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Lecture 4. Chapter 2. Structure of the Atom (Contd.)

Lecture 4. Chapter 2. Structure of the Atom (Contd.). Lecture 4. Exercise 2.1. A US penny is 19 mm across. How many Ag atoms could span this if diameter Ag atom is 2.88 Å? Note 1 Ag atom per 2.88 Å. Conversion factors: 1 = 10 -3 m/1 mm and 1 = 10 -10 m/ 1 Å.

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Lecture 4. Chapter 2. Structure of the Atom (Contd.)

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  1. Lecture 4. Chapter 2. Structure of the Atom (Contd.)

  2. Lecture 4. Exercise 2.1. A US penny is 19 mm across. How many Ag atoms could span this if diameter Ag atom is 2.88 Å? Note 1 Ag atom per 2.88 Å. Conversion factors: 1 = 10-3 m/1 mm and 1 = 10-10 m/ 1 Å. Number = 19 mm x 10-3 m x 1 Ag atom x 1 Å 1 mm 2.88 Å 10-10 m = 6.6 x 107 Ag atoms which is sixty six million Ag atoms.

  3. Diameters of Atomic Nuclei. These are on the order of 10-4 Å. So if we draw an atom on the board with a diameter of 1m, then the nucleus will be 0.1 mm across! diameter of nucleus = 0.1 mm Diameter = 1 meter

  4. Atomic numbers, mass numbers, and Isotopes. (p. 45). The chemical properties of the elements are controlled by the numbers of electrons and protons. The number of protons (= no. of electrons) is the Atomic Number (Z) of the element. Z = 1 6 8 13 protons H C O Al

  5. Mass Number The Mass Number is the sum of the number of protons plus neutrons. Atoms of a given element can have different numbers of neutrons. Thus, C (carbon) always has Z = 6, i.e. it has 6 protons, since C determines its chemistry, but can have different numbers of neutrons. Nuclei of Carbon atoms: all have six protons (red) six neutrons (blue) eight neutrons carbon-12 carbon-13 carbon-14

  6. Isotopes • Different isotopes of the same element have different mass numbers. Thus, 12C and 14C both have 6 protons (Atomic Number 6), but 12C has 6 neutrons, while 14C has 8. C Mass number: 14 Atomic number: 6 ‘carbon-14’ Usually, only the mass number is indicated, and referred to as ‘carbon-14’.

  7. Isotopes of Hydrogen (Z = 1) one proton one proton, one proton in nucleus one neutron two neutrons Hydrogen Deuterium Tritium 99.9844% 0.0156% radioactive, unstable 1 2 3 HHH 1 1 1

  8. The 12C atom: nucleus – contains 6 protons (Z = 6) and 6 neutrons 6 electrons

  9. 2.4. Atomic weights. These are expressed in amu. The mass of the 12C isotope has been defined as exactly 12 amu. 1 amu = 1.66054 x 10-24 g, or 1/6.022 x 1023 g. In these units, the 1H isotope has a mass of 1.0078 amu. The mass of the 12C isotope is exactly 12 amu. Carbon-12

  10. Atomic weights of the elements: Elements usually consist of mixtures of isotopes. The Atomic Weights for the elements are thus the average masses of these mixtures, expressed in amu. Thus, naturally occurring C consists of 98.93% 12C and 1.07% 13C. The Atomic Weight of the mixture is thus given by: Atomic weight = 98.93 x 12 + 1.07 x 13.0335 = 100 100 12.01 amu. Divide by 100 because ‘%’ = ‘per one hundred’

  11. Average Atomic Mass = Atomic Weight EXAMPLE: Si naturally occurs in three isotopes: 92.2% of 28Si (27.98 amu) 4.7% of 29Si (28.98 amu) 3.1% of 30Si (29.97 amu) what is the atomic weight of Si? 92.2 100 0.922 x 27.98 amu 0.047 x 28.98 amu 0.031 x 29.97 amu = 28.09 amu add together 4.7 100 3.1 100

  12. 2.5. The Periodic Table. The Periodic Table is the most important tool that chemists use for organizing and remembering chemical facts. The chemical properties show a periodic pattern, hence ‘periodic table’. Mendeleev in 1869 came up with the arrangement we now essentially use, possibly from the arrangement of cards in ‘solitaire’. Dmitri Ivanovich Mendeleev (1834-1907)

  13. Solitaire:

  14. Atomic Number Periodic Table Elements are sorted according to increasing atomic number 6 C 12.01 Atomic weight

  15. Groups in the periodic table: Group 5 Group 7 Group 1 Group 3 Group 1B

  16. Groups: The vertical columns in the periodic table are called GROUPS.Properties repeat down a group, such as Alkali metals (Group 1) are all soft silvery reactive metals that form basic hydroxides, while Noble gases (group 8) are all chemically unreactive gases, halogens (group 7) all exist as X2 molecules, and form salts where they are the anion X-.

