Ionic Versus Covalent Bonding

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1 Ionic Versus Covalent Bonding Ionic compounds are formed when electrons are transferred from one atom to another The transfer of electrons forms ions Each ion is isoelectronic with a noble gas Electrostatic force (ionic bond) holds atoms together Covalent bonding involves sharing of electrons to achieve noble gas configurations for the atoms involved Covalent bond = attractive force resulting from atoms attracted to a shared pair of electrons Covalent Bonding First, some background The octet rule: The atoms tend to gain, lose or share electrons to have eight electrons in the valence shells (electronic configuration of noble gas, stable configuration) Carbon (group IVA) would need to gain or lose 4 electrons to achieve noble gas configuration in an ionic compound Diatomic molecules, like O 2, Cl 2, I 2 do not transfer electrons to form ionic bonds Cl - Cl +???? Does this look OK? Covalent Bonding H 2 Another type of bonding is at work here G N Lewis (1916) theorized that noble gas configurations could be attained by electron sharing Covalent Bonding H 2 Overlapping 1s orbitals form a molecular orbital For H 2 (a) Two noninteracting H atoms, each with one electron in its 1s orbital (b) As the two H atoms approach, the spins pair and the s orbitals merge into a molecular orbital, electrons are shared Each H atom is satisfied (c) Fully formed molecular orbital 1

2 Covalent Bonding Cl 2 Multiple Covalent Bonds Each Cl atom has a 3p orbital that is singly occupied Bonding pairs Lone pairs 3p 3s 2p 2s Cl 1s Check the octets! : C l Cl : + : Cl Cl : _ Each Cl atom in Cl 2 achieves its octet with a single Cl-Cl bond or What about N? Ṅ Ṅ Ṅ Ṅ + How can nitrogen satisfy its octet? : Cl Cl : : Cl Cl : Which bond is stronger, that in Cl 2 or in N 2? We can use Lewis dot symbols to build molecules, to show the bonding between atoms, and to predict the three-dimensional molecular structure of the resulting compound Powerful tool to predict molecular structure Start thinking in 3-D How to determine the central atom for polyatomic molecules (H 2 O,, CH 4 )? Electronegativity A measure of the ability of an atom in a molecule to draw electrons to itself -An atom with large negative electron affinity and large ionization energy has large electronegativity Example: fluorine, oxygen Use a systematic approach 2

3 24 Electronegativity A Systematic Approach 1 Identify the central atom based on electronegativities of the constituents atoms The central atom is the one with low electronegativity value Exception: Hydrogen (always be the terminal atom) 2 Recognize the number of bonds each atom should make to satisfy it s octet 3 Determine the total number of valence electrons in the molecule A Systematic Approach 5 Place one pair of electrons between each pair of bonded atoms to form a single bond 6 Use any remaining pairs as lone pairs around each terminal atom (except H) so that each terminal atom is surrounded by eight electrons 7 If the central atom has fewer than eight electrons at this point, move one or more of the lone pairs on the terminal atoms into a position intermediate between the center and the terminal atom to form double or triple bonds Number of valence electrons Usual number of covalent bonds Hydrogen needs one e - to achieve a noble gas configuration thus it forms one covalent bond Carbon Nitrogen Oxygen 1A 1 e H 3A 3 e B 3 bonds 4A 4 e C 4 bonds Si 4 bonds 5A 5 e N 3 bonds P 3 bonds (5) 6A 6 e O 2 bonds S 2 bonds (4, 6) 7A 7 e F Cl (3, 5) Br (3, 5) I (3, 5, 7) 8A 8 e He Ne Ar Kr Xe Fluorine 3

4 Formaldehyde, CH 2 O Which is the central atom? Ammonia, NH 3 Which is the central atom? Check the octet of C, is it satisfied? Check the octet of N, is it satisfied? SO 2 Which is the central atom? Ethane, C 2 H 6 How many electrons remain and where do they go? Check the octets of S and O, are they satisfied? Check the octets, are they satisfied? 4

5 Ethylene, C 2 H 4 Phosgene, COCl 2 How many electrons are left over and where do they go? Check the octets, are they satisfied? Hydrazine, N 2 H 4 Isoelectronic Species NO +, N 2, CN, CO What s common between these four? Hydrogen peroxide, H 2 O 2 5

6 Covalent bonding can also occur in polyatomic ions Sulfate (SO 4 2- ), nitrate (NO 3- ), ammonium (NH 4+ ) for example To generate Lewis structures, use the systematic approach except Nitrate, NO 3 How many electrons are left over and where do they go? Check the octets of O and N, are they satisfied? Place brackets around the structure: overall charge is 1- Another example, PO 4 3- Chlorate, ClO 3 6