Nuclear Chemistry Chapter 22 The Nucleus G Protons

Nuclear Chemistry Chapter 22

The Nucleus G Protons and neutrons are collectively referred to as nucleons. G In nuclear chemistry, an atom is referred to as a nuclide, and is identified by the number of protons and neutrons. G Nuclides can be written as 228 88 Ra or as Radium-228.

An Error in Mass? ? ? G Since an atom is made up of protons, neutrons and electrons, you might assume that the mass of the atom would be equal to the sum of the individual subatomic particles. G Let’s look at 4 He: G 2 protons: (2 x 2 1. 007276 amu) G 2 neutrons: (2 x 1. 008665 amu) G 2 electrons: (2 x 0. 0005486 amu)

An Error in Mass? ? ? G When you add the components of the He atom, you get a mass of 4. 032979 amu. G However, the atomic mass of He has been measured to be 4. 00260 amu. G So, as you might have noticed the calculated mass is 0. 03038 amu more than the actual mass. G How can this be so?

Mass Defect G The difference between the measured mass and the sum of its protons, neutrons and electrons is called the mass defect. G The differences in mass can be explained by Einstein’s equation : G E= mc 2

E=mc 2 E stands for energy m stands for the mass defect c 2 stands for the speed of light squared In order to use the equation, you must convert amu to kg, to match the units for energy (kg m 2/s 2) G 1 amu= 1. 6605 x 10 -27 kg G G

Calculating Nuclear Binding Energy G So using the example of 24 He from before: G 0. 03038 amu x 1. 6605 x 10 -27 kg 1 amu equals 5. 0446 x 10 -29 kg. G E= (5. 0446 x 10 -29 kg)(3. 00 x 108 m/s)2 G = 4. 54 x 10 -12 kg m 2/s 2 or J

Nuclear Binding Energy G The nuclear binding energy, that we just calculated for He, is the energy released when a nucleus is formed from nucleons. G This binding energy per nucleon is used to compare the stability of different nuclides. G To find the binding energy per nucleon, you simply divide the nuclear binding energy by the total number of nucleons present.

Stability of the Nuclide G The higher the binding energy per nucleon, the more tightly the nucleons are held together. G Generally, the elements with intermediate atomic masses are those that are most stable.

Band of Stability G When the number of protons of stable nuclei are plotted against the neutrons, a belt-like graph is obtained.

Stability G Atoms that have low atomic numbers are most stable with a 1: 1 neutron to proton ratio. ie: Helium G As the atomic number increases, the most stable ratio increases to 1. 5: 1. ie: Lead-206 has 124 neutrons to 82 protons (1. 51: 1 ratio).

Strong Nuclear Force G The trend in stability is explained by the relationship between the nuclear force and the electrostatic forces between protons. G You know that protons repel each other as far as the electrostatic force goes. (“like repels like”). G The strong nuclear force allows protons to be attracted, but only to protons that are close to one another.

Strong Nuclear Force G As the atomic number increases, the electrostatic force between protons increases more quickly than the nuclear force. G More neutrons are required to strengthen the strong nuclear force. G However, beyond the atomic number 83 (Bismuth), no stable nuclides exist.

Magic Numbers G Stable nuclei tend to have even numbers of nucleons. G This is because the nucleus is most stable when nucleons (like electrons) are paired! G The most stable nuclides are those that have 2, 8, 20, 28, 50, 82, or 126 protons, neutrons or total nucleons.

Nuclear Shell Model G This theory says that nucleons exist in different energy levels, or shells, in the nucleus. G 2, 8, 20, 28, 50, 82, 126 are representative of completed nuclear energy levels and are called magic numbers.

Nuclear Reactions G A nuclear reaction, or nuclear decay reaction, is a reaction that affects the nucleus of an atom. G Unstable nuclei undergo spontaneous changes that change the number of protons and neutrons in an atom. G Large amounts of energy are given off in this process.

How to Solve Nuclear Reactions G 1) Remember that the total of atomic numbers and mass numbers should be the same on each side of the equation. G *Notice that when the atomic number (number of protons) changes, the identity of the element changes. This is called transmutation.

Helpful symbols G Neutrons are represented as 10 n G Electrons are represented as -10 e G Protons are represented as 11 p

Examples: G 212 Po 84 42 He + _____ G Mass number: 212 - 4 = 208 G Atomic number: 84 - 2 = 82 G So the missing nuclide must have a mass of 208 and an atomic number of 82. G Answer: 208 Pb 82

Complete these on your own: G 1) G 2) G 3)

History of Radioactive Decay G 1896 - Henri G Even though the Becquerel plate was protected discovered from sun exposure, radioactivity when a it was still exposed uranium compound due to the was left on a radioactive x-rays photographic plate. given off by uranium.

Radioactive Decay G Defined: the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by the emission of particles, electromagnetic radiation, or both. G This is what happened to the photographic plate in Becquerel’s experiment.

