SL STANDARD D

LEWIS STRUCTURES&

MOLCULAR GEOMETRY

TYPES OF COVALENT BONDSSINGLE BONDS• LONGEST OF THE 3 TYPES• WEAKEST OF THE 3 TYPES• CONTAINS ONE PAIR OF ELECTRONS (2 ELECTRONS)

DOUBLE BONDS• LENGTH IS GREATER THAN TRIPLE BUT LESS THAN SINGLE• STRENGTH IS GREATER THAN SINGLE BUT LESS THAN TRIPLE• CONTAINS TWO PAIRS OF ELECTRONS (4 ELECTRONS)

TRIPLE BONDS• SHORTEST OF THE 3 TYPES• STRONGEST OF THE 3 TYPES• CONTAINS THREE PAIRS OF ELECTRONS (6 ELECTRONS)

 LONGEST BONDS(SINGLE) ARE THE WEAKEST

SHORTEST BONDS(TRIPLE) ARE THE STRONGEST 

VSERP THEORYVALENCE SHELL ELECTRON REPLUSION THEORY

VSERP THEORY states that electron pairs found in the outer energy level or valence shell of atoms repel each other and thus position themselves as far apart as possible

The VSERP theory helps to predict the shape of the molecule. The following points should be considered in predicting shape (molecular geometry):

• Double or triples bonded electron pairs are oriented together and so behave in terms of repulsion as a single unit know as a ‘negative charged center’.

• The repulsion applies to both bonding and non- bonding (lone) pairs of electrons

• The total number of charge centers around the central atom will determine the geometrical arrangement of the electrons

• The shape of the molecule is determined by the angles between the bonded atoms

• Because non-bonded (lone) pairs are not shared between two atoms they cause more repulsion than bonded pairs

• The repulsion decrease in the following order:

lone-lone pair>lone-bonding pair>bonding-bonding pair

FOR EXAMPLE, IN THE LEWIS STRUCTURE OF WATER, THERE ARE TWO NON-BONDED (LONE) PAIRS OF ELECTRONS ON THE OXYGEN ATOM. THUS THE NON-BONDED (LONE) PAIR OF ELECTRONS REPEL MORE THAN THE LONE-BONDED PAIR BETWEEN THE OXYGEN AND HYDROGEN AND THE BONDED-BONDED PAIR OF ELECTRONS BETWEEN THE TWO HYDROGENS HAVE THE LEAST REPULSION.

NON-BONDED (LONE)&NON-BONDED (LONE) PAIR OF ELECTRONS

NON-BONDED(LONE)& BONDED PAIROF ELECTRONS

BONDED-BONDED PAIR OF ELECTRONS

BOND ANGLESDue to the fact that lone pair of

electrons repel more than bonded pairs, the bond angles will be different

in different shaped molecules

BEND(with 2 lone pair of e-) <TRIGONAL PYRAMIDAL<TETRAHEDRAL<BEND(with 1 lone pair of e-)<TRIGONAL PLANAR<LINEAR

****KNOW THE BOND ANGLES OF THIS SHAPES

LINEARTRIGONAL PLANAR

TETRAHEDRAL

BENT (w/ 2 lone pairs of e-)

TRIGONAL PYRAMIDAL

.. S // \:O: :O: ..

BENT (w/ 1 lone pairs of e-)

180o

120o

109.5o

109.5o

(107.5o ) 120o (119o)

109.5 o (104.5o)

ETHANOIC ACIDCH3COOH

104.5o

TETRAHEDRAL

TRIGONAL PLANAR

BENT

CAN A MOLECULE BE NONPOLAR AND STILL HAVE POLAR BONDS WITHIN THE MOLECULE???FIRST THINK ABOUT WHAT MAKES A BOND POLAR!

NOW THINK ABOUT WHAT MAKES A MOLECULE NONPOLAR!

THE DIFFERENCE IN ELECTRONEGATIVITY VALUES (CHARGE SEPARATION) DETERMINE IF A BOND WILL BE POLAR OR NONPOLAR.

0-.3 difference is NONPOLAR.3-1.7 difference is POLAR

IF A MOLECULE IS SYMMETRICAL OR IF THERE IS NO NET DIPOLE MOMENT THEN IT IS NONPOLAR

EXAMPLE: CCl4

THE C-Cl BOND IS POLAR BUT THE MOLECULE IS TETRAHEDRAL SO IT IS SYMMETRICAL AND

THUS NONPOLAR

YES

WHICH OF THE FOLLOWING BONDS IS THE MOST POLAR

B-C or C-O or N-O or O-F

ANSWER:C-O

b/c they are further apart on the periodic table which means thatthey have the greatest charge separation.