Chapter 21 Nuclear Chemistry Section 21.1 Types of Radioactivity

Objectives: Analyze Common Sources of Background Radiation, Compare and Contrast Alpha, Beta and Gamma Radiation, Apply the concept of Half-Life of a Radioactive Element

In a chemical reaction, what is the main subatomic particle involved In a chemical reaction, what is the main subatomic particle involved? The ELECTRON

In a nuclear reaction, what is the main area of the atom involved? The NUCLEUS https://www.youtube.com/watch?v=hORaebYWDwk

The Nucleus Remember that the nucleus is comprised of the protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom.

Nuclear Notation A nucleus can LOSE or GAIN protons and neutrons (Adding or losing Protons changes the identity of an element) When writing nuclear equations, it is important to indicate the isotopes of the given elements.

Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238

Discovery of Radioactivity Henri Becquerel discovered that uranium compounds spontaneously give off radiation.

Discovery of Radioactivity (cont.) Marie and Pierre Curie concluded that a nuclear reaction was taking place within the uranium atoms. Radioactivity is the spontaneous emission of radiation by an unstable atomic nucleus. **Objects do not become radioactive when subjected to radiation unless they actually absorb radioactive elements.

Radiation vs. Radioactivity: What’s the difference? Nuclear radiation is made up of matter or energy that has been released by a substance

During a nuclear reaction, what can an unstable atomic nucleus do? It can gain or lose protons and/or neutrons

Radioactivity It is not uncommon for some nuclides of an element to be unstable, or radioactive. We refer to these as radionuclides. There are several ways radionuclides can decay into a different nuclide.

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Types of Radioactive Decay

Radioactive Decay Radioactive Decay – the release of radiation by radioactive isotopes (radioisotopes)

Alpha Decay Streams of alpha particles Helium nuclei which consist of 2 protons and 2 neutrons    Alpha radiation does NOT deeply penetrate into matter and is easily stopped; the particles have a + charge and are relatively large in comparison to other forms of radiation A new nucleus with an atomic number that is 2 less, and a mass number that is 4 less than the original nucleus

He U Th He Alpha Decay: + 4 2 238 92  234 90 4 2 Alpha Particle- consist of a He nuclei with 2 protons and 2 neutrons (α) Loss of an -particle (a helium nucleus) He 4 2 U 238 92  Th 234 90 He 4 2 + Must be balanced! Sum of mass numbers and atomic numbers must be the same on the left and right.

Try Thorium 230 (Th-230) and Radon 222 (Rn-222) 230Th → 226Ra + 4He 90 88 2 222Rn → 218Po + 4He 86 84 2

Radioactive Decay (cont.) Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.

Beta Decay Beta particles : high energy electron with a 1- charge; represented 2 ways:   Smaller & faster than alpha, so greater penetrating power; stopped by thick materials A beta decay results in a new nucleus with an atomic number that is 1 greater than that of the original and a mass number that is the same This electron is NOT from outside the nucleus, it is produced by the change of a neutron into a proton and an electron Carbon-14 undergoes beta decay, so does Sulfur -35

Beta Decay: Beta Particle- a high energy electron with a 1- charge Loss of a -particle (a high energy electron)  −1 e or I 131 53 Xe 54  + e −1

Try Magnesium 27 (Mg-27) and Sulfur 35 (S-35) 27Mg → 27Al + 0 e 12 13 -1 35S → 35Cl + 0 e 16 17 -1

Can be stopped by heavy clothing, sheets of metal, blocks of wood Beta Blockers Can be stopped by heavy clothing, sheets of metal, blocks of wood

Gamma Decay Gamma Ray : high energy form of electromagnetic radiation w/o mass or charge Most penetrating and causes the greatest damage Energy is the only “product” of gamma decay; represented by: Occurs with other types of decay, and does not affect mass number or atomic number

Gamma Decay: Gamma Ray – high-energy form of electromagnetic radiation without charge or mass (γ) (Gamma typically does not occur alone – occurs with β or α) Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 

Can be stopped by thick blocks of lead or concrete Gamma Blockers Can be stopped by thick blocks of lead or concrete

Positron Emission Is a type of radioactive decay called beta plus decay. Positron emission results in a new nucleus with an atomic number that is 1 less than that of the original and a mass number that is the same A proton becomes a neutron. From this process a positron and a neutrino are ejected from the nucleus.

e C B e Positron Emission: + Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 1 C 11 6  B 5 + e 1

Positron Emission Example: 124Ba 124Cs + 0e 56 55 1 56 55 1 Try Si -26 and La -125

Practice Problem Cont… Write a balanced equation for the nuclear reaction which neon-23 decays to form sodium-23 and determine the type of decay.

Example Problems p. 747 #1 and 2 #1) Alpha 226Ra → 222Rn + 4He 88 86 2 #2) Beta 23Ne → 23Na + 0 e 10 11 -1

Practice Problem… Write the balanced equation for the nuclear equation for the nuclear reaction in which uranium-234 decays to form thorium-230 and determine the type of decay.

https://www. youtube. com/watch https://www.youtube.com/watch?v=TJgc28csgV0&index=11&list=PLKEmXepzBsY9Zx4EHN27HHSQyvb4Xysb4

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Rates of Decay

Neutron-Proton Ratios Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart.

Neutron-Proton Ratios Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons is an important factor.

Neutron-Proton Ratios For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

Stable Nuclei There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission.

Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Some Trends Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.

Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

Particle Accelerators These particle accelerators are enormous, having circular tracks with radii that are miles long.

Detecting Radioactivity Radioactivity cannot be seen, heard or touched – it has no smell or taste

Section 21.1 Radioactive Decay (cont.) There are several methods used to detect radiation. photographic film scintillation counters A Geiger counter detects ionizing radiation, which is radiation energetic enough to ionize matter with which it collides.

Half-Life and Radioisotope Dating The rate of spontaneous nuclear decay cannot be changed. Radioactive decay rates are measured in half-lives. The half-life is the time it takes for half of a given amount of a radioactive isotope to undergo decay. (T1/2) Can be fractions of a second or billions of years

Radioactive Dating Organisms take in carbon during their lifetime. After death, no new carbon is taken in. C-14 remaining is measured, compared with how much was in the material when it was alive. Age of object can be estimated. T1/2 of C = 5730 yrs Other materials : K-40 (1.25 billion years), U-238 (4.5 billion years), Rb-87 (48 billion years)

Half-Life and Radioisotope Dating (cont.)

Half-Life and Radioisotope Dating (cont.) Four different isotopes are commonly used for dating objects: carbon-14, uranium-238, rubidium-87, and potassium-40. Carbon-14 dating is commonly used to measure the age of fossils. To date objects that are more than 60,000 years old, carbon-14 dating cannot be used since there is very little carbon left to measure.

Example Problems p. 755 #3 and 4 #3) A rock was analyzed using K-40. The half life of K-40 is 1.25 billion years old. If the rock had only 25% of the K-40 that would be found formed in a rock today – Calculate how long ago the rock was formed. 25% of K-40 100% → 50% → 25% 1 2 1.25 x 2 = 2.5 BY

#4) Ash from an early fire pit was found to have 12 #4) Ash from an early fire pit was found to have 12.5% as much carbon-14 as would be found in a similar sample of ash today. How long ago was the ash formed? 12.5 % of C-14 100% → 50% → 25%→ 12.5 1 2 3 5730 x 3 = 17,190 yrs

So… ***The end result of all types of decay will be a stable nucleus***

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