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Classroom notes for: Radiation and Life 98.101.201 Professor: Thomas M. Regan Pinanski 207 ext 3283 Basic Atomic Theory An atom can be defined as the most basic unit of a chemical element – Remember: an element can be informally defined as something with unique physical and chemical properties and something that cannot be broken down into any other substances. – For example: one hydrogen atom is the smallest subdivision that exists of the element hydrogen, which has properties different from all other elements. – The word “atom” is a derivation of the Greek words “tomos” (‘to cut’) and “a” (‘not’); a result of the original thought that atoms are indivisible. http://www.nzedge.com/heroes/rutherford.html Remember Democritus and Dalton? Atoms are tiny. Object Dia. (m) atom 1.00E-10 apple 8.00E-02 earth 1.27E+07 atom/appl 1.25E-09 appl/earth 6.28E-09 An average atom has a diameter of about 10-10 m. (Radiation and Health, Luetzelschwab, p. A4) A non-SI unit of length traditionally used by chemists is the angstrom, which equals 10-10 meters. (General Chemistry, Ebbing and Wrighton, p. 13) Roughly, the size of an atom is to an apple as the size of an apple is to the earth. No one has ever viewed an atom using visible light. Electron Microscope To resolve detail that is the size of angstroms, we need a wavelength on the order of angstroms. X-Rays have wavelengths in this range, but so far no practical means have been found for focusing them. Electrons, on the other hand, are readily focused with electric and magnetic fields. The German physicist Ernst August Friedrich Ruska (1906-1988) used this wave property to construct the first electron microscope in 1933 (for which he shared the 1986 Nobel Prize for physics). (General Chemistry, Ebbing and Wrighton, p. 254) and (Asimov’s Chronology of Science and Discovery, Asimov, pp. 585-586) http://www.mos.org/sln/sem/tour20.html Transmission Electron Microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light. What you can see with a light microscope is limited by the wavelength of light. TEMs use electrons as "light source" and their much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope. You can see objects to the order of a few angstrom (10-10 m). For example, you can study small details in the cell or different materials down to near atomic levels. The possibility for high magnifications has made the TEM a valuable tool in both medical, biological and materials research. Magnetic Lenses Guide the Electrons A "light source" at the top of the microscope emits the electrons that travel through vacuum in the column of the microscope. Instead of glass lenses focusing the light in the light microscope, the TEM uses electromagnetic lenses to focus the electrons into a very thin beam. The electron beam then travels through the specimen you want to study. Depending on the density of the material present, some of the electrons are scattered and disappear from the beam. At the bottom of the microscope the unscattered electrons hit a fluorescent screen, which gives rise to a "shadow image" of the specimen with its different parts displayed in varied darkness according to their density. The image can be studied directly by the operator or photographed with a camera. TEM Scanning Tunneling Microscope The scanning tunneling microscope consists of a tungsten metal needle with an extremely fine point (the probe) placed close to the sample to be viewed. If the probe is close enough to the sample, electrons can tunnel from the probe to the sample. The probability for this can be increased by having a small voltage applied between the probe and sample. Electrons tunneling from the probe to the sample give rise to a measurable electric current. The magnitude of this current depends on the distance between the probe and the sample (as well as on the wave function of the atom in the sample). By adjusting this distance, the current can be maintained at a fixed value. As the probe scans the sample, it moves toward or away from the sample, in effect following the contours of the sample. (General Chemistry, Ebbing and Wrighton, pp. 257-258) The probe must move incredibly small distances, and this is done by an ingenious mechanism. Certain solids (piezoelectric crystals) generate a voltage when their length is changed (as when a phonograph needle rides over the grooves of a record). The reverse also occurs. Small voltage variations applied to a piezoelectric rod can generate