  17. Group 1B, the coinage metals: Cu Ag Au Copper penny American silver South African Eagle Gold Krugerrand They are all relatively inert, corrosion-resistant metals which have been used for minting coins. They are all excellent conductors of electricity. They all can form chlorides of the composition MCl (M = metal), such as CuCl, AgCl, and AuCl.

  18. Numbers Down side Indicate period Rows or Periods in the Periodic Table: (these run across the periodic table) First period contains only H and He third period 5th period

  19. Noble Gases Alkaline Earth Metals Transition metals Alkali Metals Halogens HW: 37 Periodic Table Most elements are metals VIII A I A II A III A IV A VA VI A VIIA Non- metals

  20. Molecules and Molecular Compounds Some elements exist as diatomic molecules at room temp. H2 N2 O2 F2 Cl2 Br2 I2

  21. Periodic table helps predict formulas of compounds: e.g. fluorides of metallic elements: Group 1 HF, LiF, NaF, KF, RbF, CsF Group 3A. BF3, AlF3, GaF3, InF3 Group 2. BeF2, MgF2, CaF2, SrF2, BaF2 Group 3B. ScF3, YF3, LaF3, AcF3. Group 2B. ZnF2, CdF2, HgF2 Metallic elements

  22. Periodic table helps predict formulas of compounds: e.g. Hydrides (compounds with hydrogen) of non-metallic elements: Group 7. HF, HCl, HBr, HI. Group 6. H2O, H2S, H2Se, H2Te inert Group 5. NH3, PH3, AsH3, SbH3 Group 4. CH4, SiH4, GeH4

  23. Atoms of the first four elements in the periodic table: 1 4 7 9 1H 2He 3Li 4Be Hydrogen Helium Lithium Beryllium protons: 1 2 3 4 Neutrons: 0 2 4 5 Electrons: 1 2 3 4

  24. Ions are formed when a neutral atom - gains electrons or - loses electrons e- + Li Li Cation - e- Anion Br Br Atoms and ions: An atom can lose one or more electrons to become a positively charged cation, or gain electrons to become a negatively charged anion:

  25. This has an effect when we count electrons in elements and Ions one extra compared to Z neutral atom, same as Z two less compared to Z number of protons (Z) number of electrons number of neutrons 16 16 32S 32 – 16 = 16 35 + 1 = 36 80 – 35 = 45 80Br- 35 20 40Ca2+ 20 - 2 = 18 40 – 20 = 20 ‘-’ charge results in gain of one electron ‘2+’ charge results in loss of two electrons

  26. Molecules and Molecular Compounds The periodic table helps predict how elements combine Empirical Formulas Molecular Formulas Only gives relative number of atoms in compound Show actual numberand types of atoms in a molecule Subscripts are smallest whole-number ratios

  27. Empirical Formulas Molecular Formulas HO H2O2 Hydrogen peroxide C4H10 C2H5 Butane C3H8 C3H8 Propane

  28. Empirical and molecular formulae of glucose hydrogen (white) oxygen (red) Empirical Formula: CH2O Molecular Formula: C6H12O6 (= empirical formula x 6) carbon (cyan) If you count the atoms in this model of glucose you will see 6 carbons, 12 hydrogens, and 6 oxygens glucose

  29. Perspective drawing Ball-and-Stick model Different representations of methane (CH4) Structural formula Space-filling model

  30. Ions are formed when a neutral atom - gains electrons or - loses electrons e- + Li Li Cation - e- Anion Br Br Ions and Ionic Compounds

  31. Which Elements form Anions, which Cations? Metalstend to form Cations Nonmetalstend to form Anions VIII A I A II A III A IV A VA VI A VIIA

  32. Cations and Anions can combine to form Ionic Compounds: Sodium cation, Na+ Chloride anion, Cl- Sodium chloride crystal Sodium chloride (Ionic Crystal – not a discrete molecule)

