Understanding Molecules and Compounds: Chemical Bonding, Formulas, and Comparisons

1. Chapter 19 Molecules and Compounds Section 19.1

2. Most matter is in the form of compounds. • Most atoms are unstable unless combined. • Atoms combine in molecules.

3. Chemicalbonds are formed when atoms combine. • Valenceshellelectrons (outermost) are involved in bonding.

4. What are valence shell e-? • e- are found in energylevels (E.L.) within electron cloud surrounding nucleus. • Highest E.L., highest energy.

5. Energy Levels • First E.L. can hold upto 2 e-. What is this element?

6. Second E.L. can hold upto 8 e-. What is this element?

7. Third E.L. can hold upto 8 valence shell e-. How many more e- can this atom hold in its valence shell?

8. Octet Rule • Chemically stable atoms have 8 valence electrons (2 for 1st E.L. only). • Atoms form bonds to complete their octet and become stable. • Known as octetrule.

9. Use the PT to find # of valence e-. • Column (group / family) tells how many valence e- . • All atoms in a group have same # of valence e-, with exception of transition metals.

10. Dot Diagrams • Dotdiagrams are used to represent valence e-. • Element symbol represents nucleus & all e- except for valence e-.

11. Dot Diagrams, cont. • Dots around symbol = valence e-. • Place one dot for each valence e-on each side of the symbol before pairing up the e-.

12. Examples: H C Cl Mg N Ar Al O

13. Types of Chemical Bonds • Recall that atoms gain, lose, or share e- to gain 8 valence e-. • so that they become stable.

14. IonicBonds-formed when atoms gain or lose electrons. • Ex: Na has 1 valence e- =

15. When Na loses that electron, its 2nd E.L. becomes full and stable with 8 e-. sodium atom sodium ion Atoms with an electrical charge are called ions.

16. Chlorine has 7 valence electrons. • If Cl gains 1 electron, its valence shell will be full and stable. chlorine atom chlorine ion

17. Opposites attract, so… • Are these two ions likely to bond?

19. Covalent Bonds • Most atoms share electrons to gain an octet. • Covalentbonds are formed when e- are shared.

20. Covalent Bonds, cont. • Covalent bonds can form between 2 different types of atoms or between 2 or more atoms of the same type.

21. Diatomicmolecules: • – covalently bonded atoms of the same type. • Exs: H2 Cl2 F2 I2 O2 Br2 N2

22. How can you tell whether a bond is ionic or covalent? In GENERAL: • Ionic – metal / nonmetal • Covalent – 2 nonmetals

23. Electronegativity • Sometimes e- are shared unequally in covalent bonds. • Electronegativity – attraction atom has for shared pair of e-.

24. Chapter 19: Molecules and Compounds Section 19.2 Chemical Formulas

25. ChemicalFormula: • Ratioofatomsbonded togetherinacompound, i.e. X:Y • General Form:AxBywhere x and y are calledsubscripts.

26. Recall NaCl (sodium chloride)… • Formula shows that atoms combine in a 1:1 ratio. • Na1Cl1 = 1:1 • Why in that ratio?

27. Tobestable… • the net electrical charge of compounds mustbezero.

28. OxidationNumber: • Indicates how many valencee- are lost,gained, orsharedwhenbonding. • (+) or (–) symbol is written after the number, i.e. 1+ or 2-

29. Writing Chemical Formulas – monatomic ions • Symbol of (+) ion always written 1st. • Symbol of (–) ion always written 2nd. • Add subscripts so sum of oxidation #’s is zero.

30. Example: • Write formula for binary (2element) compound made of iron(III) and oxygen.

31. 1. Find oxidation #’s of elements: iron(III)= Fe3+ oxygen = O2- How do you make a cmpd electrically neutral?

32. Calculate Fe3+ ions needed to combine with O2- ionsto make electrical charges equal zero. 2 (Fe3+) added to 3 (O2-) = 0 2 (3+) added to 3 (2-) = 0

33. 2. To determine ratios to write chemical formulas…Use the Criss-crossMethod 3+ 3+ 2- 2- Fe O 2 3

34. Naming binary ionic compounds • Write name of 1st element or polyatomic ion. • Write root name of 2nd element and add -ide. • Exs: chlor-ine = chlor-ide phosph-orus = phosph-ide

35. Naming ionic cmpds with polyatomic ions • Write name of (+) ion 1st. Use PT or Table 19.2 • Write name of (-) ion 2nd. Use PT or Table 19.2

36. Naming binary covalent compounds • Specify number of each element by using prefixes (Figure 19.25, pg 332). • If only one atom of 1st element, don’t use mono-

37. Examples: • CO carbon monoxide • CO2 carbon dioxide • PCl5 phosphorus pentachloride • N2S6 dinitrogen hexasulfide

38. Chapter 19: Molecules and Compounds Section 19.3 Comparing Molecules

39. COMPARING DIFFERENT MOLECULES • How does the mass of different molecules compare?

40. Do you think that a molecule of water has the same mass as a molecule of calcium carbonate?

41. Recall atomic mass units (amu)… 6CCarbon12.0107 24 • Atoms are assigned a relativemassbased on carbon as the standard. • Known as atomic mass unit

43. Chemical formula gives 3 pieces of info: • types / numbers of atoms. • if polyatomic ions are present. • allows calculation of mass of 1 molecule of a compound relative to mass of other compounds.

44. Formula Mass • Way to compare masses of molecules of different compounds. • Calculate by adding up atomic masses of all atoms in a compound.

45. Example: Figuring Formula Mass • H2O means 2H and 1O • 2 (1.01 amu) = 2.02 +1 (16.00 amu) = 16.00 • Formula mass= 18.02 amu of H2O

46. An amu is very small, so to be usable in measurements, we equate the numbervalue of the formulamass in amu to an equalamount in grams.

47. Avogadro’s Number • The formula mass in grams of any element or compound contains 6.02 x 1023 atoms or molecules. • Known as Avogadro’s # or a “mole” of the substance.

48. Calculate the formula mass of calcium carbonate. • Write formula

49. 2. List number of atoms and atomic mass of each: CaCO3 • 1 Ca = 1(40.08) = 40.08 • 1 C = 1(12.01) = 12.01 • 3 O = 3(16.00) = 48.00

50. 3. Add up values to calculate formula mass • 40.08 12.01 + 48.00 100.09 amu for CaCO3