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Atomos (Democritus c. 460 – c. 370 B.C.) The first suggestion that atoms make up all matter The Law of Conservation of Matter (Lavoisier 1743-1794) Matter is neither created nor destroyed in a chemical reaction. The Law of Constant Composition (Joseph Proust 1754-1826) A given compound always contains the same elements in the same proportions by mass. Example: H2O is always 88.9% oxygen and 11.1% hydrogen Dalton’s Atomic Theory of Matter (John Dalton 1766-1844) Postulates 1-4 (pg. 92 in book) 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical, but they differ from those of any other element. 3. Atoms are neither created nor destroyed in any chemical reaction. They are dissociated, combined or recombined. 4. Compounds are formed when atoms of different elements combine with one another. A given compound always has the same relative numbers and kinds of atoms. Dalton’s Model Described atom as a Hard Dense, Indivisible sphere Thomson’s Model Credited with the discovery of the electron using a cathode ray tube under the influence of an electric field. Described atom using his “Plum Pudding Model” where Pudding = + positively charged Plums = - negatively charged Rutherford’s Model First discovered alpha, beta and gamma radiation. Used alpha radiation in the Gold Foil Experiment to discover the NUCLEUS. (This led to the proton discovery) Bohr (Orbital) Model “Electrons exist in energy levels outside the nucleus. The further away the electron is from the nucleus, the more energy it requires to exist there. + Chadwick (Rutherford’s student) then discovered the neutron. The History of Atomic Structure. J.J. Thomson’s (1856-1940) Discovery of the ELECTRON Stream of particles Cathode Ray Tube Battery or other source of electricity The History of Atomic Structure. J.J. Thomson’s (1856-1940) Discovery of the ELECTRON Electric Field Stream of particles (+) Cathode Ray Tube (-) Battery or other source of electricity The History of Atomic Structure. Earnest Rutherford (1871-1937) Discovery Alpha, Beta & Gamma radiation Magnetic Field Lead Block (+) b g a (-) Radioactive material The History of Atomic Structure. Earnest Rutherford (1871-1937) Gold Foil Experimentation Lead Block Radioactive material The History of Atomic Structure. Earnest Rutherford (1871-1937) Gold Foil Experimentation Lead Block Radioactive material The History of Atomic Structure. Earnest Rutherford (1871-1937) Gold Foil Experimentation Lead Block Radioactive material The nucleus is a very small positively charged core containing protons and neutrons. Negatively charged electrons are extremely tiny and occupy the vast majority of the atom’s volume. • The smallest particle of an element that retains the chemical identity • composed of three main parts. Proton: •Positively charged •Found in the nucleus •Atomic number (z) = protons Electrons: Neutron: •Neutral (no charge) •Found in the nucleus •Atomic mass – atomic number = neutrons. Protons and neutrons give the atom its mass. Particle Proton Neutron Electron Charge (C) +1.602x10-19 0 -1. 602x10-19 •Negatively charged •Exist in electron cloud •Number of electrons = number of protons. (in a neutral atom) Electron Cloud gives the atom its volume. Mass (g) 1.673x10-24 1.675x10-24 9.109x10-28 Mass (amu) 1.0073 = 1 1.0087 = 1 0.0006 = 0.0 The atomic number (z) describes the number of protons in an element. (The number of protons is equal to the number of electrons in a neutral atom) 19 K 39.098 Potassium The atomic number is the smallest of the two numbers seen in each box on the periodic table. The mass number describes the average mass of the element isotopes. 19 K 39.098 Potassium The mass number (m) is the largest number in each box on the periodic table. The mass number – the atomic number is equal to the number of neutrons in an atom. m – z = neutrons •Atoms that have the same number of protons but different numbers of neutrons. •Isotopes of an element exhibit identical chemical behavior. •This is why H-1 and H-2 both bond with oxygen to form water. •Carbon-14 and carbon-12 bond with oxygen to form CO2 Isotope symbols Carbon-12, C-12 or P N E Protium Hydrogen’s 3 isotopes Deuterium Tritium 2 1H 1 1H H-1 12 C 6 H-2 3 1H H-3 Ions: electrically charged atoms. o Atoms that gain electrons are negatively charged Nonmetals gain electrons o Atoms that lose electrons are positively charged Metals lose electrons Potassium 19 protons 19 electrons 20 neutrons (m – z: 39 – 19) If potassium loses an electron: Potassium: + 19 protons (+ charges) - 18 electrons (- charges) +1 overall charge of the potassium ion (also called the oxidation number or state) Ion representations Cs+1 Cs+ 133 +1 Cs 55 All three of these indicate that cesium has given 1 electron away. Isoelectric: Indicates the number of electrons in an ion matches the number of electrons in the noble gas it wants to become to increase stability. So what Noble Gas is Cs+1 isoelectric with? Xenon Isotope symbols Cesium-133 Cs-133 133 55 Cs How many protons, neutrons, and electrons are present in the 56 +2 Fe 26 P = 26 N=m–z 56 – 26 = 30 E = 24 +2 indicates two more protons than electrons. What is the chemical symbol for the ion with 15 protons and 18 electrons? P-3 31 15 P-3 Atomic Mass Scientist’s chose to define an atomic mass unit in terms of an arbitrary standard—a carbon-12 atom. This means that 1 atomic mass unit is 1/12 of the mass of a carbon-12 atom. 1 amu = 1/12 (mass of C-12 atom) = 1.673 x 10-24 g Isotopes of Three Common Elements Element Symbol Mass Mass (amu) Number Carbon C-12 C-13 12 13 12 13.003 98.89% 1.11% 12.01 Chlorine Cl-35 Cl-37 35 37 34.969 36.966 75.53% 24.47% 35.45 Si-28 Si-29 Si-30 28 29 30 27.977 28.976 29.974 92.21% 4.70% 3.09% Silicon % Average abundance Atomic mass 28.09 How did you get the value for the Mass? Get the mass of the isotope and multiply it by the % abundance. For example: To determine the actual mass, as seen of the periodic table, for Chlorine: 1) Determine the mass in amu of each isotope and then find the total mass by adding up the masses of each isotope. 