This chapter deals with matter, in its various forms, on the nanoscale.

Transcription

1 Chapter 2 (Hill/Petrucci/McCreary/Perry This chapter deals with matter, in its various forms, on the nanoscale. While we believe that all matter is composed of protons, electrons and, usually, neutrons, we encounter matter as elements in the form of atoms (and, occasionally, molecules), as compounds in the form of molecules and crystal lattices and as ions, charged particles. Flow chart for classification of matter this should be a review. Alchemy ( A.D.) Study and classification of a large number of substances. Separations: crystallization, distillation. Relationship between experiment and theory. Transmutation" of "base" metals into gold Galileo. Chemical changes involve rearrangements of parts too small to be seen Separations: crystallization, distillation; Relationship between experiment and theory; Transmutation" of "base" metals into gold. Francis Bacon Heat originates from the motion of very small particles. Robert Boyle ( ) Theory must be backed up by scientific "facts" (beginnings of modern chemistry). Boyle's definition of an element: one must test an "element" to see if it can be chemically broken down into simpler substances Use of atomic concepts to explain physical phenomena. "Phlogiston" Theory ( ) Georg Stahl Phlogiston theory: an attempt to explain fire and burning process ("to set on fire") Process of burning involved loss of "phlogiston" to air, a carrier for phlogiston Combustion and rusting - the same Rusting metals gained weight - "negative" phlogiston Joseph Priestley ( ) Prepared/described a number of previously unknown gases - oxygen (1774) mercury oxide + heat " mercury + O 2 Discovery of oxygen critical to: 1.clear distinction between various pure gases and air 2.understanding most fundamental process: FIRE!!

2 Antoine Lavoisier ( ) Masses of chemicals which reacted in closed vessels were not changed - Used Boyle's definition of elements to identify a number of new elements - Elementary Treatise on Chemistry - Combustion is a reaction between a substance and oxygen -- NO PHLOGISTON!!! - Established chemistry as a quantitative science -- accurate weighings Law of Conservation of Matter (Mass) Matter neither gained nor lost in a chemical reaction Substances can be created or destroyed, but matter cannot. Joseph Proust (1799) Chemical compounds have constant, unvarying compositions with respect to weights of elements which make them up. Law of Constant Composition (Definite Proportions) "In a compound, elements are always present in a definite proportion by weight." Thus, water, a compound is always 11 parts H and 89 parts O by weight (mass) John Dalton s Atomic Theory ( ) A comprehensive theory (a synthesis) to account for the observations of Lavoisier and Proust Dalton's atomic theory had several postulates: 1. all matter is composed of small, invisible particles called atoms 2. in chemical reactions, atoms are neither created nor destroyed 3. atoms of each element have unique properties - all atoms of a given atom are identical and have identical masses and other properties 4. chemical reactions involve the uniting or the separation of atoms of different elements Dalton's experiments with various elements led him to propose relative atomic "weights" for some of the more common elements: hydrogen, 1; carbon, 6; oxygen, 8. Because Dalton did not and could not know that some elements (, O 2, N 2, Cl 2, Br 2, I 2 ) were diatomic in nature, his atomic weights were wrong. Water, for example, which we now know is O, Dalton thought it was HO. Nevertheless, his weights did work for CO and for CO 2 The other error in Dalton's thinking involved the idea that atoms of a given element had exactly the same mass. They don t... necessarily...

3 DIVISIBLE ATOMS The atomistic view of matter was seen to be correct but, if so, what were atoms? truly indivisible? As experiments toward the end of the 19th Century were to demonstrate, atoms were divisible. Protons +1 charge, relative mass = 1.00 Neutrons 0 charge, relative mass = 1.00 Electrons -1 charge, relative mass = 1/1836 ( ) All neutral atoms of a given element have the same numbers of protons and electrons... zero charge! ISOTOPES We now know that isotopes exist for various elements. Isotopes are forms of an element whose atoms differ in mass because their nuclei have different numbers of neutrons. Using an instrument called a mass spectrometer, we can now determine atomic masses (in units called atomic mass units or a.m.u. s) for various isotopes with extreme accuracy. ISOTOPIC NOTATION A way of describing atomic "makeup for nuclei of atoms # PROTONS (P) determine type of atom #NEUTRONS (N) determine isotope EXAMPLE: 18X8 Upper number is "mass number", A A = # P + # N Lower number is "atomic number," Z Z = # P Thus, A - Z =??? and, A - Z = # N (number of neutrons) Why is this important?? because the number of neutrons determines what isotope we are talking about. EXAMPLE: 18X8 A = 18 Z = 8 # P = 8 (atomic number) # N = 18-8 = 10 a The element is oxygen (O). Z = 8

