Section 22-1: The Nucleus Objectives Explain what nucleons are. Explain what a nuclide is, and describe the different ways it can be written. Define nuclear binding energy. Explain the relationship between nucleon number and stability of nuclei.
The Nucleus • The nucleus is composed of nucleons • Protons • Neutrons • A nucleus is characterized by two numbers • mass number (A; total # of nucleons) • atomic number (Z; number of protons) ZAE
1327Al • total number of nucleons is 27 • total number of protons is 13 • the number of neutrons is 14 • in nuclear chemistry, an atom is referred to as a nuclide.
Subatomic Particlesone atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom
Einstein’s Equation • Energy and mass can be interconverted • E = mc2 • E- energy, m-mass, c-speed of light • When protons & neutrons are packed together to form a nucleus, some of the mass is converted to energy and released.
Einstein’s Equation • Nuclear Binding Energy – the energy released when a nucleus is formed from protons and neutrons. • Can also be thought of as the amount of energy required to break apart the nucleus. Of an existing atom. • The higher the binding energy, the more tightly the nucleus is held together.
Binding Energy Curve • graph peaks at A=56 • the more BE released per nucleon, the more stable the nucleus • mass number of 56 is maximum possible stability
How Many Neutrons? • Stable nuclides have certain characteristics • The number of neutrons in a nucleus can vary as we have seen • Range limited by the degree of instability created by: • having too many neutrons • too few neutrons • Stable nuclei do not decay spontaneously • Unstable nuclei have a certain probability to decay
Nuclear Stability Facts • 265 stable nuclides • For light elements (Z 20), Z:N ratio is ~1 • Example: helium-4 (2 neutrons, 2 protons) • Z:N ratio increases toward 1.5 for heavy elements • Example: lead-56 (124 neutrons, 82 protons) • For Z > 83 (bismuth), all isotopes are radioactive
Nuclear Stability Facts • Stable nuclei tend to have an even number of nucleons. • Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons. • Only 4 nuclides have odd numbers of both.
The most stable nuclides are those having: 2, 8, 20, 28, 50, 82, 126 • Protons, neutrons or total nucleons • Examples: • Sn (Z=50) has 10 isotopes; In (Z=49) & Sb (Z=51) have only 2 isotopes • Pb-208 has a double magic number (126n, 82p) & is very stable • “Magic numbers” of protons or neutrons which are unusually stable
Nuclear Reactions • Nuclear Reaction – a reaction that affects the nucleus of an atom. • Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons. • They also give off a large amount of energy and increase their stability in the process.
Nuclear Reactions • In a nuclear reaction, the total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation. • Example: • 49Be + 24He 612C + 01n
49Be + 24He 612C + 01n • Note that when the atomic number changes, the identity of the element changes. • Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.
Sample Problem Identify the product that balances the following nuclear reaction: 84212Po ? + 24He
Classwork Problems 1-3, page 704
Homework Page 724 Problems: 31, 33 and 40
Section 22-2 Radioactive Decay
Radioactivity Objectives Define the terms radioactive decay and nuclear radiation. Describe the different types of radioactive decay. Define the term half-life, and how it relates to stability.
Radioactivity • The spontaneous decay of an unstable nucleus into a more stable nucleus. • Energy is released. • Certain isotopes are just not stable and will spontaneously decay.
Types of Radioactive Decay • All elements with 84 or more protons (Polonium) are unstable, and will undergo radioactive decay. • Naturally occurring radioactive isotopes decay in three primary ways: • Alpha particle emission • Beta particle emission • Gamma radiation emission
Alpha Emission Alpha particle – is two protons and two neutrons bound together and emitted from the nucleus. They are helium nuclei with a charge of +2. It has no electrons. Represented by the symbol: 24He Restricted to heavy elements: Ex. uranium
Alpha Emission Process which is effective to lose a lot of mass form the element. Example: 84210Po 82206Pb + 24He The atomic number decreases by two (a new element) and the mass number decreases by four.
Alpha Emission • 92235U 90231Th + 24 • Quick way for a large atom to lose a lot of nucleons
Beta Emission Beta particle – is essentially an electron that’s emitted from the nucleus: -10e In the nucleus, a neutron is converted (decayed) into a proton and an electron. The electron is emitted as a beta particle. Represented by the symbol: -10e or -10b A good way to decrease the number of neutrons.
Beta Emission By decreasing the number of neutrons you improve the neutron/proton ratio. Example: 53131I 54131Xe + -10b The mass number stays the same in going from I-131 to Xe-131but the atomic number increases by 1. Loss of neutron!
Beta Emission • 1940K 2040Ca + -10b • Identity of atom changes
Gamma Emission Gamma rays – are high energy electromagnetic waves emitted from the nucleus. There is no mass change with gamma emission, only radiation. Usually occurs immediately following other types of decay. Not shown in a balanced nuclear reaction.
Gamma Emission • An example is cobalt-60 (Co-60) which gives off a large amount of gamma radiation. • Co-60 used in the radiation treatment of cancer.
Electromagnetic Radiation • Electromagnetic radiation is a form of energy that can pass through empty space • It is not just a particle, and it is not just a wave. It may be both.
Electromagnetic Radiation • Gamma rays are similar to x-rays – high energy, short wavelength radiation.
Positron Emission Positron particle – is essentially an electron that has a positive charge. A useful way to decrease the number of protons. Represented by the symbol: +10e Doesn’t occur with naturally occurring radioactive isotopes.
Positron Emission Example: 1938K 1838Ar + +10e Notice that the atomic number decreases by one but the mass number stays the same. Can be viewed as the opposite of beta emission.
Half-Life Half-life - the amount of time it takes for one-half of a radioactive sample to decay is called the half-life of the isotope It is given the symbol: t1/2 No two radioactive isotopes decay at the same rate
Half-Life Useful application of half-life is radioactive dating. Used to determine the age of things. Carbon-14 dating can be used to determine the age of something that was once alive. Examples include animal and plant species. Cannot be used to determine the age of rocks.
Half-Life Radium-226 has a half-life of 1599 years. Half of a given amount of radium-226 decays in 1599 years. In another 1599 years, half of the remaining radium-226 will decay. This will continue until there is a negligible amount of radium-226 remaining.
Half-Life Decay of radium-226
Half-Life • Each radioactive element has its own half-life. • More stable elements decay slowly and have longer half-lives. • Less stable elements decay quicker and have shorter half-lives
Half-Life The time required for half of a sample to decay
Half-Life Problem: Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope. First determine the number of half-lives that have elapsed!
Classwork Problems 1-6, page 709
Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission. • The product of such an emission is itself radioactive and will undergo a further decay process. • Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product. Decay Series
Trying To Reach Nuclear Stability Decay Series – a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached. The heaviest nuclide of each series is called the parent nuclide. The nuclides produced are called daughter nuclides.