Radioactive Decay What is Radioactivity? http://explorecuriocity.org/explore/articleid/3033 http://explorecuriocity.org/explore/articleid/3035 http://explorecuriocity.org/explore/articleid/2160

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

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 Barium Uranium Chlorine Atomic number 56 92 17 # of protons 56 92 17 # of electrons 56 92 17 Mass number 137 238 35 # of neutrons 81 146 18 Standard Atomic Notation 137 56 Ba 238 35 92U 17Cl

What about these? 16 O 17 O and 8 8 and 18 8 O All three are forms of the oxygen atom found in nature but each have different numbers of neutrons. 16 8O 17 8O has 8 protons and 8 neutrons has 8 protons but 9 neutrons Isotopes can also be written as Oxygen-18 for 18 8 O 18 8O has 8 protons and 10 neutrons

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 100 0 C but heavy water (water with atoms) boils at 101.42 0 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 nucleidisintegrate 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 Consist of 2 protons and 2 neutrons Rutherford found an alpha particle to be a helium atom,

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 * Notice the conservation of mass holds (no particles are destroyed). Nuclear Equation 238 234 U Th 4 He 92 90 2 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

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: 234 234 Th Pa 0 e v 90 91-1 Electron Antineutrino

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: 22 22 Na Ne 0 e v 11 10 1 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

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 238 U 234 Th 92 He 90 2 Uranium atom New isotope Alpha particle 234 Th 234 Pa 0 e 90 91-1 Thorium atom New isotope Electron 4

Activity Cut and paste handout in your groups Then put them in order for uranium-238 decay!