4 See EXAMPLE and Exercises in Hill, p. 41 ATOMIC MASSES If we measure the mass of a single hydrogen atom, it may vary, depending on the kind of hydrogen atom we have. Hydrogen has three isotopes: 1 H 1, 2 H 1, and 3 H 1. The isotope 3 H 1, called tritium, has 1 proton and 2 neutrons; deuterium, 2 H 1, has 1 proton and 1 neutrons, while protium, 1 H 1, has only 1 proton. The various isotopes of an element are present in fixed percentages in nature.. isotopic abundances The various isotopes of an element are present in fixed percentages, e.g. hydrogen (3 isotopes) 1H %) amu 2H (0.015%) amu 3H 1 Negligible See and do Average Atomic Weights Exercise Now look at Example 2.4 and Exercises 2.4A and 2.4B, p. 43 The Periodic Table - Observations Mendeleev (1865) Organization of known elements in order of increasing atomic mass and grouped by recurring chemical and physical properties into groups or families Prediction of new elements in gaps periodicity concept Mendeelev s order failed (K, Ni, I) The Periodic Table - Revisited Moseley (1914) Determined the number of protons in atoms using x-rays. Arranged elements by their numbers of protons (order number) or atomic number Elements now fit the table correctly Modern Periodic Law: Chemical and physical properties of elements are periodic functions of the number of protons in the nuclei of their atoms

5 Molecules And Compounds Operational Definitions... molecule: a group of two or more atoms held together by chemical bonds, in which atoms share electrons molecular compound: a pure substance consisting of molecules, and in which different elements are present in fixed proportions by mass and in fixed atomic ratios molecular formula: a formula, consisting of elemental symbols and numerical subscripts, that fully describes a unit of a molecular compound empirical formula: the simplest whole-number formula for a molecular compound Example. C 6 H 6 C 1 H 1 molecular formula (benzene) empirical formula for benzene structural formula: a formula that shows how atoms are connected together in a molecule of a molecular compound this type of formula, however, does not necessarily tell us anything about the 3-D shape of the molecule See examples. Chemical Nomenclature... Chemical Formulas and Chemical Names Binary compounds: compounds that consist of only two elements, e.g O (molecular) or NaCl (ionic) Naming Binary Molecular Compounds Binary molecular compounds consist of a combination of two nonmetals, e.g. CO 2. Numeric Prefixes Indicate How Many Atoms 1 mono- 6 hexa- 2 di- 7 hepta- 3 tri- 8 octa- 4 tetra- 9 nona- 5 penta- 10 deca- See Table 2.3, p be sure to learn these! Recall: Binary molecular compounds consist of a combination of two nonmetals, e.g. CO 2 Naming: prefix + nonmetal 1 + prefix + nonmetal 2 prefix + -ide

6 Examples. P 2 O 5 C 3 H 8 diphosphorus pentoxide tricarbon octahydride See Practice Problems for Ionic Formulas and Nomenclature, #4 and #5 More Examples. N 2 O SiF 4 O NBr 3 CO CO 2 Molecules and Compounds: Ions and Ionic Compounds ion: a chemical species that has an excess or a deficit of electrons, and hence, a net charge cation: an ion with a net positive charge, e.g. Na + A monatomic cation can be formed from a neutral atom when the atom loses one or more of its electrons Example. Na 0 (11 protons (+), 11 electrons (-1) ) Net charge = = 0 Na + (11 protons (+), but only 10 electrons (-1) ) Net charge = = +1 anion: an ion with a net negative charge, e.g. S 2- A monatomic anion can be formed from a neutral atom when the atom gains one or more additional electrons Example. S 0 (16 protons (+), 16 electrons (-1) ) Net charge = = 0 S 2- (16 protons (+), but 18 electrons (-1) ) Net charge = = - 2 See Figure 2.10 (p. 51) for monatomic ions Naming Binary Ionic Compounds Recall: Binary ionic compounds consist of a combination of a metal and a nonmetal, e.g. NaCl Naming: metal name + nonmetal name + -ide NO NUMERIC PREFIXES!! Examples. CaBr 2 calcium bromide FeCl 2 iron(ii) chloride Li 2 S lithium sulfide CuI copper(i) iodide

