9-3 Covalent Bonds (Sections 12.1, 12.6, 12.7) And you

A covalent bond is an equal sharing (“co”) of valence electrons (“valent”) and is typically formed between two nonmetal

Terms Central atom X – X – X Single bond (a pair of shared e - ) X : X = X-X Double bond (4 shared e - ) X : : X = X=X Triple bond (6 shared e - ) X : : : X = X X Lone pair (nonbonding pair) : X-X Duet Rule ~ H will have only 2 shared electrons (no more) to reach s 2. Octet Rule ~ ALL ATOMS, except H, will have 8 shared electrons to reach s 2 p 6. * with exceptions

Covalent Lewis Structures Like ionic Lewis structures, the covalent dot structure of a molecule shows how the outermost energy level electrons are arranged among the atoms in the molecule. Remember, stable compounds have atoms with noble gas configurations (s 2 p 6 ). To draw an accurate covalent Lewis structure, follow these 4 (or 5) steps:

Step 1: Place Atomsexample NF 3 Odd man out is central atom NF F F

Step 2: Count Valence e - s N = 5 F = 7 Σ = 26

Step 3: Single Bonds (2 e- for each) Create a single bond between each atom N F F F

Step 4: Octets and Duets (start on most EN) Everyone gets an octet!!!! N F F F

Step 5: Multiple Bonds (if needed) If not all atoms have octets, change a lone pair from a surrounding atom into a bonding pair with the central atom. I will show you this in more detail in a multiple bonds ppt. When you count up the e- s in our “guess model” you find that we have 26. That is the same number of e-s that we counted when we summed the total valence electrons for all the atoms in the molecule. – so we are done here

Supplemental notes (not included in your notes notebook

Example O 2 1)Each oxygen atom has 6 valence electrons – which gives us 12 total electrons to work with 2)Start by arranging the atoms involved so that there is a single pair of shared electrons between the atoms and that each atom has an octet.

Like this:

But wait, that is too many electrons! That is correct. We only can have 12 electrons total, but here we have 14 So here we move electrons to create multiple bonds

HEY The oxygen on the left has 10 electrons – what now? This allows us to remove 2 electrons from the oxygen that has a surplus – so that each atom has an octet, and we have 12 total electrons involved.

We would show things like this:

More supplemental notes on multiple bonds is posted Now lets get back to the workbook and try to do the next example: C 2 H 4

C2H4C2H4 1) Place atoms 2) How many valence e- s total? ) Place single bonds between each of the atoms (recall that H can never have more than 2 valence e-s so it can never be a central atom in a structure. Then complete the octets and duets.

12 valence e- But wait – that is 14 e-s in our first guess – we need to get rid of 2!!!! Move 2 to make a double bond then remove 2 e- s from the atom with the 10 e-s. CCH HH H :::.. :

Practice HOClNH 4 +1 N 2

HOCl

NH 4 +

Polyatomic ions Polyatomic ions are a group of atoms covalently bonded together that act as unit. What makes these groups of atoms unique is that they have either gained or lost one or more electrons, making an ion. The bonding of polyatomic ions with other species is therefore ionic in nature. WOW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Example:CaCO 3 is a combination of Ca +2 with CO 3 -2 CaC O OO : : :: :.. : : Resonance – double or triple bond can be in different places Example: CO 3 -2 experiments find all the bonds are the same – kind of like all are 1 1 / 3 bonds – anytime you have a variety of ways, the real structure is somewhere in between

Exceptions to the Octet Rule Fewer than 8 electrons (violation of octet rule) Occurs with B and Be. – but readily binds with other compounds like water or ammonia. BF 3 B F F F.. : : : : : :

BeCl 2

Greater than 8 electrons Occurs for nonmetals only in periods 3 or higher with empty d orbitals. SF 6

PCl 5

NORMS: Carbon generally central atom (or chains) Odd man out in center – build around Halogens normally only form one bond (think of the valence e-s here) Oxygen normally forms 2 (think of valence e-s) Do not forget brackets and charge for poly. Ions Be and B exceptions (think of valence e-s here – fewer than 8) P3 and below exceptions (more than 8)