travel farther through matter than either alpha or beta particles.

Nuclear Decay How does radiation affect the nucleus of an unstable isotope? Anytime that an unstable nucleus emits alpha or beta particles, the number of protons or neutrons. Nuclear-decay equations are similar to those used for chemical reactions.

Nuclear Decay, continued Gamma decay changes the energy of the nucleus. There is no change in the atomic number or the atomic mass of the element. The atomic number changes during beta decay. The mass number before and after the decay does not change. The atomic number of the product nucleus increases by 1. The atom changes to a different element.

Nuclear Decay, continued A beta decay process occurs when carbon-14 decays to nitrogen-14 by emitting a beta particle.

Nuclear Decay, continued Both atomic mass and number change in alpha decay. The atomic mass decrease by 4. The atomic number decreases by 2. The process of the alpha decay of radium-226 is written as follows.

Math Skills Nuclear Decay Actinium-217 decays by releasing an alpha particle. Write the equation for this decay process, and determine which element is formed. 1. Write down the equation with the original element on the left side and the products on the right side. X = unknown product; A = unknown mass; Z = unknown atomic number

Math Skills, continued 2. Write math equations for the atomic and mass numbers. 217 = A + 4 89 = Z + 2 3. Rearrange the equations. A = 217 – 4 Z = 89 – 2 A = 213 Z = 87 4. Rewrite the equation with all nuclei represented. The unknown decay product has an atomic number of 87, which is francium.

Radioactive Decay Rates How do scientists predict when an atom will undergo radioactive decay? It is impossible to predict the moment when any particular nucleus will decay, but it is possible to predict the time required for half of the nuclei in a given radioactive sample to decay. half-life: the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope

Radioactive Decay Rates, continued Half-life is a measure of how quickly a substance decays. Doctors use isotopes with short half-lives to help diagnose medical problems. Scientists can also use half-life to predict how old an object is. Geologists calculate the age of rocks by using the half-lives of long-lasting isotopes, such as potassium-40.

Half-Life If 100g of carbon-14 decays until only 25g of carbon is left after 11,460 y, what is the half-life of carbon-14? Initial mass of sample: 100g Final mass of sample: 25g Total time of decay: 11,460 years Unknown: number of half-lives Total time of decay = number of half lives X number of years half-life Number of years = total time of decay half-life number of half-lives

Calculate how many half-lives have passed during decay Final mass of sample = 25.0g = 1/4 Initial mass of sample 100g After one half life = ½ After two half lives = ½ X ½ = ¼

2. Total time of decay = number of half lives X number of years half-life Number of years = total time of decay half-life number of half-lives Number of years = 11,460 years = 5730 years half-life 2 half-lives

Half Life The half life of a radioactive material is the amount of time required for ½ of the sample to decay into another element or isotope. Half lives are calculated according to the equation: a = a0(½)x

Half Life a = amount of material left at any time a0 = amount of material that you begin with x = the number of half lives that have passed since you have begun counting This type of decay is said to be exponential since it can be described graphically as a hyperbola Alta Physics

Sample Problem Carbon-14, a radioactive isotope of carbon, has a half life of 5730 years. If a 20 gram sample of carbon-14 is allowed to decay for 10,000 years, how much remains at the end of this period? Alta Physics

Solution a = a0(½)x a0 = 20 grams x = 10,000 yrs/5730 yrs/half life = 1.75 So a = 20 grams(½)1.75 = 5.95 grams