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powerpoint slides slides for bonding and structures of chemicals

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York University
CHEM 1000
William Pietro

Chemical Bonding • Petrucci, Harwood and Herring: Chapters 11 and 12 •ms: – To look at bonding and possible shapes of molecules • We will mainly do this through Lewis structures – To look at ionic and covalent bonds – Use valence shell electronic structure to predict shapes of molecules CHEM 1000A 3.0 Chemical Bonding I 1 Chemical Bonding • The most important concepts here are: – Electrons, particularly valence electrons play a fundamental role in chemical bonding. – When elements combine to produce compounds they are attempting to achieve a valence shell like a noble gas CHEM 1000A 3.0 Chemical Bonding I 2 •1 Chemical Bonding • The driving force for the production of chemical bonds is the need for an atom to complete a valence shell (usually 8 electrons) – Often electrons can be transferred from one atom to another to make ions. The atoms are then held together by coulombic forces in an ionic bond – More often the only way an atom can gain electrons is by sharing. This sharing produces a covalent bond CHEM 1000A 3.0 Chemical Bonding I 3 Lewis Symbols • This is a way of representing the valence electrons in an element – It does not include the inner shell electrons – It does not include the spin of an electron – e.g. Si ([Ne]3s 23p ) Si – N ([He]2s 22p ) N CHEM 1000A 3.0 Chemical Bonding I 4 •2 Lewis Structures • These are the combination of Lewis symbols that represents the sharing or transfer of electrons in a molecule –Exmps – Ionic bond Na x + Cl [Na]+ [ Cl ]- x x – Covalent bond H + Cl H Cl CHEM 1000A 3.0 Chemical Bonding I 5 Ionic Compounds • We don’t usually see isolated ionic compounds – Normally they are in crystals where one anion (negative) is attached to several cations (positive) and vice versa. Electrical neutrality means the total number of each ion is the same. CHEM 1000A 3.0 Chemical Bonding I 6 •3 Ionic Compounds CHEM 1000A 3.0 Chemical Bonding I 7 Covalent Compounds • Here electrons are shared between two atoms. – (May actually be more than just a couple of shared electrons) – The electrons spend “time” around each atom in the covalent bond – The overall effect is that each atom has “more” (usually a full shell of) valence electrons. CHEM 1000A 3.0 Chemical Bonding I 8 •4 Covalent Compounds •ml – Single covalent bond HCl Lone pairs Bond pair H Cl H Cl Note that there are 6 electrons around the Cl that are not involved in bonding. It is normal to talk about these as lone pairs, in contrast to bond pairs CHEM 1000A 3.0 Chemical Bonding I 9 Multiple Covalent Bonds • Often the sharing of one electron does not “fill” the valence shell of an atom. • e.g. 2 N N N N N N N N CHEM 1000A 3.0 Chemical Bonding I 10 •5 Multiple Bonds • We use a number of different ways to describe multiple bonds • For 1 bond pair For 3 bond pair – Single bond triple bond – bond order =1 bond order =3 • For 2 bond pairs – Double bond – bond order =2 CHEM 1000A 3.0 Chemical Bonding I 11 Bond Length • Bond Length – The distance between two atoms joined by a covalent bond – As the bond order increases the bond length decreases Bond Length/pm Bond Length/pm C-C 154 N-N 145 C=C 134 N=N 123 C C 120 N N 110 CHEM 1000A 3.0 Chemical Bonding I 12 •6 Bond Energy • To separate two atoms that are joined by a covalent bond, energy must be supplied. This is the Bond Dissociation Energy (D). • This is equal to the energy released when the bond is formed. H2(g) 62H(g) ∆H = D(H-H) = +435.93 kJ mol -1 • Listed bond energies are usually an average over a number of compounds CHEM 1000A 3.0 Chemical Bonding I 13 Polarity of Bonds In many molecules the electrons in the covalent bond are not shared equally between the two atoms. – They are associated more strongly with one of the atoms. – This leads to a polar covalent bond. CHEM 1000A 3.0 Chemical Bonding I 14 •7 Polarity of Bonds The molecules are symmetric,the centre of positive and negative charge are the same This molecule is not symmetric. The electrons in the bond are more closely associated with the Cl. Therefore the centre of the negative charge in the bond is not the same as the center of the positive charges. H Cl H Cl This gives the molecule a permanent dipole (moment) δ+ δ- CHEM 1000A 3.0 Chemical Bonding I 15 Polarity of Bonds • The ability to attract electrons in a bond appears to be related to electron affinity but we use a molecular property to describe it. • Electronegativity is a quantitative measure of an atom’s ability to compete for electrons with other atoms to which it is bonded CHEM 1000A 3.0 Chemical Bonding I 16 •8 Polarity of Bonds: Electronegativities CHEM 1000A 3.0 Chemical Bonding I 17 Polarity of Bonds: Ionic Character The larger the electronegativity difference between the atoms, the more ionic the bond CHEM 1000A 3.0 Chemical Bonding I 18 •
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