Drawing Lewis Structures and VSEPR

Draw basic Lewis dot structures of atoms and compounds. Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram. Additional KEY Terms

3 F- Al+3 As atoms combine in definite proportions: Aluminum Fluorine Valence electrons determine atom reactivity

S Ca Electron Dot Diagrams (Lewis Dot) Quick way to show bonding electrons (valence) - atomic symbol represents nucleus and inner shell electrons - only valence electrons are used - dots placed clockwise, ending in pairs S Ca

STEPS / RULES TO LEWIS DOT: 1. Position first atom in the centre. ~ usually first in formula (unless H or F) CH2ClF, SeCl2, O3 (CO2, NH3, PO43-) 2. Join other atoms with line bond to the central. Cl – Se – Cl 3. Determine total valence electrons for all atoms. ~ watch for negative/positive ions The central atom is supposed to be have the lowest EN value

4. Place remaining e- around atoms to fill valence. SeCl2 6 +7 + 7 = 20 - 4 = 16 Subtract 2 electrons for every line bond formed        Cl – Se – Cl          4. Place remaining e- around atoms to fill valence. Fill surrounding atoms BEFORE central atom 5. Check that each atom has a complete octet. Remember: “8 is great, but 2 will do” – Hydrogen is an exception

Draw the Lewis structure for CO2: CO2 : 4 + 6 + 6 = 16 - 4 = 12 Not “full”      – – O –  O – C       Too few electrons? - give central atom a complete octet by creating a double or triple bond. Create multiple bonds by moving an electron pair from a surrounding atom into a line bond with the central

Place brackets around charged structures Draw the Lewis structure for SO42- : SO42- : 6 + (4)6 + 2 = 32 - 8 = 24 2-  S – O O –    Place brackets around charged structures

MgCl2 : 2 + 7 + 7 = 16 - 4 = 12 Ionic Bond Cl – Mg – Cl 2+ Mg 1- Cl 1-   Cl – Mg – Cl 2+ Mg 1- Cl  1- Cl  Ionic Lewis dots are indicated by using brackets to distinguish the “give/take” nature of the bonding

Valence Shell Electron Pair Repulsion Theory VSEPR Valence Shell Electron Pair Repulsion Theory Predicts 3D structures of molecules Minimizes the electrostatic repulsion between electrons in valence sublevels Shape calculated by comparing bonding and lone pair electrons around the central atom

A line bond counts as 1 bonding pair SeCl2 : 6 +7 + 7 = 20 - 4 = 16  Cl – Se – Cl VSEPR shape is determined by counting bonding pairs and lone pairs around the CENTRAL atom only. A line bond counts as 1 bonding pair A lone pair is a pair of non-bonding electrons

You are given this table – count bonding pairs and lone pairs and read the shape

AXE method: A: Central atom Xn: # of bonding pairs En: # lone pairs This is just a different why of representing the same table information

(Does not need a full octet) Central Atom with only bonding pairs: Draw the Lewis structure for BF3: BF3 : 3 + (3)7 = 24 - 6 = 18   *Exception (Does not need a full octet) – F F – B F –  3 – bonding pairs 0 – lone pairs Trigonal Planar

Draw the Lewis structure for CH4: 4 – bonding pairs (X4) 0 – lone pairs Tetrahedral Draw the Lewis structure for PCl5: 5 – bonding pairs (X5) 0 – lone pairs Trigonal Bipyramidal Draw the Lewis structure for SF6: 6 – bonding pairs (X6) 0 – lone pairs Octahedral

Its drawn linear but the lone electrons Central Atom with LONE pairs: Lone pairs have a greater repulsion force and distort the predicted bond angles. H2S : 1 + 1 + 6 = 8 - 4 – H H –  S 2 – bonding pairs 2 – lone pairs Bent Its drawn linear but the lone electrons distort the shape

Notice again the distorted shape from the lone pair of electrons Draw the Lewis structure for PF3: PF3 : 5 + (3)7 = 26 - 6    – F F – P F – 3 – bonding pairs 1 – lone pairs  Trigonal Pyramidal Notice again the distorted shape from the lone pair of electrons

Double and triple bonds are still counting as ONE bonding pair - 4   O – – C – O – Double Bonds   2 – bonding pairs 0 – lone pairs Linear Double and triple bonds are still counting as ONE bonding pair

CAN YOU / HAVE YOU? Draw basic Lewis dot structures of atoms and compounds. Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram. Additional KEY Terms