1. Chapter 3 The Composition of Molecules Department of Chemistry and Biochemistry Seton Hall University

2. 2 Molecules The sizes and shapes of molecules are a key determinant of their physical and chemical properties A variety of chemical properties can be displayed by compounds consisting of only a few elements Even simple molecules can have vastly different properties (consider O 2, CO and CO 2

3. 3 Representing molecules Ways of indicating structure –structural formulas –ball and stick structures –space-filling structures –line structures Each has its uses; you will need to learn all of them

4. 4 Chemical formulas indicate relative numbers of atoms of each element for substances that form discrete molecules, the chemical formula is also the molecular formula contains elemental symbols to represent the elements subscripts indicate the number of each element

5. 5 Chemical formulas Since chemical compounds contain two or more different elements, the order of the elements in the formula is an issue Element that occurs furthest to the left of the periodic table is normally listed first

6. 6 Structural formulas give number of atoms and how they are connected together atoms are connected together by electrostatic forces called bonds in discrete molecules, bonds consist of pairs of electrons shared by two atoms bonds are represented by a straight line

7. 7 Structural formulas

8. 8 Ball and Stick

9. 9 Space-filling model

10. 10 Line structures used for carbon-based compounds to simplify structure Rules –All bonds except C-H are shown as lines –C-H bonds are not shown –Single, double and triple bonds are indicated by the number of lines between the atoms –Carbon atoms are not labeled

11. 11 Line structures con’t –All atoms except C and H are labeled with their element symbols –Hydrogen atoms are labeled when they are attached to atoms other than C

12. 12 Physical states of elements Reference condition is 1 atmosphere of pressure and 25 °C Gaseous elements - H 2, N 2, O 2, F 2, Cl 2, noble gases Liquid elements - Hg, Br 2 All other elements are solids Some elements occur as atomic clusters (S 8, P 4 )

13. 13 Binary compound nomenclature Binary compounds contain two different elements Three types of binary compounds –Metals exhibiting only one oxidation state forming a compound with a nonmetal –Metals exhibiting two or more oxidation states forming a compound with a nonmetal –Compounds of nonmetals and nonmetals

14. 14 Metals with only one oxidation state Groups of metals with only one common oxidation state –alkali metals - +1 –alkaline earths - +2 –Ag - +1 –Cd, Zn - +2 –Al - +3 All other metals can exhibit more that one oxidation state

15. 15 Anions in negative oxidation states Nonmetallic anions usually exhibit one negative oxidation state –halogens -1 –chalcogens -2 –pnicogens -3 –carbides -4

16. 16 Forming compounds Metal and nonmetal combine to neutralize charge Consider - Al 3+, O 2- –cross multiply charges –2 Al 3+ + 3 O 2- = Al 2 O 3

17. 17 Naming compounds Use name of metal with no changes Change the name of the anion by taking the “stem” and add the suffix -ide Examples –NaCl - sodium chloride –MgCl 2 - magnesium chloride

18. 18 Metals with multiple oxidation state Two systems Stock system –metal name and the oxidation state in Roman numbers in parenthesis –Fe 2+ = iron(II) Form compound by balance charge of metal with correct number of nonmetals –CoCl 3 = cobalt(III) chloride

19. 19 Classical nomenclature Metals in multiple oxidation states usually have one or two common oxidation states First row metals are +2 and +3 (except Cu 2+ and Cu + ) use -ous suffix for lower common oxidation state use -ic suffix for higher common oxidation state

20. 20 Examples CoCl 3 - cobaltic chloride NiCl 2 - nickelous chloride For metals with Latin names, use them CuCl - cuprous chloride FeBr 3 - ferric bromide

21. 21 Nonmetals + nonmetals Name nonmetal further to the left of the periodic table first with no changes Name nonmetal further to the right of the periodic table second with the -ide suffix Use Greek prefixes to indicate the number of each one

22. 22 Greek prefixes NumberPrefixes 1mono 2di, bi 3tri 4tetra 5penta 6hexa 7hepta

23. 23 Examples N 2 O 3 - dinitrogen trioxide CO 2 - carbon dioxide P 2 O 5 - diphosphorus pentoxide

24. 24 Oxy anions anions composed of oxygen and another elements other elements can be a metal or a nonmetals Examples –SO 4 2-, NO 2 - Note that oxyanions that have 2- or greater negative charge can bind a proton –HCO 3 - - hydrogen carbonate – HSO 4 - - hydrogen sulfate

25. 25 Naming Need common oxidation states –most common oxidation state for nonmetals is the group number (except for the halogens) –next most common oxidation state is the group number minus one use -ate suffix for higher oxidation state and -ite suffix for next higher oxidation state

26. 26 Examples SO 4 2- - sulfate SO 3 2- - sulfite NO 3 - - nitrate NO 2 - - nitrite Salts with these oxyanions –Na 2 SO 4 - sodium sulfate –KNO 3 - potassium nitrate

27. 27 Acids Binary acids –name begins with hydro –then add stem of nonmetal plus -ic –end with acid Examples –HCl - hydrochloric acid –H 2 S - hydrosulfuric acid

28. 28 Oxyacids Take oxyanion suffix and convert –change -ate to -ic –change -ite to -ous Do not use hydro- in the beginning Examples –H 2 SO 4 - sulfuric acid –H 2 SO 3 - sulfurous acid