Chapter 8 H H H H. Molecular Compounds & Covalent Bonding. Why do covalent bonds form? 8.1 Molecular Compounds. Properties of Molecular Compounds

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1 Chapter 8 Molecular Compounds & Covalent Bonding Why do covalent bonds form? If only group 5A, 6A, 7A atoms existed, ionic bonds can t form. NNMETALS Each atom needs electrons so they are not willing to lose any. If two ydrogen atoms are locked in a room together, what happens? Both atoms have 1 unpaired electron The electrons pair up. Covalent Bond formed. 8.1 Molecular Compounds Molecule: neutral group of atoms joined together by covalent bonds. (Sharing electrons) Consists of two or more nonmetals!!! Diatomic Molecule: a molecule consisting of two identical atoms Does it contain ionic or covalent bonds? Formula Ionic or Covalent Explanation CaCl 2 C 2 CaS Mg 3 (P 4 ) 2 NaBr Properties of Molecular Compounds Lower melting and boiling points than ionic compounds. Most are gases or liquids at room temperature. Atoms are attached by more than just electrical attraction. Why, No metals? 1

2 Molecular compounds are made of molecules, not INS! Ionic compounds are expressed as formula units, not molecules. A molecular formula is the chemical formula of a molecular compound. Molecular Formulas The chemical formulas of covalent compounds are correctly described as molecular formulas Chemical formula for molecular compounds. Shows how many atoms of each element a molecule contains. Subscripts are not always lowest whole number ratios. > (No simplification) Does not give the structure of the molecule. Ionic vs. Covalent 8.2 The Nature of Covalent Bonding Formula Unit Transfer electrons Metal Nonmetal Solid Crystals Good electrical conductor igh melting point Molecule Share electrons Nonmetal Nonmetal Solid, liquid, gas Poor electrical conductor Low melting point A single covalent bond is formed when a pair of electrons is shared between two atoms. Element xygen Nitrogen Carbon Electron Distribution (Show Boxes) 1s 2 2s 2 2p 4 1s 2 2s 2 2p 3 1s 2 2s 2 2p 2 Dot Structure Electrons needed Unpaired Eelctrons Electron configurations are slightly different when atoms form covalent bonds. Remember, a covalent bond is formed by the unpaired electrons in two atoms. For example, Carbon needs to form 4 bonds with ydrogen. So it must have 4 half filled orbitals instead of the neutral electron configuration. 1s 2 2s 1 2p 3, not 1s 2 2s 2 2p 2 1s 2 2s 1 2p 3 Carbon 2

3 Gilbert Lewis Stated... Sharing of electrons occurs if the atoms involved acquire the electron configurations of noble gases. Become stable by sharing. Single Covalent Bond ne shared pair of electrons. Each atom donates 1 electron to the bond. Represented by 1 dash. F F F Shared Pair F Bonding Rules Carbon: 4 unpaired electrons needs 4 electrons to be stable must form 4 covalent bonds xygen: 2 unpaired electrons needs 2 electrons to be stable must form 2 covalent bonds Bonding Rules Nitrogen: 3 unpaired electrons needs 3 electrons to be stable must form 3 covalent bonds Fluorine: 1 unpaired electron needs 1 electron to be stable must form 1 covalent bond Bonding Rules ydrogen: 1 unpaired electron needs 1 electron to be stable must form 1 covalent bond Chlorine: 1 unpaired electrons needs 1 electron to be stable must form 1 covalent bonds 3

4 Structural formulas show the arrangement of atoms in molecules and polyatomic ions. Chlorine bonding to Chlorine Dot Formula Structural Formula Dashes are used 1 dash: 2 shared electrons 2 dashes: 4 shared electrons 3 dashes: 6 shared electrons Dot Formula Cl Cl Structural Formula Cl Cl ow many electrons are donated by each chlorine? ow many unshared pairs are in the molecule? ow many electrons are being shared? ow many shared pairs are in the molecule? The pairs of valence electrons that are not shared between atoms are called unshared pairs of electrons, or unshared pairs. They are also called lone pairs or nonbonding pairs. Double and Triple Covalent Bonds Double covalent bonds involve two shared pairs of electrons. Represented by 2 dashes Triple covalent bonds involve three shared pairs of electrons. Represented by 3 dashes 4

