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Compounds & Molecules

Learn about the properties, formulas, and modeling of compounds and molecules. Understand molar masses, ion charges, and the formation of ionic compounds.

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Compounds & Molecules

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  1. NaCl, salt Compounds & Molecules Molecule: The smallest identifiable unit that retains the physical and chemical properties of the pure substances. Ethanol, C2H6O Buckyball, C60

  2. Compounds & Molecules • COMPOUNDis a combination of 2 or more elements in definite ratios by mass. • The character of each element is lost when forming a compound (e.g., think of NaCl). • MOLECULESare the smallest units of a compound that retains the characteristics of the compound.

  3. MOLECULAR FORMULAS • Formula for glycine is C2H5NO2 (description of the composition) • In one molecule there are • 2 C atoms • 5 H atoms • 1 N atom • 2 O atoms

  4. CONDENSED FORMULAS • Formula for glycine is NH2CH2CO2H (composition and functional groups) • In one molecule there are • 1 NH2 (amine group) • 1 CH2 group • 1 CO2H group

  5. STRUCTURAL FORMULAS • Show how the atoms are attached within a molecule • The lines between atoms represent chemical bonds that hold the atoms together.

  6. WRITING FORMULAS • Can also write glycine as the condensed formula H2NCH2COOH showing functional groups (atom ordering and connectivity) • or in the form of a structural formula showing how atoms are attached to each other (bond orders)

  7. MOLECULAR MODELING An even higher level of structural detail Drawing of glycine Ball & stick Space-filling

  8. Resources for Molecular Modeling • Modeling software • CAChe (General Chemistry Interactive CD-ROM) • Rasmol • Molden • Gaussview • Maestro

  9. MOLECULAR WEIGHT AND MOLAR MASS Molecular weight = sum of the atomic weights of all atoms in the molecule. Molar mass = molecular weight in grams per mol.

  10. What is the molar mass of ethanol, C2H6O? 1 mol contains 2 moles of C (12.01 g C/1 mol) = 24.02 g C 6 moles of H (1.01 g H/1 mol) = 6.06 g H 1 mol of O (16.00 g O/1 mol) = 16.00 g O TOTAL = molar mass = 46.08 g/mol

  11. Tylenol • Formula = • Molar mass = C8H9NO2 151.2 g/mol

  12. Molar Mass

  13. How many moles of alcohol (C2H6O) are there in a “standard” can of beer if there are 21.3 g of C2H6O? (a) Molar mass of C2H6O = 46.08 g/mol (b) Calc. moles of alcohol

  14. How many moleculesof alcohol are there in a “standard” can of beer if there are 21.3 g of C2H6O? = 2.78 x 1023 molecules We know there are 0.462 mol of C2H6O.

  15. How many atoms of Care there in a “standard” can of beer if there are 21.3 g of C2H6O? = 5.57 x 1023 C atoms There are 2.78 x 1023 molecules. Each molecule contains 2 C atoms. Therefore, the number of C atoms is

  16. Molecular & Ionic Compounds NaCl Heme Molecular compounds consist of discrete molecules Ionic compounds consist of discrete ions

  17. IONS AND IONIC COMPOUNDS • IONSare atoms or groups of atoms with a positive or negative charge. • Taking away an electron from an atom gives a CATION with a positive charge • Adding an electron to an atom gives an ANION with a negative charge.

  18. Forming Cations & Anions A CATION forms when an atom loses one or more electrons. An ANION forms when an atom gains one or more electrons F + e- --> F- Mg --> Mg2+ + 2 e- reduction oxidation

  19. oxidation reduction

  20. PREDICTING ION CHARGES In general • metals (Mg) lose electrons ---> cations • nonmetals (F) gain electrons ---> anions

  21. -4 -3 -2 -1 +1 +2 Charges on Common Ions Anion charge=group #-8 Cation charge=group # +3 By losing or gaining e-, atom has same number of electrons as nearest Group 8A atom.

