6 - Shapes of molecules and intermolecular forces Flashcards
electron pair repulsion theory
- e- surrounding a central atom determine the shape of the molecule
- the e- pairs repel one another so they are arranged as far as possible
- the arrangement of e- pairs minimises repulsion and holds the atoms in a definite shape
state the trend of increasing repulsion in molecules
why
increasing repulsion downwards
bonded pair and bonded pair
bonded pair and lone pair
lone pair and lone pair
- lone pair of electrons are slightly closer to central atom and occupy more space, so the lone pair repels more strongly than a bonding pair
BP: 2
LP: 0
linear
180
BP: 3
LP: 0
trigonal planar
120
BP: 4
LP: 0
tetrahedral
109.5
BP: 6
LP: 0
octahedral
90
BP: 3
LP: 1
pyramidal
107
BP: 4
LP: 3
square planar
BP: 5
LP: 0
trigonal bipyramidal
explanation for shape of a molecule
State bonding pairs and lone pairs in molecule
Electron pairs around the central atom repel as far apart as possible and lone pairs repel more than bonding pairs and decrease the bond angle (by 2.5)
greater the electron pairs the smaller the bond angle
electronegativity
the attraction of a bonded atom for the pair of electrons in a covalent bond
how does EN increase in the periodic table
across the table nuclear charge increases and atomic radius decreases
therefore
en increases across the table and up the table
therefore
most EN elements are O, F and Cl
non-polar bond
bonded electron pair is shared equally between the bonded atoms
- happens when bonded atoms have the same of similar EN
polar bonds
the bonded electron pair is shared unequally between the bonded atoms
- happens when bonded atoms have different EN
polar molecules
- have one polar bond (permanent dipole)
- or has more than one permanent dipole that act in different directions but don’t oppose each other so don’t cancel each other
- molecule is not symmetrical
non-polar molecules
dipoles oppose each other and cancel each other out
- the molecule is symmetrical
three intermolecular forces
induced dipole-dipole
permanent dipole-dipole
hydrogen bonding
intermolecular forces
weak interactions between dipoles of different molecules
induced dipole-dipole
Movement of electrons produces a changing
dipole in a molecule, creating an instantaneous
dipole
The instantaneous dipole induces a dipole on a
neighbouring molecule
The induced dipole induces further dipoles on
neighbouring molecules, which then attract one
another
the strenght of induced
- the larger the instantaneous and induced dipole, the greater the induced dipole-dipole so stronger attractive forces between molecules
- larger number of e- means larger induced dipoles
-> more energy is needed to overcome the IF
permanent dipole-dipole
interactions between the permanent dipoles in different polar molecules
simple molecular substances
substances made of simple molecules - small units containing a definite number of atoms with a definite molecular formula
simple molecular lattice
- molecules are held in place by weak IF
- atoms in each molecule are bonded by covalent bonds strongly
simple molecular substances Tm and Tb
- low Tm
- as IF are weak, so little energy is needed to overcome them
Simple molecular substances solubility
Non-polar
-> in a non-polar solvent = IF forms between the simple substance and the solvent and this weakens the IF in the simple molecular lattice. the compound dissolves
-> in polar solvent = little interactions between simple substance and the solvent so IF in the simple molecular lattice is too strong
Polar
-> may dissolve in polar solvents as they can attract each other
—- sugar, HCL
-> it does depend on the strength of the dipole. eg ethanol contains polar OH and non-polar carbon chain so can dissolve in both non-polar and polar
electrical conductivity of simple molecular substances
- there are no mobile charged particles in simple molecular structures
- so there is nothing to complete an electrical circuit
hydrogen bond
A hydrogen bond is an attraction between a
lone pair of electrons on an electronegative
atom in one molecule and a hydrogen atom in
another molecule attached to an electronegative
atom
strongest IF
why is ice less dense than water
Ice is less dense than water
-Because H2O molecules in ice are held apart by
hydrogen bonds in open lattice structure
- so water molecules in ice are further apart in ice than water. so it is less dense
-
water has a high Tm and Tb
Because hydrogen bonds are stronger than
other intermolecular forces so more energy is
needed to break the hydrogen bonds
