Post on 16-Dec-2015
Drawing Lewis Structures and VSEPRDrawing 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
CH2ClF, SeCl2, O3 (CO2, NH3, PO43-)
• Write other atoms around central with line bond.~line represents 2 e- (must be subtracted from total)
Se – ClCl –
SeCl2 : 6 +7 + 7 = 20 - 4 = 16
Se – ClCl –
• 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 CO2:
CO2 : 4 + 6 + 6 = 16
C – OO –
- 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 – OO ––
O– O
Draw the Lewis structure for SO42- :
SO42- : 6 + (4)6 + 2 = 32 - 8 = 24
2-
• place brackets around charged structures (ions)
BeCl2 : 2 + 7 + 7 = 16
– ClCl –
Be
- 4 = 12 Ionic Bond
-1
Cl
+2
Be
-1
Cl
• all atoms of an ionic bond are in brackets to distinguish the “giving” and “taking” nature of the bond.
VSEPRValence 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.
SeCl2 : 6 +7 + 7 = 20 - 4 = 16
Se – ClCl –
• 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 atomXn: # of bonding pairsEn: # lone pairs
Draw the Lewis structure for BF3:BF3 : 3 + (3)7 = 24
- 6 = 18
– FF – B
F
–
*Unusual(Does not have octet)
3 – bonding pairs (X3)0 – lone pairs
Trigonal Planar
Central Atom with only bonding pairs:
Draw the Lewis structure for CH4:
Draw the Lewis structure for PCl5:
Draw the Lewis structure for SF6:
4 – bonding pairs (X4)0 – lone pairs
Tetrahedral
5 – bonding pairs (X5)0 – lone pairs
Trigonal Bipyramidal
6 – bonding pairs (X6)0 – lone pairs
Octahedral
Central Atom with LONE pairs:Lone pairs have a greater repulsion force and distort the predicted bond angles.
H2S : 1 + 1 + 6 = 8
– HH –
S
- 4
2 – bonding pairs (X2)2 – lone pairs (E2)
Bent
*Predict Linear
Draw the Lewis structure for PF3:
PF3 : 5 + (3)7 = 26
- 6
– FF – P
F
–
*Predict Trigonal Planar
3 – bonding pairs (X3)1 – lone pairs
Trigonal Pyramidal
Treat double/triple bonds as single bonds:
CO2 : 4 + 6 + 6 = 16 - 4
Double Bonds
2 – bonding pairs (X2)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