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

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

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) CH 2 ClF, SeCl 2, O 3 (CO 2, NH 3, PO 4 3- ) Se – Cl Cl – The central atom is supposed to be have the lowest EN value 3. Determine total valence electrons for all atoms. ~ watch for negative/positive ions 2. Join other atoms with line bond to the central.

SeCl = 16              Se – Cl Cl – 4. Place remaining e - around atoms to fill valence. Fill surrounding atoms BEFORE central atom = 20 Subtract 2 electrons for every line bond formed 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 CO 2 : CO 2 : = 16 C – O O – - 4 = 12           – – 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. Not “full”

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

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

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

SeCl 2 : = = 16             Se – Cl 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 X n : # of bonding pairs E n : # lone pairs This is just a different why of representing the same table information

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

Draw the Lewis structure for CH 4 : Draw the Lewis structure for PCl 5 : Draw the Lewis structure for SF 6 : 4 – bonding pairs (X 4 ) 0 – lone pairs Tetrahedral 5 – bonding pairs (X 5 ) 0 – lone pairs Trigonal Bipyramidal 6 – bonding pairs (X 6 ) 0 – lone pairs Octahedral

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

Draw the Lewis structure for PF 3 : PF 3 : 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

CO 2 : = Double Bonds 2 – bonding pairs 0 – lone pairs Linear   C – OO –         –– 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