The Curies G Marie and Pierre found that of the elements known in 1896, only uranium and thorium were radioactive. G Marie is credited with the discovery of Radium and Polonium, both of which are also radioactive.

Types of Radioactive Decay Type Symbol Alpha particle Beta particle Positron Gamma ray Charge +2 Mass (amu) 4. 00260 -1 0. 0005486 +1 0. 0005486 0 0

Alpha Emission G An alpha particle has 2 protons and 2 neutrons bound together and is emitted during some types of nuclear decay. G They can be stopped by clothing or paper because they are so large. G Example reaction:

Beta Emission G A beta particle is an electron emitted from the nucleus during some kinds of radioactive decay. G They can be stopped by aluminum foil or thin metals. G Example reaction:

Positron Emission G A positron is a particle that has the same mass as an electron but has a positive charge. G They can be stopped by aluminum foil and thin metals as well. G Example reaction:

Electron Capture G In electron capture, an inner orbital electron is captured by the nucleus of its own atom. G Example reaction:

Gamma Emission G Gamma rays are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state.

Gamma Rays G Unlike the other forms of radiation we have discussed, it takes a lot to stop gamma rays from penetrating your skin. G Dense materials, like lead or concrete, are used to stop gamma rays.

Decay Series G Most of the decay G The heaviest nuclide reactions we have of each series is studied thus far only called the parent involve one nuclide. transformation. G All the nuclides G However, it is often produced by the necessary for a parent are called series of these daughter nuclides. reactions to occur before a stable nuclide is reached.

Uranium-238 Decay Series

Artificial Transmutations G Some radioactive G They are made by nuclide cannot be bombardment of found naturally on stable nuclei with the earth, so they charged and are called artificial uncharged particles. radioactive nuclides. Rutherford’s apparatus

Artificial Transmutations G Neutrons are often G When alpha used because of particles or protons their neutral charge. are used, they must be accelerated to obtain the energy needed to approach the positively charged nucleus.

Getting Radioactive elements G Some radioactive substances are found in nature. G However, some can be created through artificial transmutation. G Examples:

Radiation Exposure G Roentgen- unit used to measure nuclear radiation, equal to the amount of radiation that produces 2 x 109 ion pairs when it passes through 1 cm 3 of dry air. G rems- (roentgen equivalent, man) measures radiation damage to human tissue G 1 rem is the quantity of ionizing radiation that does as much damage to human tissue as 1 roentgen of high voltage X-rays.

Exposure to Radiation G The long term effects of continued radiation damage include cancer and DNA mutations, causing genetic abnormalities. G Everyone is exposed to background radiation. The amount of exposure depends on certain activities. The average exposure in one year is 0. 1 rem. The maximum permissible dose is 0. 5 rem per year. G http: //www. epa. gov/rpdweb 00/understand /calculate. html

Radiation Sickness G The larger the dose received at once the greater the effect on the whole body. G Generally- 25 rem and under cannot be detected G 100 rem reduces white blood cell count temporarily G >100 rem person experiences nausea, vomiting and a reduction in white blood cell count G >300 rem white cell count at zero and diarrhea, hair loss and infection occur

Lethal Dose G The lethal dose of a substance is often referred to as LD 50. This means that it is expected to cause death in 50% of the people with exposure. G The LD 50 for humans of radiation is 500 rems. G Dosages of 600 rem would be fatal to all humans within a few weeks.

LD 50 for life forms other than humans G G G Insects- 100, 000 rems Bacterium- 50, 000 rems Rat- 800 rems Humans- 500 rems Dog- 300 rems

Detecting radiation in your surroundings G Geiger counterdetect radiation by counting electric pulses, kind of sounds like a metal detector G Film Badges- used to measure exposure of people working with radiation

Cloud Chambers G Used to detect alpha and beta particles G Chamber is filled with ethanol or water vapor. When the particles collide with air, ions are formed. G The vapor condenses around the path of ions and makes it visible.

Applications of nuclear Chemistry G 1) Radioactive dating- age is measured by accumulation of daughter nuclides or disappearance of parent nuclides G 2) Radioactive tracers- radioactive atoms included in substances so that movement can be followed by radiation detectors G ie: technetium-99 can be used to detect bone cancer.

Applications of Nuclear Chemistry G 3) Irradiation of food G Using radiation to kill bacteria on food, like producetomatoes, blueberries, mushrooms, and even spinach G This is in response to the food borne illnesses that have been plaguing the US. G E. coli G Listeria G Salmonella

Radiation Doses for Therapeutic Procedures G Lymphoma- 4500 rem G Skin cancer- 50006000 rem G Lung cancer- 6000 rem G Brain tumor- 60007000 rem G Remember that these are not whole body dosages. G Many external beam cancer treatments are targeted to specific areas of the body.