Chapter 8: Periodic Properties of the Elements Chemical Bonds ionic bond covalent bond metallic bond

Chapter 8: Periodic Properties of the Elements The formation of chemical bonds involves valence electrons Lewis Symbols: “shorthand” for depicting valence electrons Gilbert Lewis ( )

Chapter 8: Periodic Properties of the Elements Lewis dot structures: H Li He BeBCNOFNe NaMgAlSiPSClAr For representative (main group) elements: group number = number of valence electrons 1A 2A3A4A5A6A7A 8A

Chapter 8: Periodic Properties of the Elements Noble gases, except helium, have When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence electrons (an “octet” of electrons) K Cl + K+K+ +

Chapter 8: Periodic Properties of the Elements Na + Cl - Na + Cl - Formation of an ionic lattice

Chapter 8: Periodic Properties of the Elements Na + (s) + Cl - (g) NaCl (s) very exothermic  H<0

Chapter 8: Periodic Properties of the Elements Na + Cl - Na + Cl - HH

Chapter 8: Periodic Properties of the Elements charge on ions distance between charges Strength of ionic bond depends on E el the larger E el, the stronger the bond

Chapter 8: Periodic Properties of the Elements The stronger the ionic bond the the melting point SrF 2 +2, -2 66, , , o C r1r1 r2r2

Chapter 8: Basic Concepts of Chemical Bonding Covalent Bonds … HH + HH FF + FF

Chapter 8: Basic Concepts of Chemical Bonding Covalent Bonds HH + HH FF + FF A shared electron pair is drawn as a dash (two electrons!) Unshared electrons are drawn as dots

Chapter 8: Basic Concepts of Chemical Bonding Single and Multiple Bonds FF + F F OO + C + O C O N N N + N

Chapter 8: Basic Concepts of Chemical Bonding Single and Multiple Bonds X

Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity Cl non-polar covalent bond: equal sharing of electrons When both atoms attract bond electrons equally, electrons are shared equally

Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity H Cl polar covalent bond: unequal sharing of electrons ++ If one of the atoms attracts bond electrons more strongly, electrons are shared unequally 

Chapter 8: Basic Concepts of Chemical Bonding Bond Polarity and Electronegativity H Cl polar covalent bond: unequal sharing of electrons ++ For comparison:  ionic bond: electrons are not shared Na + Cl -

Chapter 8: Basic Concepts of Chemical Bonding How do we know when a non-polar, polar, or ionic bond is formed? The tendency of atom to attract electrons when forming a molecule is summarized in the concept of electronegativity Increase in Electronegativity

Linus Carl Pauling ( ) developed concept of electronegativity the electronegativity scale assigns a value of 4 to Fluorine and 0.7 to Cesium, the least electronegative element. least electronegative atoms are those that give up electrons most easily. Chapter 8: Basic Concepts of Chemical Bonding

How do we know when a non-polar, polar, or ionic bond is formed? If the difference in Electronegativity,  EN, is… …smaller than 0.5  EN nonpolar bond …greater than or equal to 0.5 AND smaller than   EN polar bond …greater than or equal to 2.0  EN  2.0 => ionic bond

Chapter 8: Basic Concepts of Chemical Bonding Among the following examples, which bond is most polar? S

Chapter 8: Basic Concepts of Chemical Bonding Among the following examples, which bond is shortest? S Bond length depends on (a) radii of the bonded atoms (b) the number of bonds between atoms remember that the atomic radii decrease along a period in the P.T.

Chapter 8: Basic Concepts of Chemical Bonding If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break? S

Chapter 8: Basic Concepts of Chemical Bonding Drawing Lewis Structures of Molecules If the compound contains more than 2 atoms: how are the atoms bonded and, if there are nonbonding electron, where are they?

Chapter 8: Basic Concepts of Chemical Bonding Molecules with a central atom : NH 3, PCl 3, CHCl 3 central atom is generally the first in the molecular formula

Chapter 8: Basic Concepts of Chemical Bonding …unless the first element is Hydrogen : H2OH2O HCN (same order as in formula)

Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (4) place any leftovers from (1) on the central atom check that central atom has octet (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds NH n/a + 2 n/a Rules for Drawing Lewis Structures

Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds CO (4) place any leftovers from (1) on the central atom check that central atom has octet

Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds SF 2 (4) place any leftovers from (1) on the central atom check that central atom has octet

Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet Ions NH 4 + for ions, the charge is generally indicated by square brackets and the sign

Chapter 8: Basic Concepts of Chemical Bonding (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet Ions ClO 2 - for ions, the charge is generally indicated by square brackets and the sign

Chapter 8: Basic Concepts of Chemical Bonding Exceptions to the Octet rule On occasion, an atom in a molecule does not have an octet of valence electrons: If the molecule has an odd number of valence electrons an atom may have less than an octet [mainly Be, B] an atom may have more than an octet [period (n)  3]

Chapter 8: Basic Concepts of Chemical Bonding Exceptions to the Octet rule: odd number of electrons (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet NO 2

Chapter 8: Basic Concepts of Chemical Bonding Exceptions to the Octet rule: less than an octet (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet BF 3

Chapter 8: Basic Concepts of Chemical Bonding Exceptions to the Octet rule: more than an octet (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet BrF 5

Chapter 8: Basic Concepts of Chemical Bonding Resonance (1) sum valence electrons from all atoms: these are the ones that need to be distributed (3) complete "octets" of atoms bound to central atom (2) connect atoms by covalent bonds: count electrons used up (5) If there are not enough electrons to give the central atom an octet, try multiple bonds (4) place any leftovers from (1) on the central atom check that central atom has octet SO 3

Chapter 8: Basic Concepts of Chemical Bonding Which of the following molecules exhibits resonance? NO 3 - HCNSO 2 Draw Lewis structures!

Chapter 8: Basic Concepts of Chemical Bonding If you had to guess, which of the following bonds do you think would be most difficult (require the largest energy) to break? S

Chapter 8: Basic Concepts of Chemical Bonding Bond Enthalpy… …is the energy required to break a covalent bond …is always positive (g) C (g) + 4 H (g)  H = 1660 kJ/mol C H (g) C (g) + H (g) D (C-H) = 1660/4 kJ/mol = 415 kJ/mol per C-H bond:

Chapter 8: Basic Concepts of Chemical Bonding Bond Enthalpy is the energy required to break a covalent bond … and it offers yet another way to estimate the  H o rxn  H o rxn = Σ bond enthalpies of bonds broken – Σ bond enthalpies of bonds formed  H o rxn = Σ n x D broken – Σ m x D formed

Chapter 8: Basic Concepts of Chemical Bonding Estimate  H o for the following reaction: CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g)

Chapter 8: Basic Concepts of Chemical Bonding Estimate  H o for the following reaction: CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (g)  H o rxn = Σ n x D broken – Σ m x D formed  H o rxn = [4 x x 498] – [2 x x 463] = kJ/mol