6 - Shapes of molecules and intermolecular forces Flashcards
What is electron pair repulsion theory?
model used in chemistry for explaining & predicting shapes of molecules & polyatomic ions
- electron pairs surrounding central atom determine shape of molecule/ion
- electron pairs repel -> arranged as far apart as possible
- arrangement minimises repulsion & holds atoms in definite shape
- diff numbers of electron pairs results in different shapes
Bonded-pair vs lone-pair repulsion
Lone pair of electrons slightly closer to central atom and occupies more space than bonded pair. So lone pair repels more strongly than bonding pair.
(Increasing repulsion)
bonded-pair/bonded-pair < bonded-pair/ lone pair < lone pair/lone pair
Shape of molecule with 4 bonded pairs, 0 lone pairs and bond angle
Tetrahedral
109.5 degrees
Bond angle and Shape of molecule with 3 bonded pairs, 1 lone pair
Pyramidal
107 degrees
Bond angle and Shape of molecule with 2 bonded pairs, 2 lone pairs
Non linear
104.5 degrees
Bond angle and Shape of molecule with 2 bonding regions, 0 lone pairs
Linear
180 degrees
Bond angle and Shape of molecule with 3 electron regions, 0 lone pairs
Trigonal planar,
120 degrees
Bond angle and Shape of molecule with 6 bonding regions, 0 lone pairs
Octahedral
90 degrees
Bond angle and Shape of molecule with 5 bonding regions, 0 lone pairs
Trigonal Bipyramidal
90 degrees and 120 degrees
Bond angle of CH4
109.5 degrees
Bond angle of NH3
107 degrees
Bond angle of H2O
104.5 degrees
Define electronegativity
The ability of an atom to attract the bonding electrons in a covalent bond.
How is electronegativity measured?
Using Pauling electronegativity values
Electronegativity increases towards F
- going right = more protons, Nuclear charge becomes more positive, more electronegative
– going up = smaller radius, more electronegative
As you go left to right, atom size slightly decreases because more protons, and electrons are more attracted to them
What makes an element more electronegative?
1) nuclear charge.
When there is more positive charge it is more electronegative
2)radius of atom.
Smaller radius atoms are more electronegative because electrons are closer, so there is less shielding
Electronegativity difference = 0
(Pure) covalent
Electronegativity difference = 0 to 1.8
Polar covalent
Electronegativity difference > 1.8
Ionic bond
What is a nonpolar bond
The bonded electron pairs are shared equally between the bonded atoms.
The bonded atoms are the same OR have same/similar electronegativity
Eg;
Cl2
Hydrocarbons
What is a polar bond?
The bonded electron pair is shared unequally between bonded atoms. Bonded atoms are different and have different electronegativity values resulting in a polar covalent bond.
Electronegativity difference must be greater than 0.5.
Dipoles in polar bonds
In H – Cl bond, H has a small partial positive charge and Cl has a small partial negative charge.
Separation of opposite charges is called a dipole
A dipole in a polar covalent bond does not change. PERMANENT DIPOLE.
Polarity in polar molecules
For molecules with 2+ atoms there may be 2+ polar bonds
Polar molecule requires polar bonds with dipoles that do not cancel due to their direction. CO2 does not have a permanent dipole but H2O does
Define intermolecular forces. What types?
An attractive force between molecules // weak interactions between dipoles of different molecules
Can be London forces, permanent dipole-dipole interactions or hydrogen bonding
Covalent vs intermolecular forces
Covalent: strong, holds atoms in a molecule together. Determine identity and chemical reactions of molecules
Intermolecular: weak interactions between dipoles of different molecules. largely responsible for physical properties like melting & boiling points