CHEMICAL BONDING
***Occurs when atoms of elements combine together to form compounds.*****
Formation of compounds involve adjustments in the position of one or more valence electrons. PE is lower in bonded atoms. Attractive force that develops is called "chemical bond process is called "bonding" and occurs during chem. reactions
Two (or three) methods Ionic bonding - attraction of ions Covalent bonding - shared pairs of electrons (Metallic bonding) - alloys (metals) not compounds
Covalent Bond a. an attractive force that develops between atoms that are sharing pairs of electrons EN < 1.7 b. Hydrogen H 2 H + H H:H (dot diagram) Structural formulas use a dash H - H
Bond energy Energy required to break a bond. Bonds form to lower PE, so breaking bonds will increase PE. Depends on the size of atoms and number of electrons shared.
Breaking Bonds is always endothermic Energy is required. ALWAYS.
Bond Length
Why not ionic? The difference in electronegativity is less than 1.7 Electrons are not pulled away from either atom. They are shared.
Fluorine F2
Ammonia -NH3
Special Types of Covalent Bonds Multiple bonds Occasionally atoms share more than one pair of electrons Double bond two shared pairs Ex. O 2 O=O or
- Triple bonds - two atoms share three pairs of electrons. Ex. N 2 N N or
Coordinate Covalent Bonds Occurs when one atom donates both of the shared electrons in a pair Is most commonly seen in polyatomic ions. Ex. NO 3-1
How to write Lewis Structures Set out the atoms think symmetry. Count all the valence e - Insert single bonds first, then fill rest. Usually, the e - are paired. each nonmetal atom requires an octet. H only requires 2 e -. Multiple bonds may be needed. Readily formed by C, N, O, S, and P.
Exceptions to the octet rule Resonance occurs when more than one Lewis structure is possible. Ex. polyatomic ions Expanded valence some elements in the third and fourth period have the ability to have more than 4 pairs of electrons around the central atom. Ex. PCl 5 or SF 6
Resonance more than one Lewis diagram is possible. + + O O O - - O O O
Exceptions to the Octet Rule Expanded valence. Cl P Cl Cl Cl Cl P Cl Cl Cl F F F F S F F
Polarity An unequal sharing of electrons due to difference in electronegativity. Polar bond Any bond with EN 0.5-1.7 Polar molecule Has a positive end and a negative end. Called dipoles. Occurs if there are polar bonds and the molecule is asymmetrical. Causes intermolecular attraction to increase.
An ionic bond -(electrovalent) a. Definition - An attraction that forms between oppositely charged ions b. + ions + - ions neutral compound EN > 1.7
An ionic bond -(electrovalent) c. dot diagrams of ions 1) formation A + IE positive ion (A + ) + e- B + e- negative ion (B - ) + EA 2) bonding A+ + B- AB
Ionic Solids Have strong attractions in all directions Metal-nonmetal (or polyatomic ions) High MP and BP Tend to be hard, solid, and brittle Some dissolve readily Non conductors as solids, but will conduct when molten or dissolved
Ionic solids are brittle due to Crystal lattice + - + - - + - + + - + - - + - +
Ionic solids are brittle Strong Repulsion breaks crystal apart. - + - + + - + - - + - +
Lattice Energy (ionic) More correct than bond energy for crystals. Lattice Energy = k(q 1 Q 2 / r) k is a constant that depends on the structure of the crystal. Q s are charges. r is internuclear distance. Lattice energy is greater with more highly charged ions.
Molecular Substances Covalently bonded substances show more variety in phases and properties. Tend to be insulators(nonconductors). Nonmetal-nonmetal combinations 1. Nonpolar tends to be gases at room temp. have only dispersion (Vanderwaals) forces. Have low MP and BP and high VP 2. Polar (dipoles) tend to be liquids or solids at room temp. Have MP and BP and VP 3. H- bonding very strong type of polar
Metallic Bonding In metals the valence electrons are not bonded to any specific atom. (delocalized) Able to move freely over the positive centers. Referred to as a sea of electrons Moving charge allows current, malleability, ductility,etc.
Sea of Electrons Electrons are free to move through the solid. Metals conduct electricity. + + + + + + + + + + + +
Malleable + + + + + + + + + + + +
Malleable Electrons allow atoms to slide by. + + + + + + + + + + + +
Shapes of Molecules 3 or more atoms Results from VSEPR and/or hybridization of orbitals. Can affect properties of the molecule (polarity)
VSEPR VALENCE SHELL ELECTRON PAIR REPULSION A theory which describes the shapes of molecules based on the idea that pairs of electrons will repel each other as much as possible.
H H C H H VSEPR Single bonds fill all atoms. There are 4 pairs of electrons pushing away. Farthest apart is at 109.5º.
4 atoms bonded Basic shape is tetrahedral. A pyramid with a triangular base. Same shape for everything with 4 pairs. H H H C 109.5º H
3 bonded - 1 lone pair Still basic tetrahedral but you can t see the electron pair. Shape is called pyramidal. H N H H N H H H <109.5º
2 bonded - 2 lone pair Still basic tetrahedral but you can t see the 2 lone pair. Shape is called bent. H O O H H H <109.5º
3 atoms no lone pair The farthest you can get the electron pair apart is 120º. Shape is flat and called trigonal planar. H H C 120º C O H O H
2 atoms no lone pair With three atoms the farthest they can get apart is 180º. Shape called linear. 180º O C O
VSEPR Table Example #of BP #of LP shape bond angle BeH 2 2 0 linear 180 BH 3 3 0 trigonal 120 (planar) CH 4 4 0 tetrahedral 109 NH 3 3 1 pyramidal 107 (unshared pairs take up more space) H 2 0 2 2 bent 105 See table 6-5 in text for more info
Polar molecules are asymmetrical and have polar bonds. Nonpolar molecules are symmetrical and may or may not have polar bonds.
Intermolecular Attractions (Vanderwaals forces) Attractive forces between molecules Happens to covalent molecules. Strongest - hydrogen bonds Medium - dipoles/ polar Weakest - dispersion or London forces ( All molecules have London forces)
Hydrogen bonding In some highly polar compounds, a H atom is attracted to, and forms a weak bond with, an adjacent molecule. Only occurs in compounds where there is: H-F (strongest) H-O H-N (weakest)
Hydrogen Bonding d + d - H O H d +
Hydrogen bonding H O H
H-bonding Gives water its very unusual properties. High MP and BP Holds the DNA molecule together. Provides stability and shapes for proteins, enzymes, etc. Strongest type of intermolecular attraction.
London dispersion (Van der waals) Forces Weakest type of intermolecular attraction. Develops between nonpolar molecules due to momentary shifts in the electron positions. Strength of attraction is directly proportional to the number of electrons (wax is a nonpolar molecule but really big)
Network Solids Large arrays of covalently bonded crystals. Do not conduct, very high MP and BP, hard solids Examples: diamond, graphite, SiO 2 (very few easiest to just memorize) More on this later
Bond length and energy Bond length depends on the size of the atoms/ions and the number of bonds between them C-C is longer than C=C is longer than C C Bond energy is related to bond length. Longer bonds tend to have lower BE
Breaking Bonds is always endothermic Energy is required. ALWAYS.
Endothermic reactions
Forming bonds is exothermic Energy is released. ALWAYS
Exothermic reactions