6-2 Covalent Bonding

What is a MOLECULE ?

Why would nature favor forming a covalent bond?

Beaker Breaker Based on the their locations on the periodic table are the following covalent bonds or ionic bonds MgCl 2 KCl NO 3 CO 2

Attraction: nucleus of one atom to the electron cloud of the other atom Repulsion: both nuclei repel…as do both electron clouds As atoms approach, attraction increases and potential E goes down Beyond a certain point, repulsion increases and E goes up ∴ bottom of valley on E curve is where there is a balance between attraction and repulsion

MOLECULE: neutral group of atoms held together by covalent bonds MOLECULAR COMPOUND: a cmpd that is made of molecules MOLECULAR FORMULA: shows the kinds & #s of atoms making up a molecule i.e. H 2 O DIATOMIC MOLECULE: molecule containing only 2 atoms i.e. H 2

Characteristics of the Covalent Bond Bond Length: average distance between 2 bonded atoms (min potential E) In the H 2 molecule, the 1s orbitals overlap and result in a increased electron density between the nuclei.

Bond energy: Energy required to break a chemical bond and form neutral isolated atoms Measured in kJ/mol (kilojoules/mole) Positive value Bond energy and bond length values will vary depending on what atoms an element is bonded to…ave values found on tables P.168)

Octet Rule Chemical cmpds tend to form so that each atom, by gaining, losing or sharing electrons, has an octet (8) of electrons in its highest (outermost) energy level Example of exceptions: –H: only has 2 valence e - s to reach stability –B: only has 6 valence e - s “ “ –Some elements have > 8 (occurs when atom bonds with highly electroneg elements & some “d” e - s are involved in bonding

Electron Dot Diagrams Electron configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol F: 1s 2 2s 2 2p 5

“other” dot diagram format Mg: [Ne]3s 2 “paired” electrons are shown as a pair

Lewis Structures Formulas in which…. – atomic symbols represent nuclei and inner- shell electrons –Dot-pairs or dashed between 2 atoms represent electron pairs in covalent bonds –Dots adjacent to only one atomic symbol represent unshared electrons

Lewis structure Structural Formula: indicates The kind, number, arrangement, and bonds..but NOT the unshared Pairs of the atoms in a molecule

Lewis structure Single bond: covalent bond produced by the sharing on ONE pair of electrons between 2 atoms

Draw the Lewis structure for carbon tetrachloride CCl 4 See page for rules Total # of valence electrons: –C: 1 x 4 e - = 4 e - –Cl: 4 x 7 e - = 28 e - Total # = 32 e -

or…..

Draw the Lewis structure for the following molecules: IBr CH 3 Br F 2 O SiCl 4 C 2 HCl (????)

Beaker Breaker Draw the Lewis structures showing the bonds for: HBr CH 4

Multiple Covalent Bonds DOUBLE BOND –sharing of TWO pairs of electrons between 2 atoms TRIPLE BOND –sharing of THREE pairs of electrons between 2 atoms Bond Energies: triple > double > single …..triple bonds are stronger and shorter

Draw the Lewis structure for sulfur trioxide, SO 3

S: 6 e - O: 3 x 6 e - Total # = 24 e - ?????

Resonance Structures bonding in molecules or ions that cannot be correctly represented by a single Lewis structure the experimental properties (observed properties) do NOT support the paper- and-pen structure suggested by the Lewis structure

Draw the resonance structure for sulfur trioxide, SO 3 How many valence electrons? How are the atoms arranged relative to each other (skeleton structure)? How must you distribute the valence electrons so all four atoms comply with the octet rule?

Draw the resonance structure for sulfur trioxide, SO 3 S: 6 e - O: 3 x 6 e - Total # = 24 e - (see next slide)

- Lab evidence shows that all three S-O bonds are the same….not 1/3 double and 2/3 single bonding! - The arrow shows that the real structure is an average of all three resonance structures

Covalent-network Bonding Covalent bonding exists throughout a large network of atoms…will be discussed in chapter 12 i.e. carbon atoms in a diamond

Look at sample problems p (if extra time at end of class)