Unit 2: Atomic Theory Notes The changing of a substance into one or more new substances is known as a chemical reaction. Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes Law of definite proportions: a chemical compound contains the same elements in exactly the same proportions regardless of the amount or source of the compound (water is always H2O) Law of multiple proportions: if two or more different compounds are composed of the same two elements, then the ratio of the masses are always a ratio of small whole numbers (CO2, not CO1.5) All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created, or destroyed. Atoms are divisible into even smaller particles. A given element can have atoms with different masses. An atom is the smallest particle of an element that retains the properties of that element. The nucleus is a very small region located at the center of an atom (contains protons and neutrons). The electron cloud is a region outside the nucleus where electrons are most likely to be found (regions of probability).
Particle Name Proton Symbol Charge Mass Location p + +1 1 amu Nucleus Neutron n o Neutral 1 amu Nucleus Electron e - -1 -- Electron Cloud Discovery of the Electron Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles. These particles were named electrons. Cathode Ray Tube Experiment J.J. Thomson s cathode- ray tube experiment discovered the electron.
Robert Milikan s Oil Drop Experiment Robert A. Millikan s oil drop experiment measured the charge of an electron.
Discovery of the Nucleus Rutherford s Gold Foil Experiment The results of the gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge. Rutherford called this positive bundle of matter the nucleus. Composition of the nucleus Most atomic nuclei are made of protons and neutrons. Atoms are electrically neutral because they contain equal numbers of protons and electrons. A neutron has no charge. Atomic Number Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. The atomic number (Z) of an element is the number of protons of each atom of that element.
Isotopes Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. Mass Number The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. Hyphen notation: The mass number is written with a hyphen after the name of the element. uranium- 235 Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number. The Sub- Sub Atomic Particles 235 92 U Quarks elementary particles that are the basic building blocks of protons and neutrons. It is believed that neutrons and protons are each composed of 3 quarks. 6 flavors of quarks up, down, charm, strange, top, and bottom Quarks were hypothesized in 1964 and proven to exist in 1968, the last of the quarks (top) was discovered in 1995. Proton composed of 3 quarks (up, up, and down)
Neutron composed of 3 quarks (up, down, and down) Colors of the quarks are not important. Nuclear Chemistry Nuclear Chemistry is based on Albert Einstein s famous equation: E = mc 2 In this equation E (energy) is equal to the product of the mass (m) of the material multiplied by the square of the speed of light (c). Speed of light = 3.0 10 8 m/s This equation shows that a small amount of mass can produce a huge amount of energy. The work of Einstein and others on the Manhattan Project led directly to the development of the atomic bomb and the end of WWII. August 6, 1945 1 st atomic bomb dropped on Hiroshima, Japan (Little Boy). In the Little Boy bomb, less than 1 kg of U underwent nuclear fission. Applying Einstein s equation, E = mc 2 E = (1 kg) (3.0 10 8 m/s) 2 E = 9 10 16 kg. m 2 /s 2 = 9 10 16 Joules (J)
August 9, 1945 2 nd atomic bomb dropped on Nagasaki, Japan (Fat Man) Over 246,000 people were killed as a direct result of these atomic bombs. Many thousands more (the exact number is unknown) died of side effects in the months and years afterward. In addition, many others suffered life- long consequences due to radiation exposure. The United States remains the only country to ever use an atomic weapon. Chemical reactions involve the electrons of an atom. A nuclear reaction is a reaction that affects the nucleus of an atom. A transmutation is a change in the identity of a nucleus as a result of a change in the number of its protons. Radioactive decay is the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, radiation, or both. All of the nuclides beyond atomic number 83 are unstable and thus radioactive. Types of Radioactive Decay Alpha Emission An alpha particle (α) is two protons and two neutrons bound together and is emitted (released) from the nucleus during some kinds of radioactive decay.!!he Alpha emission is restricted almost entirely to very heavy nuclei. Beta Emission A beta particle (β) is an electron emitted from the nucleus during some kinds of radioactive decay. To decrease the number of neutrons, a neutron can be converted into a proton and an electron. Gamma Emission!!!n! p +!!! β Gamma rays (γ) are high- energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state.
Comparing Alpha, Beta, and Gamma Image Source: http://wpscms.pearsoncmg.com/wps/media/objects/3661/3749680/aus_content_01/fig01-21.jpg Nuclear Fission In nuclear fission, a very heavy nucleus splits into more- stable nuclei of intermediate mass. Enormous amounts of energy are released. Nuclear fission can occur spontaneously or when nuclei are bombarded by particles.
Nuclear Fusion In nuclear fusion, low- mass nuclei combine to form a heavier, more stable nucleus. Nuclear fusion releases even more energy per gram of fuel than nuclear fission. If fusion reactions can be controlled, they could be used for energy generation. Average Atomic Masses of Elements Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. The average atomic mass of an element depends on both the mass and the relative abundance of each of the element s isotopes. Formula for Average Atomic Mass (mass of element % abundance) + (mass of element % abundance) 100