5 xygen bonding to xygen Dot Formula Structural Formula Dot Formula Structural Formula ow many electrons are donated by each oxygen? ow many unshared pairs are in the molecule? ow many electrons are being shared? ow many shared pairs are in the molecule? Dot Formula 2 Structural Formula Dot Formula Structural Formula ow many electrons are donated by each hydrogen? ow many electrons are donated by the oxygen? ow many unshared pairs are in the molecule? ow many electrons are being shared? ow many shared pairs are in the molecule? C 4 Dot Formula Structural Formula C C C ow many electrons are donated by each hydrogen? ow many electrons are donated by the carbon? ow many unshared pairs are in the molecule? ow many electrons are being shared? ow many shared pairs are in the molecule? 5

6 Dot Formula Structural Dot Formula Fluidity of Shared Electrons C C Dot Formula C 2 Structural Formula Dot Formula C Structural Formula C ow many electrons are donated by each oxygen? ow many electrons are donated by the carbon? ow many unshared pairs are in the molecule? ow many electrons are being shared? ow many shared pairs are in the molecule? C 2 4 Dot Formula Structural Formula Two chemists go into a restaurant. The first one says "I think I'll have an 2." The second one says "I think I'll have an 2 too" and he died. 6

7 Why don t metals usually form covalent bonds? Mg has 2 valence electrons. ow many covalent bonds must it form to be stable? ow many electrons does it have to donate? ow about Aluminum? Why don't metals form covalent bonds? Mg K ow many more electrons does each atom need to be stable? ow many covalent bonds can each atom form? Bonding of diatomic molecules Diatomic molecules are more stable together than apart. F, I, N,, Br, Cl, Examples page 222 Electron Dot Structures Bonding of Diatomic Molecules Coordinate Covalent Bonds Emergency Bonds Carbon monoxide example Electrons are fluid (mobile) nce formed, they act as normal covalent bonds. Polyatomic ion formation. Fluidity of Shared Electrons 7

8 Coordinate Covalent Bonds C C C C C Bond 1 carbon with 1 oxygen Carbon is unstable. nly 6 surrounding electrons. Carbon needs 2 more electrons, but xygen is stable. Carbon is sharing 2 more electrons, but didn t have to donate any of them. C= C= C= xygen is stable! 8 valence electrons & 2 unshared pairs. xygen lets carbon use 1 of it s unshared pairs. xygen is still stable. It donated both electrons being shared in the Coordinate Bond. Coordinate Covalent Bonds S 2 S S S S S A coordinate covalent bond is formed when one atom contributes both bonding electrons in a covalent bond. Arrows are used to indicate a coordinate covalent bond Ex.) C, N 4 +, 3 +, S 3, S 4 2 N N N N + + The unshared pair is now a bond, not an unshared pair. 8

9 Negative Polyatomic Ions N Resonance Resonance structures occur when two or more valid electron dot formulas can be written for a molecule. Ex. 3, C 3 2 Same formula, different Structures Exceptions to the ctet Rule Sometimes it is impossible to write electron dot structures that fulfill the octet rule. ccurs whenever the total number of valence electrons in the species is an odd number or less than eight. nly certain metals: Be, Al, B 9

10 Exceptions to the ctet Rule Some metals do form covalent bonds, but result in a shortage of valence electrons. Why is BF 3 attracted to N 3? F Exceptions to the octet F F B F F B N N F P S C N

11 Chapter 8 Part 2 Molecular Shapes & Intermolecular Forces 8.3 VSEPR Theory VSEPR theory states that because electron pairs repel, molecules adjust their shapes so that the valence electron pairs are as far apart as possible. VSEPR Theory (cont.) Valence Shell Electron Pair Repulsion. Bond angles are created by this repulsion of electrons More about shapes Molecules are 3 dimensional. Molecular shape is effected by unshared pairs of electrons. Each shape has a specific bond angle. Bond Angles Tetrahedral = Linear = 180 Bent = 105 Pyrimad = 107 Trigonal Planar = 120 Molecular Shapes Bent Pyramidal Tetrahedral N C 11

12 Common Molecular Shapes Linear Triatomic: CN, C 2 All binary compounds 2 Bonds & 0 unshared pairs No unshared pairs to bend molecule Trigonal Planar: B 3, C 2 3 bonds & 0 unshared pair Bent triatomic: 2 2 bonds & 2 unshared pair Unshared pairs bend the molecule 2 unshared pair is bent most Trigonal Pyramidal: N 3 3 bonds & 1 unshared pair Tetrahedral: C 4 4 bonds & 0 unshared pair

13 P 3 CF 4 2 S Al 3 Ionic vs. Molecular Compounds Ionic Compounds form solid crystals. Why? Molecular Compounds form gases, liquids, & solids. Why? ow many electrons are shared? Which atom has a greater electronegativity? Which atom has become more negative? F F F 13

