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

OCTET RULE: atoms will gain, lose or share electrons to fill their valence shell (noble gas)  use only VALENCE electrons in Lewis dot.

STEPS / RULES TO LEWIS DOT: Determine total valence electrons for all atoms. ~watch for negative/positive ions. Position least electronegative atom in the centre. ~central atom usually first in formula CH 2 ClF, SeCl 2, O 3 (CO 2, NH 3, PO 4 3- ) Write other atoms around central with line bond. ~line represents 2 e - (must be subtracted from total) Se – Cl Cl –

SeCl 2 : = = 16              Se – Cl Cl – double check to make sure each atom in the compound has 8 electrons around it (Hydrogen is an exception – it only needs 2) Place remaining e - around atoms to fill valence Fill surrounding atoms BEFORE central atom

Draw the Lewis structure for CO 2 : CO 2 : = 16 C – O O – - 4 = 12           – – create double/triple bonds by moving an electron pair from a surrounding atom to share with the central atom. Too few electrons? - give central atom a complete octet by creating a double or triple bond.

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 (ions)

BeCl 2 : = 16 – Cl Cl –           Be - 4 = 12 Ionic Bond Cl        +2 Be Cl        all atoms of an ionic bond are in brackets to distinguish the “giving” and “taking” nature of the bond.

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

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 will always have this (check the back of the booklet) Count the bonding and lone pairs and read the shape.

AXE method: A: Central atom X n : # of bonding pairs E n : # lone pairs

Draw the Lewis structure for BF 3 : BF 3 : 3 + (3)7 = 24           - 6 = 18 – F F – B F –      *Unusual (Does not have octet) 3 – bonding pairs (X 3 ) 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 (X 2 ) 2 – lone pairs (E 2 ) Bent *Predict Linear

Draw the Lewis structure for PF 3 : PF 3 : 5 + (3)7 = 26           - 6 – F F – P F –      *Predict Trigonal Planar 3 – bonding pairs (X 3 ) 1 – lone pairs Trigonal Pyramidal 

Treat double/triple bonds as single bonds: CO 2 : = Double Bonds 2 – bonding pairs (X 2 ) 0 – lone pairs Linear   C – OO –         ––

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