Radioactive Decay

Radioactive Decay What is Radioactivity?

Quick Review • Nucleus contains protons and neutrons • Electron circles the nucleus in orbits • Proton: +1 charge, mass number (MN) of 1 • Neutron: 0 charge, MN of 1 • Electron: -1 charge, MN nearly zero

Standard Atomic Notation (SAN) • SAN tells us all we need to know about an atom • For boron, # of protons = 5# of electrons = 5 • Mass number = 9# neutrons = 4 Mass Number Atomic Symbol Atomic Number (number of protons)

Try these… Use your periodic table to state the following for each atom: • Atomic number • # of protons • # of electrons • rounded atomic mass • # of neutrons • Standard Atomic Notation 1) barium 2) uranium 3) chlorine

Answers

What about these? and and All three are forms of the oxygen atom found in nature but each have different numbers of neutrons. has 8 protons and 8 neutrons has 8 protons but 9 neutrons has 8 protons and 10 neutrons Isotopes can also be written as Oxygen-18 for

They are isotopes • Isotopes are the same atoms with different mass numbers • The different masses in the three oxygen isotopes are from extra neutrons in the nucleus • Isotopes appear in all the same compounds as the element • Isotopes have different but similar properties, e.g., ordinary water (water with atoms) boils at 1000 C but heavy water (water withatoms) boils at 101.420 C

A new type of reaction • Isotopes mean that the nucleus has changed (added neutrons) • This is new because in chemical reactions the nuclei of the reactants and products remain unchanged • A reaction that changes the nucleus of an atom is called a nuclear reaction

Unstable nuclei • Elements with atomic number <83 [except for technectium (43) and promethium (61)] have one or more stable isotopes • All elements above atomic number 83 have isotopes with unstable nuclei • The nuclei of these elements spontaneously disintegrate • The spontaneous disintegration of a nucleus is called radioactivity • The unstable nucleus that disintegrates is said to be radioactive

Nuclear disintegration • When a nucleus disintegrates it can emit: • alpha particles • beta particles (electrons or positrons) • gamma rays • energy • The new nucleus that remains may be radioactive or stable

Alpha particles • Given the Greek symbol for alpha,  (first letter of the Greek alphabet) • Fast moving, high energy • Relatively heavy, loses energy quickly • Stopped by a piece of paper • Travel 2.5 cm in air • Penetrate skin only 0.3 mm • Consist of 2 protons and 2 neutrons • Rutherford found an alpha particle to be a helium atom,

4 He 2 Alpha Decay • When an unstable nucleus emits an alpha particle it undergoes alpha decay • The resulting new nucleus is an isotope Example: Uranium-238 undergoes alpha decay Uranium-238 decays to make a new isotope - thorium and an alpha particle Nuclear Equation Th 238 U 234  92 90 * Notice the conservation of mass holds (no particles are destroyed). Alpha particle

Beta particles • Given the Greek symbol for beta, β(second letter of the Greek alphabet) • Very fast • Very light • Stopped by 0.1 mm of lead • Can travel 4.5 m in air • Penetrate skin 17 mm • Beta particles may be positively or negatively charged

__ v e Pa 234 Th 234 0   -1 90 91 Beta Decay Two types of beta decay are known: β – Decay • Neutron in the nucleus is converted into a proton • Atomic number increases by 1 (new element) • During this conversion an electron and an antineutrino are ejected from the nucleus • Also known as electron emission • An example of β- decay is: Electron Antineutrino

Ne 22 Na 22 v   11 10 e 0 1 Beta Decay β + Decay (positron emission) • Proton in the nucleus is converted into a neutron • Atomic number decreases by 1 (new element) • During this conversion a positron and a neutrino are ejected from the nucleus • Also known as positron emission • An example of β+ decay is: Positron Neutrino

Gamma Rays • Given the Greek symbol for gamma, γ (third letter of the Greek alphabet) • Type of ionizing electromagnetic radiation • Most penetrating form • Highest frequency and shortest wavelength in the electromagnetic spectrum • Produced when the nucleus of an atom is in an excited state and then releases energy, becoming more stable • When a nucleus emits an  or β particle, the daughter nucleus is sometimes left in an excited state. It can then jump down to a lower level by emitting a gamma ray

4 He 2 e 0 Th 238 U 234 Pa 234 Th 234 -1 92 90 90 91 Nuclear Reaction Equations • Nuclear reactions are just like regular chemical reactions • Mass is conserved • Nuclear reactions show the starting material, the type of radiation given off and the end products • Example: Radioactive atom new radioactive element particle emitted   Uranium atom New isotope Alpha particle  New isotope Thorium atom Electron

Radioactive Decay