14 Polar or Non Polar 8.4 Polar Bonds and Molecules Cl Cl Cl When the atoms in a bond are the same, the bonding electrons are shared equally and the bond is a nonpolar covalent bond Ex. diatomics When two different atoms are joined by a covalent bond and the bonding electrons are shared unequally, the bond is a polar covalent bond, or simply a polar bond. The atom with stronger electronegativity in a polar bond acquires a slightly negative charge. The less electronegative atom acquires a slightly positive charge. Ex. Cl, 2 Electronegativity Ability of atoms to attract electrons. Determines the reactivity and strength of polar covalent bonds. Cl: Moderately polar covalent F: Very polar covalent (Reactive) See page 177. Electronegativity of Atoms F = 4.0 Br = 2.8 = 3.5 I = 2.5 N = 3.0 C = 2.5 Cl = 3.0 S = 2.5 ydrogen =

15 Which bond is the most polar? Cl F I A molecule that has two poles is called a dipolar molecule, or dipole. Not every molecule with polar bonds is itself polar. Polar Molecules In a polar molecule one end of the molecule is slightly negative and the other is slightly positive. Dipolar molecules Ex.) Cl, 2, F Is a water molecule polar? Is a C 4 molecule polar? C Is a C 2 molecule polar? C 15

16 Non Polar Molecules When a molecule has no difference in charge between opposite ends or sides of the molecule. Not very reactive! 2, F 2, C 2, Cl 2, CCl 4 Water is only polar due to it s shape Attractions Between Molecules In addition to covalent bonds in molecules, there are attractions between molecules, or intermolecular attractions Covalently bonded atoms attracted to each other. Gases Liquids Solids Nonpolar molecules are usually gases! No attraction Nonpolar Molecules Dipole Attraction Polar Molecules Ionic Attraction Ions Form Crystals C C C C C (Between) Intermolecular Attractions old molecules together. Weaker than either an ionic or covalent bond. They are responsible for whether a molecular compound is a gas, liquid, or solid. Intermolecular attractions Van der Waals forces The weakest attractions between molecules. Not Bonds!!!!!! Three types are Dispersion forces, Dipole interations, and ydrogen bonds ydrogen > Dipole > Dispersion Attractions between polarized molecules 16

17 Dispersion Forces The weakest of all intermolecular interactions. Thought to be caused by the motion of electrons. Strength of dispersion forces increases as the number of electrons in a molecule increases Electrons are not lost or gained Dispersion forces Due to movement, the electrons move to one side and create a separation of charge. Dipole Interactions ccur when polar molecules are attracted to one another Electrostatic attractions occur between theoppositely charged regions of dipolar molecules. Similar to ionic bonding, but much weaker attraction. A covalent bond with a dipole. A cation attracted to a dipole. A dipole attracted to a dipole Most dipoles involve hydrogen. ydrogen Bonds Strongest of all intermolecular attractions. (Must involve hydrogen!) Dipole interactions with hydrogen. An atom or molecule is attracted to a ydrogen atom that is already bonded to an atom with high electronegativity. ydrogen Bonds (cont.) The covalently bonded hydrogen becomes slightly positive. Unshared electron pairs and atoms with high electronegativity become attracted to the slightly (+) ydrogen. 17

18 ydrogen Bonds (cont.) ydrogen Bonding in Water Attraction ydrogen Bonding is the attraction between polar molecules with hydrogen. Why is there so much water? Water molecules are polar. The oxygen atom becomes slightly negative and each hydrogen becomes slightly positive. This causes an intermolecular attraction between water molecules. The attraction water molecules have for one another is called ydrogen bonding. Properties of Molecular Substances The physical properties of a compound depend on the type of bonding it displays. Ionic or Covalent Network Solid: All of the atoms are covalently bonded to each other. (Crystals) No intermolecular attractions. Most stable type of molecule. Very high melting point. Ex.) Diamonds 18

19 rganic Compounds All Carbon containing compounds Except carbon oxides, carbides, and carbonates which are inorganic. ydrocarbons Simple organic compounds Contain carbon and hydrogen Carbon forms 4 bonds and hydrogen 1 bond Alkanes ydrocarbons that have only single bonds between atoms. Carbons are saturated with ydrogen atoms Alkenes Unsaturated ydrocarbons that have one or more double bonds between carbon atoms. Carbons are unsaturated with ydrogen atoms 19

20 Alkynes Unsaturated ydrocarbons that have one or more triple bonds between carbon atoms. Carbons are unsaturated with ydrogen atoms Isomers Two or more compounds that have the same molecular formula but different molecular structure. Structural Isomers Two or more compounds that have the same molecular formula but are bonded in a completely different order, therefore changing its properties. End of Part 2 Intermolecular Forces C