Nuclear Chemistry The weird world of the nucleus

Isotope Refresher Atoms of the same element all have the same number of protons Atoms of the same element all have the same number of protons Atoms of the same element may have different neutrons and therefore mass numbers (p + + n 0 ) Atoms of the same element may have different neutrons and therefore mass numbers (p + + n 0 )

Nuclear Instability Not all combinations of protons and neutrons are created equal Not all combinations of protons and neutrons are created equal Some are more unstable than others. Some are more unstable than others. If they are unstable they will do one of the following: If they are unstable they will do one of the following: Radioactive decay Radioactive decay Nuclear fission Nuclear fission Nuclear fusion Nuclear fusion

Nuclear “Reactions” Nuclear “reactions” must still be balanced. Nuclear “reactions” must still be balanced. Notice 238 = “Mass balanced” Notice 238 = “Mass balanced” Notice 92 = “Charge balanced” Notice 92 = “Charge balanced”

Radioactive Decay

Radioactive decay – the nucleus of an atom undergoes a change so that it is no longer the same element Radioactive decay – the nucleus of an atom undergoes a change so that it is no longer the same element Decay is a totally random event. Nothing has an effect when an atom decays Decay is a totally random event. Nothing has an effect when an atom decays Two Main Types of Radioactive Decay 1. Alpha decay 2. Beta decay

Alpha Decay Alpha decay – emission of an alpha particle from the nucleus Alpha decay – emission of an alpha particle from the nucleus Alpha (α) particle - a helium-4 nucleus Alpha (α) particle - a helium-4 nucleus Uranium-235, “enriched uranium,” decays by alpha decay Uranium-235, “enriched uranium,” decays by alpha decay

Beta Decay Beta decay – emission of a beta particle from the nucleus Beta decay – emission of a beta particle from the nucleus Beta (β) particle – electron Beta (β) particle – electron Thorium-231 decays by beta decay Thorium-231 decays by beta decay

Practice Decay Reactions Beta decay by cesium-137 Beta decay by cesium-137 Alpha decay by polonium-210 Alpha decay by polonium-210

Decay Series Some atoms require multiple decays to become stable Some atoms require multiple decays to become stable Most decay series of naturally occurring isotopes end in lead. Most decay series of naturally occurring isotopes end in lead.

Half Life

Half life – the amount of time it takes for ½ of a radioactive isotope to decay into something else. Half life – the amount of time it takes for ½ of a radioactive isotope to decay into something else. Notice the atoms don’t disappear, they just change their identity. Notice the atoms don’t disappear, they just change their identity.

Half Life Problem A 100.0g radioactive sample decays for 5 hours. Only 12.5g of the original isotope remains after 5 hours. How long is the half life? A 100.0g radioactive sample decays for 5 hours. Only 12.5g of the original isotope remains after 5 hours. How long is the half life? 3 half lives = 5 hours 3 half lives = 5 hours 5hours/3half lives = 1.67hours/halflife 5hours/3half lives = 1.67hours/halflife Number of half lives Mass of sample Time g g 225.0g 312.5g 5 hours

25.2 days Half Life Problem Selenium-72 has a half life of 8.40 days. How much of a 450.0g sample of selenium-72 will remain after 42.0 days? Number of half lives Mass of sample Time g g 8.40 days g 16.8 days g 33.6 days g g 42.0 days

Radiocarbon Dating Technique Uses the known half life of C-14 to estimate death of organic matter Uses the known half life of C-14 to estimate death of organic matter Based on the known ratio of C-14 to C-12 Based on the known ratio of C-14 to C-12

Nuclear Fission and Fusion

Nuclear Fission Nuclear fission – one atom’s nucleus splits apart. Nuclear fission – one atom’s nucleus splits apart. A neutron strikes a nucleus causing it to split into small pieces A neutron strikes a nucleus causing it to split into small pieces Releases lots of energy. Releases lots of energy. Extra neutrons are also produced. Extra neutrons are also produced.

Nuclear Fission The extra neutrons can cause a chain reaction The extra neutrons can cause a chain reaction

Uses of Nuclear Fission Nuclear power Nuclear power Energy produced by fission is transferred as heat to a coolant Energy produced by fission is transferred as heat to a coolant

Nuclear Power

Uses of Nuclear Fission Mushroom cloud from Nagasaki

Uses of Nuclear Fission Nuclear weaponry Nuclear weaponry “Atomic bomb” “Atomic bomb” “A-bomb” “A-bomb” Runaway fission reaction resulting in an explosion. Runaway fission reaction resulting in an explosion.

Nuclear Fusion Fusion – combining of nuclei Fusion – combining of nuclei Above reactions describe fusion in the Sun. Above reactions describe fusion in the Sun. Releases more energy than fission. Releases more energy than fission. Also requires very high temperatures. Also requires very high temperatures.

Uses of Nuclear Fusion

Fusion weapons Fusion weapons Hydrogen bomb Hydrogen bomb “H-bomb” “H-bomb” Thermonuclear bomb Thermonuclear bomb Uses a fission reaction to start the fusion reaction Uses a fission reaction to start the fusion reaction “A-bomb” is the detonator for an “H-bomb” “A-bomb” is the detonator for an “H-bomb”

Radiation

3 Primary Types of Radiation 1. Alpha radiation – α – alpha particles Low energy Low energy Result of alpha decay Result of alpha decay Low penetration Low penetration Stop with a sheet of paper Stop with a sheet of paper 2. Beta radiation – β –beta particles (electrons) Higher energy Higher energy Result of beta decay Result of beta decay Some penetration ability Some penetration ability Stop with several sheets of aluminum foil Stop with several sheets of aluminum foil 3. Gamma radiation – γ –high energy gamma rays Very high energy Very high energy Can result from any type of decay Can result from any type of decay Very highly penetrating and damaging Very highly penetrating and damaging Stop with several centimeters of lead or very thick concrete Stop with several centimeters of lead or very thick concrete

3 Primary Types of Radiation

Sources of Human Exposure