  33. 2+ 2- 6+ 6- 0 0 Empirical Formulas for Ionic Compounds: (A) determine charge on ions formed (B) add ions so that compound is neutral overall Na, O => Na+ O2- Al, O => O2- Al3+ Al3+ 2 Na+ O2- O2- Na2O Al2O3

  34. - 2+ 1 Cl Mg 2- 2+ Ca O CaO smallest possible ratio! A simple approach: simply swap the charge on one ion to be the subscript on the other charges 2- 3+ Al2O3 Al O subscripts MgCl2 Ca2O2

  35. 2.8 Naming Inorganic Compounds. • There are now more than 19 million known chemical substances. We cannot use common names for all of them, e.g. ‘rock salt’ or ‘sugar’. We need a system of naming them that indicates unambiguously what they are.

  36. Organic and Inorganic substances: • Organic substances are compounds of carbon and hydrogen (plus other elements such as N or S) associated with living things. Inorganic substances are substances not associated mainly with living things. CH2CH2OH AlCl3 Ethanol aluminum chloride Organic Inorganic

  37. Naming Ionic Compounds: These consist of metal cations (e.g. Na+) combined with non-metal anions (e.g. Cl-). 1. Positive cations: a) Cations formed from the metal are given the same name as the metal: e.g. Sodium, Aluminum, Zinc.

  38. b). If a metal can form cations of different charge, this is indicated with Roman numerals, e.g. Fe2+ = iron (II) Fe3+ = iron(III) Cu+ = copper(I) Cu2+ = copper(II). An older method still widely used uses Latin names where the suffix –OUS indicates the lower charge, and –IC indicates the higher charge: Fe(II) = ferrous Fe(III) = ferric Cu(I) = cuprous Cu(II) = cupric

  39. 2. Negative ions (Anions). a) The names of monoatomic anions are formed by replacing the ending of the name of the element with ide. H- = hydride ion Cl- = chloride O2- = oxide N3- = nitride S2- = sulfide A few simple polyatomic anions also get the suffix ide: OH- hydroxide CN- cyanide O22- peroxide

  40. Oxo-anions (sulfate, sulfite, nitrate, nitrite) b) Polyatomic anions containing oxygen end in ­ATE (more oxygens) or –ITE (fewer oxygens). more oxygens • NO3- nitrate SO42- sulfate • NO2- nitrite SO32- sulfite fewer oxygens

  41. The oxo-anions of chlorine: Prefixes are used when there are more than two different oxo-anions of the same element: Cl- chloride ClO-hypochlorite fewer oxygens = ite ClO2- chlorite ClO3- chlorate more oxygens = ate ClO4-perchlorate

  42. Carbonate and hydrogen carbonate: • c) Anions derived from other anions by addition of hydrogen: CO32- carbonate (+ H+) → HCO3- hydrogencarbonate

  43. 3. Ionic compounds. These are named with the cation name first, followed by the name of the anion: NaCl = sodium chloride Na2CO3 = sodium carbonate Al(NO3)3 = aluminum nitrate Cu(ClO4)3 = copper(II) perchlorate or cupric perchlorate

  44. Names and Formulas of Binary Molecular Compounds. Binary compounds are two-element compounds, e.g. carbon dioxide. Rules for naming these are as follows: 1. The name of the element further to the left in the periodic table is usually written first. Thus oxygen is always written last except when combined with fluorine. OF2 = oxygen difluoride, but Cl2O is dichlorine monoxide. 2. If elements are in same group, heavier element is written first, e.g. SO2 = sulfur dioxide. 3. the name of the second element is given an –IDE ending.

  45. Numerical prefixes: 4. Greek prefixes are used to indicate the number of atoms of each element in the formula: SF6 = sulfur hexafluoride NF3 = nitrogen trifluoride N2O4 = dinitrogen tetroxide. P4S10 = tetraphosphorus decasulfide. (A table of Greek prefixes is given on next slide) Note: here the ‘a’ in ‘tetra’ is omitted

  46. Table of Greek prefixes:(bottom left p. 66) mono- = 1 di- = 2 tri- = 3 tetra- = 4 penta- = 5 hexa- = 6 hepta- = 7 octa- = 8 nona- = 9 deca- = 10 Note that if the name of the second element starts with a vowel, e.g. oxide, then the -a is omitted for tetra-, penta-, and hepta-, so we have disulfur tetroxide, phosphorus pentoxide, diiodine heptoxide.

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