7 Molecules and Compounds: Polyatomic Ions and Ionic Formulas polyatomic ion: a cation or anion consisting of two or more atoms, bonded together, e.g. NO3 1- See Table 2.4 (p. 54) for common polyatomic ions that you should know! In ionic compounds, the total net charge is always 0. To write the formula for an ionic compound, begin with the charge on the cation and the charge on the anion use the absolute value of the charge on the cation as subscript for the anion and vice versa by convention, write the cation first. Molecules and Compounds: Writing Ionic Formulas Example. Write the ionic formula for potassium sulfide. K 0 K 1+ and S 0 S 2- Then, K 2 S 1 K 2 S Try: calcium oxide iron(iii) nitrate Molecules and Compounds: Hydrates hydrate: a type of compound whose formula includes a stated number of water-molecules Example. CuSO 4 5 O (blue crystals) where the dot indicates {waters of hydration. If we heat a compound like CuSO 4 5 O, we can drive off the waters of hydration: CuSO 4 5 O + heat CuSO 4 (anhydrous) blue crystals white powder anhydrous without water Molecules and Compounds: Acids acids: compounds that ionize in water to form solutions dominated by H 3 O+... acids generally have H atoms that can come off as H+ ions See Table 2.5 (p. 58) for common acids and their salts that you should know! binary acids: acids that have H combined with one other nonmetallic element (see text description, p. 58) Example. HBr hydrobromic acid hydro - + element name suffix + -ic + acid

8 Molecules and Compounds: Ternary Acids ternary acids: acids that contain H, two nonmetal atoms, one of which is usually an O atom (oxoacids)... the H atoms are attached to the O-atoms and not to the nonmetal atom. See Table 2.5 (p. 58) for common ternary acids and their salts that you should know! Note that HCl and S are the only binary acids in this table. halogen-containing oxoacids: HXOn Examples. HClO hypochlorous acid HClO 2 chlorous acid HClO 3 chloric acid HClO 4 perchloric acid Molecules and Compounds: Other Oxoacids When there are only two oxoacids for a nonmetal, the one with more O atoms ends in ic and the one with fewer O atoms ends in -ous Examples. SO 3 sulfurous acid SO 4 sulfuric acid HNO 2 nitrous acid HNO 3 nitric acid H 3 PO 3 phosphorous acid H3PO 4 phosphoric acid Molecules and Compounds: Salts salt: a compound formed from the cation of a base (a metal ion) and the anion from an acid To name salts of ternary acids and oxoacids, change ous to ite or ic to-ate in the salt Examples. SO 3 (sulfurous acid) Na 2 SO 3 (sodium sulfite) SO 4 (sulfuric acid) K 2 SO 4 (potassum sulfate) HNO 2 (nitrous acid) HNO 3 (nitric acid) NaNO 2 (sodium nitrite) Ca(NO 3 ) 2 (calcium nitrate) How do we name these salts? Examples. NaBrO 3, ZnSO 4, K 2 CO 3, KI, AgNO 3

9 Molecules and Compounds: Bases bases: compounds that ionize in water to form solutions dominated by OH1-... hydroxyl bases generally have OH1- ions that can go into solution See text (p. 57) for common bases from Group IA and IIA metals that you should know! Note how we name these hydroxides... Example. NaOH sodium hydroxide metal name + hydroxide Other kinds of bases include ammonia (NH 3 ) and carbonates (salts that contain the CO 3 2- ion), e.g. Na 2 CO 3 Molecules and Compounds: Organic Compounds You should read this section (pp ), but only in a cursory fashion. We will not emphasize it or discuss it in class.