6) Shapes of molecules and intermolecular forces Flashcards
Describe electron-pair repulsion theory (4)
- electron pairs surrounding a central atom determine the shape of the molecule or ion
- the electron pairs repel one another so that they are arranged as far apart as possible
- the arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
- different numbers of electron pairs result in different shapes
What are wedges used for?
to help visualise 3D structures
A lone pair of electrons is _ and _ than a bonded pair. This results in a lone pair _ more strongly than a bonding pair.
slightly closer to the central atom
occupies more space
repelling
How much is the bond angle reduced per lone pair?
about 2.5 degrees
Define bond angle
the angle between two bonds at an atom
Give the name of the shape, its bond angle and an example for a molecule with 3 bonded pairs and 1 lone pair
pyramidal
107 degrees
NH3
Give the name of the shape, its bond angle and an example for a molecule with 2 bonded pairs and 2 lone pairs
non-linear
104.5 degrees
H2O
Give the shape, its bond angle and an example for a molecule with 2 electron pairs / regions
linear
180 degrees
CO2
Give the name of the shape, its bond angle and an example for a molecule with 3 electron pairs / regions
trigonal planar
120 degrees
BF3
Give the name of the shape, its bond angle and an example for a molecule with 4 electron pairs / regions
tetrahedral
109.5 degrees
CH4
Give the name of the shape, its bond angle and an example for a molecule with 6 electron pairs / regions
octahedral
90 degrees
SF6
Define electronegativity
a measure of the attraction of a bonded atom fro the pair of electrons in a covalent bond
Define Pauling electronegativity value
a value assigned as a measure of the relative attraction of a bonded atom for the pair of electrons in a covalent bond
What does Pauling electronegativity value depend upon?
an element’s position in the periodic table
as you go up and right electronegativity increases
What will happen if the electronegativity difference is large?
one bonded atom will have a much greater attraction for the shared pair of electrons than the other bonded atom
the more electronegative atom will have gained control of the e- and the bond will now be ionic rather than covalent
Give the electronegativity differences for the following types of bonding:
covalent
polar covalent
ionic
0
0 - 1.8
> 1.8
Describe a non-polar bond
the bonded pair of electrons is shared equally between bonded atoms that are the same / have the same or similar electronegativity values
Describe a pure covalent bond
when bonded atoms come from the same element and the electron pair is shared equally
Describe a polar bond
the bonded electron pair is shared unequally between the bonded atoms that are different and have different electronegativity values
Define dipole
a separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge and the other has a small negative charge
The atom with the larger electronegativity value has the _ charge. The atom with the smaller electronegativity value has the _ charge.
partially negative
partially positive
Define permanent dipole
a small charge difference that does not change across a bond, with positive and negative partial charges on the bonded atoms: the result of the bonded atoms have different electronegativities
Depending on the shape of a molecule dipoles may _ or _?
reinforce one another to produce a larger dipole over the whole molecule
cancel out if dipoles act in opposite directions
Define intermolecular force
an attractive force between molecules
Define induced dipole-dipole interactions (London forces)
attractive forces between induced dipoles in different molecules
Describe London forces (3)
only temporary
exist between all molecules (polar and non-polar)
the more electrons in each molecule, the stronger the force
How do induced dipole-dipole interactions occur?
- movement of e- produces a changing dipole in a molecule
- at any instant, an instantaneous dipole will exist, 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 then attract each other
Define permanent dipole-dipole interactions
an attractive force between permanent dipoles in neighbouring polar molecules
Define hydrogen bonding
a strong dipole-dipole attraction between an electron-deficient hydrogen atom of -NH, -OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule
Name one significant instance where hydrogen bonding occurs (related to biology)
occurs between purine and pyrimidine bases in DNA to form the double helix structure
Give 3 anomalous properties of water
solid ice is less dense than liquid water
relatively high melting point and boiling point
relatively high surface tension and viscosity
Explain how solid ice is less dense than liquid water
hydrogen bonds hold water molecules apart in an open lattice structure, so that they are further apart in ice than water - making ice less dense and able to float
What is a simple molecular substance made up of?
simple molecules
Define simple molecular lattice
a 3D structure of molecules, bonded together by weak intermolecular forces
Give 3 properties of simple molecular lattice
- low melting point and boiling point
- non-polar dissociates in non-polar solvents and polar in polar depending on the strength of dipole
- non-conductors of electricity
What should you note about the melting and boiling of simple molecular lattices?
strong covalent bonds do not break on changing state, only the weak intermolecular interactions
Explain why NF3 has a permanent dipole
F is more electronegative than N so contains polar bonds; the molecule is non-symmetrical so dipoles do not cancel out
