che106_lecture4 - Chemistry 106 Lecture 4 Topics: ...

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Unformatted text preview: Chemistry 106 Lecture 4 Topics: Atomic Theory and Structure Chapter 2.1 – 2.5 John Dalton: Atomic Theory •  The BriCsh chemist John Dalton (1766- 1844) is credited with the formulaCon of the atomic theory of ma-er. •  Atomic theory offers an explanaCon of the structure of maMer in terms of combinaCons of very small parCcles. •  There are four postulates that define Dalton’s theory. Dalton’s Postulates of Atomic Theory •  Postulate #1: All maMer is composed of indivisible atoms. An atom is an extremely small parCcle of maMer that retains its idenCty during chemical reacCons. Image of iodine atoms on a platinum surface nanometers Dalton’s Postulates of Atomic Theory •  Postulate #2: An element is a type of maMer composed of only one kind of atom, each atom of a given element having the same properCes. –  Mass is one such property. Thus the atoms of a given element have a characterisCc mass. Dalton’s Postulates of Atomic Theory •  Postulate #3: A compound is a type of maMer composed of atoms of two or more elements chemically combined in fixed proporCons. •  The relaCve numbers of any two kinds of atoms in a compound occur in simple raCos. •  Water (H2O), for example, consists of hydrogen and oxygen in a 2 to 1 raCo. This is the Law of Multiple Proportions. ReacCon of Sodium and Chlorine Sodium (Na) + Chlorine (Cl) Chlorine Sodium Sodium Chloride (NaCl) Atomic Symbols •  An atomic symbol is a one– or two–leMer notaCon used to represent an atom corresponding to a parCcular element. –  Typically, the atomic symbol consists of the first leMer, capitalized, from the name of that element, someCmes with an addiConal leMer from the name in lowercase. –  Other symbols are derived from the name in another language (usually LaCn). Dalton’s Postulates of Atomic Theory •  Postulate #4: A chemical reac5on consists of the rearrangements of the atoms present in the reacCng substances to give new chemical combinaCons present in the substances formed by the reacCon. •  Atoms are not created, destroyed, or broken into smaller parCcles by any chemical reacCon. Structure of the Atom •  Although Dalton postulated that atoms were indivisible, experiments at the beginning of the 20th century showed that atoms themselves consist of parCcles. •  Experiments by Ernest Rutherford in 1910 showed that the atom was mostly “empty space.” Structure of the Atom •  These experiments showed that the atom consists of two disCnct areas: –  a nucleus, the atom’s central core, which is posiCvely charged (most of the atom’s mass) –  and one or more electrons. •  Electrons are very light, negaCvely charged parCcles that exist in the region around the atom’s posiCvely charged nucleus. Structure of the Atom: Electrons •  In 1897, the BriCsh physicist J. J. Thompson conducted a series of experiments that showed that atoms were not indivisible parCcles. •  From his experiments, Thompson calculated the raCo of the electron’s mass, me, to its electric charge, e. •  However, he could not unambiguously assign the actual mass or charge. FormaCon of Cathode Rays: Electron Beam Electron beams are invisible, but when they hit glass, they make it fluoresce (glow) green. A Magnet Bends The Beam of NegaCve ParCcles (Electrons) Structure of the Atom: Electrons •  In 1909, U.S. physicist, Robert Millikan measured the charge on the electron (1.602 x 10- 19 coulombs, a.k.a. “C”) using an Oil Drop Experiment. Structure of the Atom: Electrons •  Combining these two discoveries (from Thompson and Millikan) provided us with the electron’s mass of 9.109 x 10- 31 kg, which is more than 1800 Cmes smaller than the mass of the lightest atom (hydrogen). •  These experiments showed that the electron was indeed a subatomic parCcle. The Nuclear Model of the Atom •  Ernest Rutherford, a BriCsh physicist, put forth the idea of the nuclear model of the atom in 1911, based on experiments done in his laboratory by Hans Geiger and Ernest Marsden. •  Rutherford’s famous gold leaf experiment gave credibility to the theory that the majority of the mass of the atom was concentrated in a very small nucleus. Alpha- ParCcle ScaMering From Metal Foils •  Alpha parCcles are high- energy, posiCvely- charged parCcles (bundle of 2 protons and 2 neutrons) from spontaneous radioacCve decay. •  Most of the alpha parCcles passed straight through the foil, but some (1 in 8000) were deflected at large angles, or nearly straight back. ScaMering of Alpha ParCcles by a Gold Foil •  Most of the mass of the nucleus: 10-15 m atom is atom: 10-10 m concentrated in a posiCvely charged central nucleus. •  If the nucleus were a golf ball, the atom would be about three miles in diameter! Alpha- ParCcle ScaMering From Metal Foils What would be the feasible model for the atom if Geiger and Marsden had found that 7999 out of 8000 alpha particles were deflected straight back at the alpha-particle source? If most particles scattered straight back, then that would mean the atom contained positive particles that are scattered throughout the atom. There would be no nucleus! Nuclear Structure & Isotopes •  The end result of extensive alpha parCcle experiments was the understanding of the composiCon of atomic nuclei. •  The nucleus of an atom is composed of two different kinds of parCcles: protons and neutrons. •  Neutrons (uncharged parCcles) were discovered in 1932 by the BriCsh physicist James Chadwick. •  An important property of the nucleus is its posi*ve electric charge. Nuclear Structure & Isotopes •  A proton is the nuclear parCcle having a posiCve charge equal to that of the electron’s (a “unit” charge) and a mass more than 1800 Cmes that of the electron’s. •  The number of protons in the nucleus of an atom is referred to as its atomic number (Z). Nuclear Structure & Isotopes •  An element is a substance whose atoms all have the same atomic number. •  The neutron is a nuclear parCcle having a mass almost idenCcal to that of a proton, but no electric charge. •  Table 2.1 summarizes the masses and charges of these three fundamental parCcles. ProperCes of the Three Fundamental ParCcles Table 2.1 summarizes the masses and charges of these three fundamental parCcles. Nuclear Structure & Isotopes •  A nuclide is an atom characterized by a definite atomic number and mass number. •  The mass number is the total number of protons and neutrons in a nucleus. •  The shorthand notaCon for a nuclide consists of its atomic symbol with the atomic number as a subscript on the lel and its mass number as a superscript on the lel. Mass number sodium − 23 23 11 Na Atomic number Note: Nuclide is not synonymous with isotope. Nuclear Structure & Isotopes •  Isotopes are atoms whose nuclei have the same atomic number, but different mass numbers; that is, the nuclei have the same number of protons, but different numbers of neutrons. •  Chlorine, for example, naturally exists as two isotopes: chlorine- 35 and chlorine- 37. 35 17 Cl 37 17 Cl •  The frac5onal abundance is the fracCon of a sample of atoms that is composed of a parCcular isotope. Isotopes of Carbon and Neon Neon-20 Carbon-12 Neon-21 Carbon-13 Neon-22 Isotope Example Problem Which atom is an isotope of Atom A? Protons Neutrons Atom A 18 19 Mass # = 37 Atom B 16 19 Mass # = 35 Atom C 18 18 Mass # = 36 Atom D 17 20 Mass # = 37 Which atom has the same mass number as Atom A? Isotope Example Problem •  Naturally occurring lithium is a mixture of: 6 3 Li 7 3 Li •  Give the number of protons, neutrons, and electrons in the neutral atoms. –  Each isotope has three protons –  lithium- 6 has 6 – 3 = 3 neutrons. –  lithium- 7 has 7 – 3 = 4 neutrons. –  Each neutral isotope has three electrons. Diagram of a Simple Mass Spectrometer Separation of Neon Isotopes Ne+ Ne The Mass Spectrum of Neon •  The height of each peak in the graph indicates how much of each different isotope is present (frac5onal abundance). •  Add the peak heights and they total 1 (or 100%). •  This instrument is a very powerful analyCcal tool! 1 0.9051 0.0922 0 0.0027 Atomic Weights •  The frac5onal abundance is the fracCon of a sample of atoms that is composed of a parCcular isotope. •  We can calculate the atomic weight (average mass) of an element given the properCes of the isotopes, specifically the isotopic masses and their fracConal abundance. •  Let’s use the element boron (B) as an example. Isotopes of Boron ISOTOPE ISOTOPIC MASS (amu) B-10 10.013 0.1978 ←19.78% B-11 11.009 0.8022 ←80.22% amu = atomic mass units FRACTIONAL ABUNDANCE Total = 100% Atomic Weights Calculate the atomic weight of boron (B) from the following data: ISOTOPE ISOTOPIC MASS (amu) FRACTIONAL ABUNDANCE B- 10 10.013 0.1978 B- 11 11.009 0.8022 B- 10: 10.013 x 0.1978 = 1.981 B- 11: 11.009 x 0.8022 = +8.831 10.812 amu = atomic weight Nuclear Structure Example In the report, you also inform the science team that the gold atoms are X- 23, which have an isotopic mass of 23.02 amu, and the blue- green atoms are X- 25, which have an isotopic mass of 25.147 amu. What is the atomic mass of element X? What is the Atomic Mass of Element X? •  Use the picture to figure out how atoms are gold and how many are blue- green. –  5 atoms are gold out of 20 total •  5/20 = 0.25 –  15 atoms are blue- green out of 20 total •  15/20 = 0.75 •  Now use those fracConal abundances and the given isotopic masses to calculate the atomic mass of element X. –  (0.25 x 23.02 amu) + (0.75 x 25.147) = 5.755 + 18.860 = 24.62 amu Atomic Weights •  Dalton’s RelaCve Atomic Masses –  Since Dalton could not weigh individual atoms, he devised experiments to measure their masses relaCve to the hydrogen atom. –  Hydrogen was chosen as it was believed to be the lightest element. Dalton assigned hydrogen a mass of 1. –  For example, he found that carbon weighed 12 Cmes more than hydrogen. He therefore assigned carbon a mass of 12. Atomic Weights Example Hydrogen sulfide is a gas with the odor of rotten eggs. It is a compound of hydrogen and sulfur in the atomic ratio 2:1. A sample of hydrogen sulfide contains 0.587 g H and 9.330 g S. What is the atomic mass of S relative to H? Since the atomic ratio of hydrogen to sulfur is 2:1, divide the mass of S by ½ of the mass of hydrogen to find the relative mass of S. 9.330 g / ( ½ x 0.587 g) = 31.8/1 Atomic Weights •  Dalton’s RelaCve Atomic Masses –  Dalton’s atomic weight scale was eventually replaced in 1961, by the present carbon–12 mass scale. –  One atomic mass unit (amu) is, therefore, a mass unit equal to exactly 1/12 the mass of a carbon– 12 atom. –  On this modern scale, the atomic weight of an element is the average atomic mass for the naturally occurring element, expressed in atomic mass units. The Periodic Table •  In 1869, Dmitri Mendeleev discovered that if the known elements were arranged in order of atomic number, they could be placed in horizontal rows such that the elements in the ver5cal columns had similar properCes. The Periodic Table Periods and Groups •  The periodic table is a tabular arrangement of elements in rows and columns, highlighCng the regular repeCCon of properCes of the elements. –  A period consists of the elements in one horizontal row of the periodic table. –  A group consists of the elements in any one column of the periodic table. –  The groups are usually numbered. –  The eight “A” groups are called main group (or representaCve) elements. The Periodic Table Groups run vertical (columns) Periods run horizontal (rows) The Periodic Table •  Periods and Groups –  The “B” groups are called transi5on elements. –  The two rows of elements at the boMom of the table are called inner transi5on elements. –  Elements in any one group have similar properCes. The Periodic Table The “halogens” all have similar properties TransiCon Elements Inner TransiCon Elements The Periodic Table •  Periods and Groups –  The elements in group IA, olen known as the alkali metals, are sol metals that react easily with water. –  The group VIIA elements, known as the halogens, are also reacCve elements. –  The group VIIIA elements, known as the noble gases, generally are non- reac5ve. Metals, Nonmetals, and Metalloids •  A metal is an element that has a luster (shine), a good conductor of heat and electricity, and olen malleable and duc,le. Copper and gold are examples. •  A nonmetal is an element that does not exhibit the characterisCcs of the metal. Carbon and oxygen are examples. •  A metalloid, or semi- metal, is an element having both metallic and nonmetallic properCes. Silicon and germanium are good semiconductors. The Periodic Table Next Lecture •  Topics: Molecules, Ions, Compounds •  Text Reading: 2.6- 2.9 ...
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