chapter 6 Flashcards
electron pair repulsion theory
- electron pairs surrounding the central atom determine the shape
- electron pairs repel each other so that they are as far apart as possible
- the arrangement minimises repulsion, holding the bonded atoms into a definite shape
lone pair repulsions
lone pairs repel more strongly than bonding pairs. the angle is reduced by 2.5 for each lone pair.
4 bonding pairs, 0 lone pairs
tetrahedral
109.5
3 bonding pairs, 1 lone pair
pyramidal
107
2 bonding pairs, 2 lone pairs
non- linear
104.5
shapes from multiple bonds
each multiple bond is treated as one bonding region
2 bonding regions
linear
180
3 bonding pairs
trigonal planar
120
6 bonding pairs
octahedral
90
electronegativity
the attraction of a bonded atom for the pair of electrons in a covalent bond
3 most electronegative elements
NOF (Cl)
fluorine is the most
electronegativity and bonding
can be used to estimate the type of bonding
covalent- 0
polar covalent- 0-1.8
ionic- greater than 1.8
non-polar bonds
the bonded electron pair is shared equally between the bonded atoms, when:
the bonded atoms are the same or have the same/similar electronegativity
polar bonds
the bonded electron pair is shared unequally between bonded atoms. A bond will be polar when the bonded atoms are different and have different electronegativity values resulting in a polar covalent bond.
polar bonds- hydrogen chloride
- the chlorine atom is more electronegative than the hydrogen
- the chlorine atom has a greater attraction for the bonded pair of electrons– polar covalent bond
- this separation of charges= a dipole
- in a covalent bond it is a permanent dipole
polarity
if the molecule is symmetrical then the dipoles cancel and it is not polar
intermolecular forces
weak interactions between dipoles of different molecules
responsible for physical properties
type of bond in order of strength (H, covalent, London & permanent dipoles)
strongest- covalent
- hydrogen bonds
- permanent dipole-dipole interactions
- London forces
London forces
weak intermolecular forces that exist between all molecules, they act between induced dipoles in diff molecules.
origin of London forces
- movement of electrons produces a changing dipole in a molecule
- an instantaneous dipole but its position is constantly shifting
- the instantaneous dipole induces a dipole on a neighbouring molecule
- the induced dipole induces further dipoles on neighbouring molecules, which attract one another
strength of London forces
the more electrons in each molecule:
- the larger the instantaneous & induced dipoles
- the greater the induced dipole-dipole interactions
- the stronger the attractive forces between molecules
- more energy needed to overcome the intermolecular f forces, increasing the boiling point
HCl Vs F2 (Bp)
- fluorine molecules are non-polar and only have London forces
- HCl molecules are polar and have London forces & permanent dipole-dipole interactions
- extra energy needed to break the additional permanent dipole interactions
- Bp of HCl is larger than Fluorine
simple molecular substances
made up of simple molecules- small units containing a definite number of atoms w/ a definite molecular formula eg Ne, H2, H20 & CO2
simple molecular lattice
- molecules held together by weak intermolecular forces
- atoms within each molecule are bonded together strongly by covalent bonds
properties of simple molecular substances (mp & bp)
– low melting & boiling point (weak intermolecular forces easily broken)
solubility of non-polar simple molecular substances
soluble in non-polar solvents
-intermolecular forced form between the molecules and the solvent, the interactions weaken the intermolecular forces in the simple molecular lattice. intermolecular forces break and the compound dissolves
if added to a polar solvent there is little interaction, intermolecular bonding in the polar solvent is too strong to be broken- insoluble
solubility of polar simple molecular substances
- may dissolve in polar solvents, can attract each other
- depends on strength of dipole
electrical conductivity (simple molecular)
no mobile charge carriers = non-conductor
hydrogen bond
special type of permanent dipole-dipole interaction found between molecules containing:
- an electronegative atom w/ a lone pair of electrons eg NOF
- a hydrogen Aton attached to an electronegative atom
anomalous properties of water
- ice less dense than water; molecules in ice are held further apart in the lattice
- four bonds- open tetrahedral lattice 180 bond angle
- high mp & bp, large quantity of h bonds- strong
- high surface tension & viscosity