corresponding small changes in the length. In the tunneling microscope, a variable voltage is applied to a piezoelectric rod to shorten or lengthen it. Because the probe is attached to this rod, the probe is moved toward or away from the sample. In this way the distance of the probe to the sample is adjusted by a voltage in order to maintain a constant tunneling current as the probe scans the sample. (General Chemistry, Ebbing and Wrighton, pp. 257-258) STEM Four differently shaped corrals made by iron atoms on a copper surface. http://nobelprize.org/physics/educational/microscopes/scanning/gallery/4.html STEM http://nobelprize.org/physics/educational/microscopes/scanning/index.html The Atom’s Constituents The atom isn’t indivesible; it consists of three basic building blocks. These building blocks are the commonly known neutron, proton and electron. Protons (p+) are one. • Protons can be thought of as roughly spherical objects with a mass of approximately 1.6726 x 10–27 kg, or slightly more than one atomic mass unit (amu), and a “+1” charge. (Radiation and Health, Luetzelschwab, p. A1) – Note that one amu = 1.66043 x 10-27 kg. Measuring mass in amu is simply a more convenient method when dealing with such small objects. One amu is a tiny fraction of one kilogram, in the same way that an ounce is a fraction of a pound. Neutrons (n0 or n) are a second. • Neutrons can be thought of as roughly spherical objects with a mass of approximately 1.6750 x 10-27 kg, or slightly more than one amu, and no charge (they are electrically neutral. (Radiation and Health, Luetzelschwab, p. A1) • Both protons and neutrons are known to be made of more fundamental particles known as quarks (Radiation and Health, Luetzelschwab, p. A1); however consideration of these particles is beyond the scope of this course. Electrons (e-) are a third. • Electrons have a “-1” charge, and a mass of approximately 1/1837th of either a proton or a neutron; their size and shape in comparison to the proton and the neutron is not well understood. (Radiation and Health, Luetzelschwab, p. A1) The Nucleus The protons and neutrons coexist in the nucleus, while the electrons “orbit” around it. • This is Rutherford’s planetary model of the atom; it is not entirely accurate, but serves as a useful tool for describing the atom’s structure. In fact, the best we can do is to specify a region of space in which there is a given probability of finding an electron. The nucleus holds all of the positive charge and essentially all of the mass of the atom; but it is tiny compared with the overall size of the atom. • The diameter of a medium-sized nucleus is about 10 femtometers (10-14 m); on average, the diameter of this atom is about one-tenth nanometer (1010 m). This means that the atom as a whole is about 10,000 times larger than the nucleus, but mostly empty space. (Radiation and Health, Luetzelschwab, p. A4) • If the nucleus were a grain of sand at center court in a school gym, the electrons would be in orbit in the stands. • (1/16 inch assumed diameter)*(a factor of 10,000 times larger) / (12 inches/foot) • If the nucleus were a large marble, the electrons would be orbiting over 400 feet away.(1 inch assumed diameter)*(a factor of 10,000 times larger) / (12 inches/foot) What prevents my hand from passing through the desk/table when I hit it? The negatively charged electrons in the desk repel the negatively charged electrons in my hand! Building an Atom A chemical element is defined by the number of protons in the nuclei of its atoms. The simplest element is hydrogen (H); its atoms contain one proton in the nucleus. The next simplest element is helium (He); its atoms contain two protons in the nucleus. The most massive element that exists naturally in any significant quantities is uranium (U) (we’ll hear a lot about uranium throughout the course); its atoms contain 92 protons in the nucleus. Remember, the definition of an atom is such that the number of electrons always balances the number of protons; however, there isn’t such a strict prohibition on the numbers of neutrons, as we’ll see. Scientists have developed a shorthand notation that conveys all of this information ZX X = chemical symbol of the element. • Many symbols come from the original Latin names for the elements, or from Greek or Latin words or phrases that somehow tie in with the element. sodium: Na- natrium (L.) ruthenium: Ru- Ruthenia, “Russia” (L.) silver: Ag- argentum (L.) gold: Au- aurum, “shining dawn” (L.) lead: Pb- plumbum (L.) (Handbook of Chemistry and Physics, 53rd Edition) • Lead’s symbol, Pb, is based on the element’s original Latin name plumbum, also the source of the word “plumber.” (Chemistry in the Community 4th Ed., American Chemical Society, p. 54) Z = atomic number = number of protons in the nucleus A = mass number = total of number of protons + number of neutrons in the nucleus By this notation: • hydrogen is 1H; • helium is 2He; • uranium is 92U. and – The shorthand can be further simplified. For example, we know Z = 2 for helium, so we can omit writing it for helium: 4He. We can even write it as: He-4. The Electrons Electron Shells • The electrons exist without radiating energy in electron shells around the nucleus. • The shells are labeled: K, L, M, N, O, P, and Q. • Generally, the electron energies in each shell increase from K to Q; for instance, L shell electrons are more energetic than K shell electrons. Think of the K shell as being the “innermost” shell; every atom except hydrogen has two electrons in the K shell. The shells “beyond” the K shell can hold maximum numbers of electrons dictated by quantum mechanics, although the valence (“outermost”) shell will never contain more than eight electrons. Chemical Bonds The type of interaction between atoms depends on the electrons and is known as a chemical bond. • If atoms come together and bond, there should be a net decrease in energy, because the bonded state should be more stable and therefore at a lower energy level. (General Chemistry, Ebbing and Wrighton, p. 315) • There are three main types of chemical bonds: ionic, covalent, and metallic. (General Chemistry, Ebbing and Wrighton, p. 312) One type is an ionic bond. – Consider the example of chlorine and sodium atoms. • The chlorine has a negative charge, because it gained an electron, while the sodium atom now has a positive charge, because it lost an electron. • Both are now ions; an ion is an electrically charged particle obtained from an atom by adding or removing electrons. (General Chemistry, Ebbing and Wrighton, pp. 47-48) The chlorine atom is considered an anion (negative ion), and the sodium atom is considered a cation (positive ion). (General Chemistry, Ebbing and Wrighton, p. 313) • The oppositely charged ions attract each other and “stick together” in an ionic bond, forming sodium chloride (NaCl- table salt). covalent bond A covalent bond is formed by the sharing of a pair of electrons between atoms. (General Chemistry, Ebbing and Wrighton, p. 323) • Draw an H2 molecule as an example- neither atom has gained or lost electrons; there are no ions. • Consider the formation of a covalent bond between two H atoms to give the H2 molecule. As the atoms approach one another, their 1s orbitals begin to overlap. Each electron can then occupy the space around both atoms. In other words, the two electrons can be shared by the atoms. The electrons are attracted simultaneously by the positive charges of the two hydrogen nuclei. This attraction that bonds the electrons to both nuclei is the force holding the atoms together. Thus, while ions do not exist in H2, the force that holds the atoms together can still be regarded as arising from the attraction of oppositely charged particles: nuclei and electrons. (General Chemistry, Ebbing and Wrighton, pp. 323-324) • A polar covalent bond is a covalent bond in which the bonding electrons spend more time near one atom than the other. (General Chemistry, Ebbing and Wrighton, p. 327) Isotopes A chemical element is defined by atomic number (Z). • However, for a given atomic number, there may be several possible values for the mass number (A); i.e., the element can have nuclei with different numbers of neutrons.A useful mnemonic is: isotoPesame number of Protons • For example, the following are all isotopes of hydrogen, because each has one proton (Z=1). – 1H – 1H – 1H (H - hydrogen) (D - deuterium) (T - tritium; radioactive) • There are no isotopes of hydrogen with three neutrons because nuclei with large imbalances between the number of protons and neutrons will not exist very long; this point will be briefly touched upon again. • Isotopes of an element have nearly the same chemical properties (General Chemistry, Ebbing and Wrighton, p. 42), because they have the same number of electrons arranged in the shells in essentially the same fashion. • Thus, isotopes of an element can’t be separated by chemical means. • Isotopes of an element, if radioactive, will have different radioactive properties (remember Soddy?)