Chapter 10 – Prentice Hall Physical Science Nuclear Chemistry Chapter 10 – Prentice Hall Physical Science

Nuclear Chemistry - Prentice Hall Physical Science Chapter 10 Review All the chemistry we’ve discussed so far has involved electrons. Questions: If element X has a molar mass of 3 g/mol and element Y has a molar mass of 5 g/mol, what must be the molar mass of X2Y? If you tossed 128 coins in the air, about how many would you expect to land heads-up? What do the mass number and atomic number represent? Which subatomic particles are found in the nucleus?

Radioactivity Antoine Henri Becquerel (1896) experimented with uranium salts and discovered radioactivity Radioactivity (or nuclear decay): an unstable nucleus emits charged particles and energy Radioisotope: radioactive isotope - any atom that has an unstable nucleus. Examples: Uranium-238 (used in Becquerel’s experiment) Carbon-14 (used often in radioactive dating)

Isotope symbology Isotopes are named using the element name followed by the mass number (see examples, slide #3) The symbol for isotopes includes the element symbol, the mass number and the atomic number as follows: Mass # on top Uranium-238 Carbon-14 Polonium-210 Atomic # on bottom

3 Types of Nuclear Radiation Nuclear radiation: charged particles and energy that are emitted from the nuclei of radioisotopes Radiation Type Symbol Charge Mass (amu) Common Source Alpha particle a, 2+ 4 Radium-226 Beta particle b, 1- Carbon-14 Gamma ray g Cobalt-60

Alpha Decay Alpha particle, a 2 protons and 2 neutrons Positively charged Same as He nucleus Least penetrating type of nuclear radiation Travel only centimeters in air Can be stopped by a sheet of paper or clothing

Beta Decay Beta particle, b More penetrating than a particles 1 electron Negatively charged Produced by a neutron that decomposes into a proton and an electron More penetrating than a particles Pass through paper Stopped by a thin sheet of metal

Gamma Decay Gamma ray, g Penetrating ray of energy Like X-rays and light, only very short wavelength Most penetrating form of the three types discussed Often accompanies alpha or beta decay Several centimeters of lead or several meters of concrete required to stop it

Writing and Balancing Nuclear Reactions Similar to chemical equations, but isotope symbols are used. In a balanced nuclear equation: Mass # on the left = sum of mass #s on the right Atomic # on the left = sum of atomic #s on the right You will need to use your PERIODIC TABLES! Reactants → Products

Example: Math Skills p. 295 Write a balanced nuclear equation for the alpha decay of polonium-210. Step 1: Define reactants and products. Use letters to represent the unknown values. Step 2: Write and solve equations to find unknown atomic and mass #s. Step 3: Look up the element symbol on the periodic table using the atomic #. Step 4: Write the balanced nuclear equation and double-check your solution. + Atomic # 82 = Pb (Lead)

Effects of Nuclear Radiation Background radiation: naturally occurring in the environment Sources: Radioisotopes in air, water, rocks & living things Cosmic radiation Generally at safe levels Nuclear radiation can ionize atoms. At levels significantly above background, this can damage DNA and proteins Which type of nuclear radiation is the least harmful? Which the most?

Detecting Nuclear Radiation Geiger counters Use gas-filled tubes to measure ionizing radiation Gas produces an electric current when exposed to ionizing radiation Film badges Photographic film wrapped in paper Film is exposed with exposure to radiation like photographic film is “exposed” with exposure to visible light

Rate of Nuclear Decay Nuclear decay rate describes how fast nuclear changes take place Unlike chemical reactions, nuclear decay rate does NOT vary with external conditions – it is constant for a given radioisotope Half-life: the time required for half of a radioisotope sample to decay

Rates of Nuclear Decay (cont’d)

Rates of Nuclear Decay (cont’d) Different radioisotopes have different half-lives To determine how many half-lives have elapsed for a sample, divide the total time of decay by the half-life Known decay rates are used in radioactive dating Radioisotope Half-life Radon-222 3.82 days Iodine-131 8.07 days Carbon-14 5730 years Thorium-230 75,200 years Uranium-238 4.47x109 years

Radiocarbon dating Carbon-14 exists naturally in the atmosphere at a fairly constant ratio to C-12 CO2 absorbed while living (including some C-14) Age of fossil determines by comparing C-14/C-12 ratio in fossil to atmospheric ratio As C-14 decays, it’s replaced by C-14 absorbed from atmosphere Tree dies – no more CO2 absorbed to replace decaying C-14

Radiocarbon dating (cont’d) Used for objects less that 50,000 years old For older objects, must use different isotope with longer half-life What isotopes would work well to date a rock formation that is thought to be close to a trillion years old?

Artificial Transmutation Transmutation: conversion of atoms of one element into atoms of another Alchemists have attempted this for hundreds of years (but not through nuclear chemistry) First artificial transmutation: Ernest Rutherford (1919) turned nitrogen into oxygen-17

Artificial Transmutation (cont’d) Transmutation achieved by bombarding atomic nuclei with high-energy particles Protons, neutrons or alpha particles Example: Ernest Rutherford’s transmutation used which particle? Transuranium elements Many produced by artificial transmutation of a lighter element All are radioactive

Nuclear Forces The strong nuclear force attracts protons and neutrons. Strong nuclear forces: Electric forces: The strong nuclear force attracts protons and neutrons. Stronger than electric forces over short distances Decreases with distance (like gravity) Electric repulsions push protons apart. When a nucleus is large enough, the electric forces can overcome the strong nuclear forces. Nuclei are unstable at this point. Any atom with 83 or more protons is unstable – and, therefore, radioactive. Proton from a small nucleus Small nucleus Proton from a large nucleus Large nucleus

Fission Fission: splitting of nucleus into two smaller parts Lise Meitner, Fritz Strassman and Otto Hahn’s experiments (1939) first demonstrated nuclear fission. A small amount of the original mass is converted into a lot of energy Neutron (( )) (very unstable) Energy

Fission (cont’d) About how much energy was released from 6.2 kg of Plutonium-239 in the second atomic bomb explosion? (Note: Only about 1 kg underwent fission – the rest was scattered.) This quantity = 2.5 x 1010 kWh, or enough energy to power my house for over 3.6 million years!

Fission and Chain Reactions Fission can result in a chain reaction. Neutrons released from the first reaction can trigger another reaction, and so on – similar to a rumor spreading. Energy + Energy Neutron Energy + Energy +

Chain Reactions (cont’d) For a chain reaction to happen, each split nucleus must produce at least one neutron with enough energy to split another nucleus This only happens when a specific mass of fissionable material is available – called the critical mass. Controlled chain reactions are used to generate electricity in nuclear power plants. Uncontrolled chain reactions are used in nuclear weapons

Nuclear Fusion Fusion: nuclei of two atoms combine The sun and other stars are powered by fusion of H into He Requires extremely HIGH temperatures What state is matter in at such high temperatures? PLASMA Fusion + ENERGY (17.6 MeV)