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Fondamenti di Chimica & Educazione Ambientale Lecture 2 Atoms, Molecules & Ions “Leucippo è l’iniziatore di quel famigerato sistema atomistico, che, risuscitato modernamente, è passato come principio fondamentale dell’indagine razionale sulla natura. Esaminato questo sistema per sè, lo troviamo bensì assai povero e tale da offrire poco alla ricerca; ma deve essere riconosciuto a Leucippo il grande merito di aver distinto, come si dice comunemente nella nostra fisica, le qualità generali dei corpi sensibili, o le primarie dalle secondarie, o le essenziali dalle non essenziali.” Georg Wilhelm Friedrich Hegel Jena 1805 - 1806 Georg Wilhelm Friedrich Hegel “Lezioni di Storia della Filosofia” La Nuova Italia Editrice Vol. 1, p. 231, 1963 Elements Pure substances composed of only one type of atom. These are found and listed on the Periodic Table Atoms The smallest particle of an element that retains all of the properties of the element. Chemical Element u A species of atoms; all atoms with the same number of protons in the atomic nucleus. u A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. Sometimes this concept is called the elementary substance as distinct from the chemical element as defined under 1, but mostly the term chemical element is used for both concepts Molecules A sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by chemical bonds. Compounds A pure substance consisting of two or more different elements chemically bonded together in a fixed proportion by mass. Dmitrij Ivanovič Mendeleev 08/02/1834 - 20/01/1907 H=1[5] Li=7 Be=9,4 B=11 C=12 N=14 O=16 F=19 Na=23 Ti=50 Zr=90 ?[2]=180 V=51 Nb=94 Ta=182 Cr=52 Mo=96 W=186 Mn=55 Rh=104,4[3] Pt=197,4[4] Fe=56 Ru=104,4 Ir=198 Ni=Co=59 Pd=106,6 Os=199 Cu=63,4 Ag=108 Hg=200 Mg=24 Zn=65,2 Cd=112 Al=27,4 ?[6]=68 Ur=116[7] Au=197? Si=28 ?[8]=70 Sn=118 P=31 As=75 Sb=122 Bi=210? S=32 Se=79,4 Te=128? Cl=35,5 Br=80 J=127[9] K=39 Rb=85,4 Cs=133 Tl=204 Ca=40 Sr=87,6 Ba=137 Pb=207 ?[10]=45 Ce=92[11] ?Er=56 La=94 ?Yt=60 Di=95 ?In=75,6 Th=118? Alkaline Earths Halogens Main Group Alkali Metals Transition Metals Main Group Lanthanides and Actinides Calchogens Pnictogens Noble Gases The Periodic table http://www.meta-synthesis.com/webbook/35_pt/ pt_database.php?Button=post-2000+Formulations Dalton, John 1766-1844 Lithograph A given compound always contains the same proportion of elements, by mass. Dalton’s Atomic Theory 1. Each element is made up of atoms; 2. Atoms of a given element are identical while atoms of different elements differ; 3. Chemical compounds are made up of specific whole number ratios of atoms; 4. Reactions involve reorganization of atoms. Atoms do not change. Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies, and the Proportions in Which They Enter Into These Compound Lorenzo Romano Amedeo Carlo Avogadro Journal de Physique, 1811, 73, 58-76 Atomic Masses: A relative comparison of the average masses of atoms. Avogadro’s Hypothesis (1811): Equal volumes of a gas contain the same number of atoms under the same conditions of T and P. Atomic Structure Until the end of the 19th century, not much was known about the atom. People knew: i) ii) There were different elements Each had a unique mass and reacted in a special way. BUT, WHAT WAS THE ATOM? Introduction to Atomic Theory The atom was a black box. People knew there were different substances (different boxes), each with unique properties, but “why” was a question. Sir Joseph John Thomson (1856 - 1940) was awarded the 1906 Nobel Prize. “… in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases.” The early experiments