MOLECULES Part 2: LEWIS STRUCTURES
^What to catch from the above slide:
Central atoms are generally those with LOW ELECTRONEGATIVITY
H and F are ALWAYS terminal atom
Carbon atoms are ALWAYS central atoms
^In above example, we see that NITROGEN is the central atom, and Oxygen and Iodine are terminal atoms
^What to catch from above slide:
It is good to start roughly drawing Lewis structures right away to get more practice
Let's start with CO32- as seen in the slide:
Carbon is the central atom (less electronegative) & Oxygen atoms are terminal atoms (more electronegative) --> so draw them all and connect them using single bonds initially
Carbon has 4 VALENCE electrons, Oxygen has 6 VALENCE electrons multiplied by three because there are three Oxygens. And CO32- has a -2 charge, meaning it has extra 2 electrons: (4 + 18 + 2 = 24 VALENCE electrons in total for all the 4 atoms)
We see that Carbon does not fulfill the octet rule, so we convert a lone pair to another bond (indicated as red arrow), and now Carbon has 8 electrons to fulfill octet
RULE#1: ALL ATOMS GENERALLY REQUIRE FULL OCTET
^What to catch from above slide:
Here is another example of Lewis structure for NO2+
Notice that we start off with connecting all the atoms using single bonds
Nitrogen is the only atom that does not fulfill octet, so we need to convert lone pair on either side to add another bond on either side: we get two double bonds on either side now
Two double bonds now give Nitrogen the full octet
^What to catch from above slide:
FORMAL CHARGE (FC) is an important concept when drawing Lewis structures
VE refers to VALENCE ELECTRONS --> NBE refers to NON-BONDING ELECTRONS --> B refers to the NUMBER OF BONDS
Take a look at the left-most compound. Carbon has a FC of "0" because its VE (=4), NBE (=0) and B (=4): 4 - 0 - 4 gives you "0". Note that "0" FC is good, because it means the structure is stable
In the same left-most compound: Oxygen has a FC of "0" because its VE (=6), NBE (=4), and B (=2): 6 - 4 - 2 gives you "0".
^What to catch from above slide:
WHERE FC is required, it should be as SMALL as possible
Negative FC is favourable on electronegative atoms (e.g. Oxygen, Fluorine, etc.)
Positive FC is favourable on electropositive atoms (e.g. Sodium, Calcium, etc.)
RULE#2: FORMAL CHARGE OF "0" ON ALL ATOMS IS IDEAL (MAY NOT BE POSSIBLE IN SOME CASES)
^What to catch from above slide:
Again, we want FC to be either "0" or as close to "0" as possible
^What to catch from above slide:
Whenever there are double bonds or triple bonds only on one side, we have RESONANCE STRUCTURES where we can switch their places with single bonds
^What to catch from above slide:
Knowing the exceptions to octet rule will help you comfortably draw more tricky Lewis structures
Boron is usually "satisfied" with 6 electrons, Beryllium is "satisfied" with 4 electrons
Third-row elements and heavier can EXCEED the octet rule
Phosphorous is usually "satisfied with 10 electrons, Sulphur is "satisfied" with 12 electrons (see below)
RULE#3: Third-row elements and heavier can EXCEED the octet rule
PRACTICE DRAWING LEWIS STRUCTURES
ANSWER KEY
EXTRA PRACTICE ON ELECTRON DOT DIAGRAMS & LEWIS STRUCTURES (WORKSHEET CONTAINS ANSWERS)
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