  22. Predicting Charges on Monatomic Ions

  23. METALS M ---> n e- + Mn+ where n = periodic group Na+ sodium ion Mg2+ magnesium ion Al3+ aluminum ion Transition metals --> M2+ or M3+ are common Fe2+ iron(II) ion Fe3+ iron(III) ion

  24. Group 5A Group 6A Group 7A Group 4A C4-,carbide NONMETALS NONMETAL + n e- ------> Xn- where n = 8 - Group no. N3-, nitride O2-, oxide F-, fluoride S2-, sulfide Cl-, chloride Br-, bromide I-, iodide

  25. Ion Formation Reaction of aluminum and bromine

  26. POLYATOMIC IONSCD Screen 3.6 Groups of atoms with a charge. MEMORIZEthe names and formulas of common polyatomic ions listed in Table 3.1, page 107 (next slide)

  27. Polyatomic Ions NH4+ ammonium ion One of the few common polyatomic cations

  28. Polyatomic Ions (oxoanions) NO3- nitrate ion HNO3 nitric acid Prefix per- and suffix –ate: largest # Suffix -ate : greater # of oxygen atoms Suffix -ite : smaller # of oxygen atoms Prefix hypo- and suffix –ite: smallest #

  29. Polyatomic Ions SO42- sulfate ion SO32- sulfite ion

  30. Polyatomic Ions NO3- nitrate ion NO2- nitrite ion

  31. Polyatomic Ions CO32- carbonate ion HCO3- bicarbonate ion hydrogen carbonate

  32. Polyatomic Ions PO43- phosphate ion CH3CO2- acetate ion

  33. COMPOUNDS FORMED FROM IONS CATION + ANION ---> COMPOUND Na+ + Cl- --> NaCl A neutral compd. requires equal number of + and - charges.

  34. NH4+ Cl- IONIC COMPOUNDS ammonium chloride, NH4Cl

  35. Some Ionic Compounds Ca2+ + 2 F- ---> CaF2 Mg2+ + NO3- ----> Mg(NO3)2 magnesiumnitrate Fe2+ + PO43- ----> Fe3(PO4)2 iron(II) phosphate calcium fluoride

  36. Properties of Ionic CompoundsForming NaCl from Na and Cl2 • A metal atom can transfer an electron to a nonmetal. • The resulting cation and anion are attracted to each other by electrostatic forces.

  37. Electrostatic Forces The oppositely charged ions in ionic compounds are attracted to one another by ELECTROSTATIC FORCES. These forces are governed by COULOMB’S LAW.

  38. Electrostatic Forces COULOMB’S LAW As ion charge increases, the attractive force _______________. As the distance between ions increases, the attractive force ________________. This idea is important and will come up many times in future discussions!

  39. Importance of Coulomb’s Law NaCl, Na+ and Cl-, m.p. 804 oC MgO, Mg2+ and O2- m.p. 2800 oC

  40. ELEMENTS THAT EXIST AS MOLECULES See SCREEN 3.2 on the CD-ROM Allotropes of C

  41. Screen 3.2

  42. ELEMENTS THAT EXIST AS DIATOMIC MOLECULES (gases)

  43. ELEMENTS THAT EXIST AS POLYATOMIC MOLECULES S8 sulfur molecules White P4 and polymeric red phosphorus

  44. Molecular CompoundsCompounds without Ions CO2 Carbon dioxide BCl3 boron trichloride CH4 methane

  45. Naming Molecular Compounds All are formed from two or more nonmetals. CO2 Carbon dioxide Ionic compounds generally involve a metal and nonmetal (NaCl) BCl3boron trichloride CH4 methane

  46. Empirical & Molecular Formulas A pure compound always consists of the same elements combined in the same proportions by weight. Therefore, we can express molecular composition as PERCENT BY WEIGHT Ethanol, C2H6O 52.13% C 13.15% H 34.72% O

  47. Structure of NO2 Percent Composition Consider some of the family of nitrogen-oxygen compounds: NO2, nitrogen dioxide and closely related, NO, nitrogen monoxide (or nitric oxide) Chemistry of NO, nitrogen monoxide

  48. Percent Composition Consider NO2, Molar mass = ? What is the weight percent of N and of O? What are the weight percentages of N and O in NO?

  49. Determining Formulas In chemical analysis we determine the % by weight of each element in a given amount of pure compound and derive the EMPIRICALor SIMPLESTformula. PROBLEM: A compound of B and H is 81.10% B. What is its empirical formula?

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