of J.J. Thompson cast light on the question. An evacuated tube, containing a small amount of a gas was attached to a power supply LVIII. On the Masses of the Ions in Gases at Low Pressures. By J.J. Thomson, M.A., F.R.S., Cavendish Professor of Experimental Physics, Cambridge Philosophical Magazine December 1899 Series 5, Vol. 48, No. 295, p. 547-567 The same cathode ray was seen no matter what the tube was filled with. - + Conclusion: The cathode ray was a fundamental unit of all materials. Through a number of experiments, J.J. was able to determine the charge (e) to mass (m) ratio of the cathode ray. - + e/m = some number e/m =-1.75881962 x 1011 C/Kg. By similar analysis he was also able to look at the canal Ray, the positive piece “left over” after the Cathode ray had been ejected. - + e/m was different for each gas that was in the evacuated tube. Starting with two different atomic “Boxes” Same Positive pieces “left over” are different + + q ν 02 Δy EΔy = = 2 m EdL B dL x = ν 0t 1⎛ q ⎞ 2 1⎛ q ⎞ ⎛ x ⎞ y = ⎜ ⎟ Et = ⎜ ⎟ E ⎜ ⎟ 2⎝ m⎠ 2 ⎝ m ⎠ ⎝ ν0 ⎠ 2 ⎛ dy ⎞ ⎛ q ⎞ Ed Δy tan α = ⎜ ⎟ = ⎜ ⎟ 2 = ⎝ dx ⎠ x=d ⎝ m ⎠ ν 0 L 2 q ν 0 Δy = m EdL q ν Δy = m EdL 2 0 Eq = qνB E ν= B q ν 02 Δy EΔy = = 2 m EdL B dL The Plum Pudding Model At this point J.J was ready to talk about the constitution of the atom: a large “sea” of positive charge, imbedded with electrons. Millikan’s Oil Drop Experiment The e/me ratio (-1.75881962 x 1011 C/Kg) What does it tell us? Zap with radiation Small droplets of oil, each given a negative charge, are pushed upward by an electric force while they simultaneously fall downward under the influence of gravity. Controlled variations of the electric field show that the negative charge on each drop is always a multiple of a certain value e. Millikan’s Oil Drop Experiment Experimental data indicate that: e = -1.60217733 x 10-19 C; me = 9.1093897 x 10-31 kg. Back to Plum Pudding A “sea of positive charge” containing embedded electrons The gold foil experiment: They shot a particles through the foil. This should be equivalent to shooting a howitzer shell through paper tissue. United Kingdom and New Zealand Victoria University Manchester, U. K. b. 1871 (in Nelson, New Zealand) d. 1937 Nobel prize in Chemistry 1908 The gold foil experiment: They shot a particles through the foil. This should be equivalent to shooting a howitzer shell through paper tissue. A solar system model of the atom. Electrons circle the nucleus. The Concept of Atomic Number Z = number of protons You will notice that most of the Periodic Table is well-arranged according to Atomic Mass. A = atomic mass = #protons + neutrons Z = atomic number = # protons A Z X Isotopes: Atoms with the same number of protons (Z) but different number of neutrons (AZ) A X Different atomic masses Protium mass: 1.007825 u Deuterium mass: 2.01410178 u Composition of the Atom Electrons Nucleus But, the nucleus is made of protons and neutrons 2e- 1e- 2e- Li Be 2e- 8e- 2e- 2e- 8e- ...... 8e- 1e- 2e- 8e- 2e- Ar 8e8e2e- Ca 8e8e- Mg 8e- K ...... 2e- Ne 2e- 1e- Na 2e- ...... Law of Definite Proportion A given compound always contains the same proportion of elements, by mass. Law of Multiple Proportions When two elements form two different compounds, the mass ratio of the elements in one compound is related to the the mass ratio of the elements in the latter compound by a small whole number. Mass of Oxygen combining with 1 gram of Nitrogen Compound A Compound B 1.1428 g 2.2857 g Divide by smallest number to get ratios: 1.1428/1.1428 = 1 2.2857/1.1428 = 2 We really do not know the real formula NO N2O2 NO2 N 2O 4 Lecture 2 End Lecture 2 End