35 17 37 17 Cl 75.53% (35)(0.7553) = 26.4355 Cl 24.47% (37)(0.2447) = 9.0539 35.4894 amu or g Look at the mass on the periodic table… Do you think there are really more isotopes of Chlorine? Compute the average atomic mass for silicon Si-28 92.21% abundant 28 x 0.9221 = 25.8188 Silicon Si-29 4.70% abundant 29 x 0.0470 = 1.363 Si-30 3.09% abundant 30 x 0.0309 = 0.927 28.1088 amu or g Radioactive Decay When atoms emit alpha, beta or gamma radiation, it is undergoing a radioactive decay. Decay occurs due to instability within the nucleus. As the ratio of protons to neutrons becomes more skewed, the nucleus becomes more unstable. All isotopes with an atomic number greater than 83 are unstable. Nuclear equation: an equation that keeps track of the reaction’s components. Left side of equation has parent nuclide. Arrow means “yield” Right side of equation has daughter nuclide and radiation particle Changes in the Nucleus In Chemical reactions, atoms interact only through their outer electrons while their nuclei remain unchanged. Nuclear reactions change an atom’s nucleus while the electrons remain unchanged. Nuclear reactions produce three kinds of radiation: alpha radiation Beta radiation, gamma radiation. 4 2 0 -1 a+2 b g Alpha (a )Radiation-Emission 1. Alpha radiation consists of rapidly moving helium nucleus. a. A helium nucleus consists of 2 protons and 2-neutrons. b. A lpha radiation is represented by the following forms: 4 2 He+2 4He+2 4 a+2 There are others, but you get the drift!! 2. Alpha particles travel at about one-tenth the speed of light. 3. A thin sheet of paper or clothing can stop alpha particles. It will not penetrate the skin on your body. 4. In alpha emission, the parent nuclide decays into a daughter nuclide by emitting an alpha particle. 230 90 Th 4 a+2 2 + 226 Ra 88 **During alpha decay the emission results in two less protons. ** Also notice there is a change in mass; it decreases by 4. Beta (b- )Radiation-Emission 1. Electrons or positrons are emitted from the nucleus at a very high speed, often approaching the speed of light ; thus < 3.00x108 m/s a) A positron is a positively charged particle the same size as an electron. 2. Very high kinetic energy associated with beta or positron particle release. 3. Beta radiation is stopped by a few millimeters of aluminum. It will penetrate the outside of your body only 1-5 mm. It will do more damage to the inside of your body a. Beta radiation is represented by the following forms: 0 -1 e 0e 0b-1 There are others, but you get the drift!! 4. The nuclear equation to represent the radioactive decay neon-23 by b- emission is: 23 10 Ne 0 -1 b + 23 Na 11 **During the decay of Neon, the emission results in the conversion of a neutron to a proton. ** Also notice there is no change in mass. 5. Nuclide that decay by beta emission have too many neutrons in the nucleus for the number of protons present. Gamma (g )Radiation-Emission 1. Gamma radiation is a form of electromagnetic radiation similar to, but more energetic than,X-rays. 2. Gamma radiation is a pure energy form and the most dangerous of the three radiation types. 3. Gamma radiation penetrates the farthest; several centimeters of lead, or an even greater thickness of concrete. It will travel right through our bodies and thus has the potential to do much damage. 4. Gamma radiation can be emitted along with alpha radiation and/or along with beta radiation. a. Gamma radiation is represented by: 0 g , Write an equation for the alpha decay of uranium-238 235 92 U 231 90 Th + 4 +2 a 2 Write the equation for the beta decay of sodium-24. 24 11 Na 24 12 Mg + 0b -1 The amount of time a radioactive isotope needs to decompose to half it’s initial mass. AE = A0• 0.5 t t½ •AE is the amount of substance left •A0 is the original amount of substance •t is the elapsed time •t1/2 is the half-life of the substance 100.0 g of isotope W has a half-life of 60 seconds. How much of isotope W is left after 3.0 minutes. Show your work. AE = A0• 0.5 t t½ 3.0 1.0 = (100.0g)(0.5 ) = 12.50g Sig. figs depend on given mass. Solve the original equation for t½ AE = A0• 0.5 T1/2 = log 0.5 t t½ Given 150.0g of isotope Y. IF over a period of 10 .0days the decay left only 12.0 g, what would Y’s half-life be? Show your work. * 10.0 days log 12.0g 150.0 g Ans. 2.74 days Solve the original equation for t AE = A0• 0.5 t= t t½ log 20.0g 250.0 g log 0.5 The half-life of isotope X is 10.0 years. How many years would it take for a 250.g sample of X to decay to 20.0 g? Show your work. * 10.0 days Ans. 36.4 years