2 Lesson 1:Covalent Bonding Covalent bonds: atoms held together by sharing electrons.Molecules: neutral group of atoms joined together by covalent bonds.Diatomic molecule: molecule consisting of 2 atoms. Remember them: F2, Cl2, I2, Br2, H2, N2, O2Molecules tend to have lower melting and boiling points than ionic compounds.YouTube - Making Molecules with Atoms
3 Molecular FormulaShows how many atoms of each element a molecule contains.Naming binary molecular compoundsComposed of two nonmetals; often combine in more than one way. Ex. CO and CO2Prefixes are used to name binary molecular compounds.PrefixMono-Di-Tri-Tetra-Penta-Hexa-Hepta-Octa-Nona-Deca-Number12345678910
4 Binary Compounds Containing Two Nonmetals To name these compounds:give the name of the less electronegative element first with the Greek prefix indicating the number of atoms of that element presentAfter give the name of the more electronegative non-metal with the Greek prefix indicating the number of atoms of that element present and with its ending replaced by the suffix –ide.Do not use the prefix mono- if required for the first element.
7 Naming Binary Compounds YesMetal Present?NoYesMoleculeUse GreekPrefixesDoes the metal formmore than one cation?NoYesIonic compound (cation has morethan one charge) Determine theCharge of the cation; use a Romannumeral after the cation name.Ionic compound (cation hasone charge only)Use the elementname for the cation.Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 98
9 Lesson 2: The Nature of Covalent Bonding Introduction with balloon activityoctet rule: electron sharing occurs usually so that atoms attain the electron configurations of noble gases.Single covalent bond: two atoms held together by sharing a pair of electrons. Shown as two dots or as a long dash.A pair of valence electrons that is not shared between atoms is called an unshared pair.
11 Double bonds: covalent bond formed by sharing two pairs of electrons Triple bonds: covalent bond formed by sharing three pairs of electrons.
12 A dash may replace a pair of dots. Covalent bonding with equal sharing of electrons occurs in diatomic molecules formed from one element.chlorineiodinehydrogennitrogenA dash may replace a pair of dots.
14 Lesson 3:Molecular Structure Structural formula: uses symbols and bonds to show relative position of atoms.Steps to determine Lewis structures for moleculesPredict the location of certain atoms.Hydrogen is always an end atomThe least electronegative atom is the central atom (usually the one closer to the left on periodic table)Find the total number of electrons available for bonding. (# of valence electrons of atoms in molecule)Determine the number of bonding pairs by dividing the total number of electron by 2
15 Place one bonding pair (single bond) between central atom and terminal atoms. Subtract pairs used in step 4 from bonding pairs in step 3. Place lone pairs around each terminal atom bonded to the central atom to satisfy the octet rule. Any remaining pairs are assigned to the central atom.If the central atom does not have an octet, convert one or two of the lone pairs on the terminal atoms to a double or a triple bond between central and terminal atom. Some elements like Be, B, Al do not form a complete octet, S and P can have more than 8 valence electrons.
16 Ex. 1 Draw the lewis structure for ammonia, NH3 Hydrogen is an end atom and nitrogen is the central atom.Total number of valence electrons:(1 nitrogen x 5 valence electrons)+ (3 hydrogens x 1 valence electron)= 8 valence electrons.Total number of bonding pairs= 8/2 = 4Draw single bond from each H to NNH
17 Ex. 1 Draw the lewis structure for ammonia, NH3 Subtract the number of pairs of electrons used from the total pairs of electrons: =1 pair availableOne lone pair remains, hydrogen can have only one bond, assign the lone pair to the central atom, N.NH
18 Ex. 2 Draw the lewis structure for carbon dioxide, CO2 Oxygen atoms are end atoms and carbon is the central atom.Total number of valence electrons:(1 carbon x 4 valence electrons)+ (2 oxygen x 6 valence electron)= 16 valence electrons.Total number of bonding pairs= 16/2 = 8Draw single bond from each C to OCO
19 Ex. 1 Draw the lewis structure for carbon dioxide, CO2 Subtract the number of pairs of electrons used from the total pairs of electrons: =6 pair availableAdd three pairs of electrons to each oxygen.CO
20 Ex. 1 Draw the lewis structure for carbon dioxide, CO2 No lone pairs remain for carbon. Carbon does not have an octet, use a lone pair from each oxygen to form a double bond with the carbon atom.COCO
22 Lesson :4 Exception to octet rule Some molecules have an odd number of valence electrons and cannot form an octet around each atom.Some molecules form with fewer than eight electrons present around an atom. Ex. BoronSome compounds have central atoms with more than 8 electrons. This is called an expanded octet. Ex. S, Xe and P
23 Ex. 3 Draw the lewis structure for XeF4 (exception octet rule) F is an end atom and nitrogen is the central atom.Total number of valence electrons:(1 xenon x 8 valence electrons)+ (4 fluorines x 7 valence electron)= 36 valence electrons.Total number of bonding pairs= 36/2 = 18Draw single bond from each F to XeXeF
24 Ex. 1 Draw the lewis structure for XeF4 (exception octet rule) Subtract the number of pairs of electrons used from the total pairs of electrons: =14 pairs available14 lone pairs remain, place them around each fluorine so that each fluorine has 8 valence electronsXeF
25 Ex. 1 Draw the lewis structure for XeF4 (exception octet rule) There are 2 pairs of electrons still available, place around Xe which is capable of having more than 8 valence electron.XeF
26 Molecular Shape VSEPR (Valence shell electron pair repulsion) Model The repulsion between electron pairs in a molecule result in atoms existing at fixed angles from each other. (Remember balloon activity)Shared electron pairs repel each otherA greater repulsion occurs between unshared electron pairs and shared electron pairs.
27 Bonding and Shape of Molecules: Count number of bonds and unshared pairs of electrons AROUND CENTRAL ATOM and then use table below to determine shape of molecule.Numberof BondsNumber ofUnshared PairsCovalentStructureShapeExamples23412-Be-LinearTrigonal planarTetrahedralPyramidalBentBeCl2BF3CH4, SiCl4NH3, PCl3H2O, H2S, SCl2BCN:O:
28 Use table on last slide to determine shape of molecule. SO2HOShape: bent2 bonds and 2 unshared pairs
29 Carbon tetrachloride Cl Cl C Cl Cl Cl C CCl4 Shape: Tetrahedral 4 bonds and 0 unshared pairs.Carbon tetrachloride – “carbon tet” had been used as dry cleaning solventbecause of its extreme non-polarity.
30 Classwork: do in your notebook Determine the shape for the following molecules (first draw the lewis structure for the molecule and then use the table on slide #7 to determine the shape taking in consideration the number of bonds and unshared pairs of electrons around the CENTRAL ATOM.)BF OCl CF44. NH BeI2