Chapter 8 Covalent Bonding Section 8.1 The Covalent Bond * Section 8.2 Naming Molecules Section 8.3 Molecular Structures * Section 8.4 Molecular Shapes Section 8.5 Electronegativity and Polarity

The stability of an atom, ion or compound is related to its energy: lower energy states are more stable. Atoms achieve noble gas configuration by forming compounds (substances composed of two or more different elements chemically combined). They can be either ionic compounds or molecular compounds. A covalent bond results from sharing electrons (usually between nonmetallic elements. A molecule is formed when two or more atoms bond covalently and is lower in potential energy than its constituent atoms.

A molecule is the smallest electrically neutral unit of a substance that still has the properties of the substance and is held together by covalent bonds. Molecules of one compound are identical, but different from molecules of other compounds. water H2O 2 Hydrogen atoms and 1 oxygen atom carbon dioxide CO2 1 carbon atom and 2 oxygen atoms

“start at 7, make a 7, plus hydrogen” Seven nonmetallic elements that exist as diatomic molecules Chlorine Cl2 Bromine Br2 Iodine I2 Hydrogen H2 Fluorine F2 Oxygen O2 Nitrogen N2 “start at 7, make a 7, plus hydrogen” forget At2, astatine! because its radioactive, unstable, half life of 8 hrs

Different ways to display molecular formulas Perspective Drawing Molecular Formula Lewis structure Space-filled Molecular Model Structural Formula Ball-and Stick Molecular Model Strategy to draw Lewis structure see also p. 254 5

is one shared pair of valence electrons “Lewis electron dot structure” A single covalent bond is one shared pair of valence electrons H H or “Lewis electron dot structure”

Every single dot is looking for one other single dot of another atom Pairs of valence electrons that are not shared between atoms are called unshared, free, lone or nonbonding electron pairs. Every single dot is looking for one other single dot of another atom (like holding hands)

s s Sigma bonds are single covalent bonds. Sigma bonds occur when the pair of shared electrons is in an area centered between the two atoms. s s = Sigma (lower case “s” in Greek alphabet)

Multiple Covalent Bonds Double bonds form when two pairs of electrons are shared between two atoms. Triple bonds form when three pairs of electrons are shared between two atoms.

A multiple covalent bond consists of one sigma bond and at least one p bond. The pi (p) bond is formed when parallel orbitals overlap and share electrons, and it occupies the space above and below the line that represents where the two atoms are joined together.

(Sigma (s) bond along the line not shown) Two sigma and two pi bonds pi (p) bond, (Sigma (s) bond along the line not shown) Two sigma and two pi bonds

Drawing Lewis dot structures: Can you find a different way Ex. C6H6 Drawing Lewis dot structures: Put atoms with many single dots in the center, every dot can make one bond. Atoms with only one single dot are usually on the outside, because they form only one “regular” covalent bond (H, F, Cl, Br, I). Leftover single dots should be used for double or triple bonds. Often more than one formula is possible (isomers). C2H4O2 Can you find a different way to “connect the dots”? Example acetic acid

Lewis structure of polyatomic ions Atoms within a polyatomic ion are covalently bonded.  same procedure, but first add or remove dots corresponding to charge (-) add, (+) remove Symmetry often stabilizes a molecule or particle

Coordinate covalent bond is a bond in which one atom contributes both bonding electrons. also known as dative bond (latin: to give), dipolar bond or semipolar bond

Resonance structures of NO3- Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. (move electrons but not atoms!!) The molecule or ion behaves as though it has only one average structure. The bond lengths are identical to each other and intermediate between single and double covalent bonds. Resonance structures of NO3- Electrons that can be pushed around in a molecular structure are called delocalized and stabilize the molecule or ion.

Lewis resonance structures for carbonate CO3 2- The double arrow is used between resonance structures (one line but with arrow heads on both end) Benzene C6H6 Draw the Lewis resonance structures for NO2- SO2 O3 Sulfur trioxide SO3

Three Exceptions to the Octet Rule 1) A small group of molecules might have an odd number of valence electrons, for example NO2 (5 + 6 + 6 = 17) Resonance structures help to stabilize the molecule

Exceptions to the Octet Rule 2) Few compounds form stable configurations with less than 8 electrons around the atom a suboctet. Boron trihydride is reactive and will form coordinate bonds or “dimerize”

Exceptions to the Octet Rule 3) Elements in period 3 or higher can use d-orbitals to form more than 4 covalent bonds (more than 8 valence electrons), called an expanded octet. (most common exception) Draw the expanded octet Lewis structure for ClF3 A rare compound, containing the noble gas xenon

The Strength of Covalent Bonds depends on the distance between the two nuclei, or bond length. The shorter the bond the stronger it is, the greater the energy required to break it. The amount of energy required to break a bond is called the bond dissociation energy

Every chemical reactions involves a change in energy Exothermic Reaction: Energy is being released (test tube getting hot) more energy is released than is required to break the bonds in the initial reactants CH4 (g) + 2 O2 (g) -- CO2 (g) + 2 H2O (g) + energy Endothermic Reaction: Energy is being absorbed (test tube getting cold) 2 H2O (l) + energy -- 2H2 (g) + O2 (g) a greater amount of energy is required to break a bond in reactants than is released when the new bonds form in the products.

Exothermic!

What is it called when one or more correct Lewis structures can be drawn for a molecule? A. suboctet B. expanded octet C. expanded structure D. resonance

Where do atoms with expanded octets occur? A. transition metals B. noble gases C. elements in period 3 or higher D. elements in group 3 or higher