CHAPTER 6 - SHAPES OF MOLECULES AND INTERMOLECULAR FORCES Flashcards
What is electron pair repulsion theory?
- the number of electron pairs surrounding the central atom determine the shape of the molecule
- electron pairs repel as far as possible so that they are arranged equally
- the arrangement of electron pairs minimise repulsion and hold the shape of the molecule
- lone pairs repel other pairs more than bonded pairs
What factors determine the shape of a compound or ion?
- the number of electron pairs around the central atom
- the nature of the pairs (bonding / lone)
What are the features of bonding pairs?
- they repel equally
- involved in bonding
What are the features of lone pairs?
- more compact so repel other pairs more than bonding pairs
- not involved in bonding
- for each lone pair, bond angle decreases by 2.5
Explain what the different wedges represent when drawing molecules.
- normal : bond in plane of paper
- dotted : bond going into paper
- bold : bond coming out of paper
What is the octet rule?
- 8 electrons in pairs maximum around one atom
Explain shrinking octet.
- not enough electrons to pair and form an octet so….
- unpaired electrons pair up
- octet is not achieved
- e.g. boron trifluoride
Explain expanding octet.
- bonding atoms have 8+ electrons in outer shell
- occurs period 3 down only in groups 15, 16, 17
- e.g. SF6-sulfur hexafluoride
Describe a linear shaped molecule and give an example.
- one or two bonding pair
- bond angle 180
- e.g. H2 or CO2
Describe a trigonal planar shaped molecule and give an example.
- 3 bonding pairs
- bond angle 120
- e.g. BF3
Describe a pyramidal shaped molecule and give an example.
- 3 bonding pairs
- 1 lone pair
- bond angle 107
- e.g. NH3
Describe a non-linear shaped molecule and give an example.
- 2 bonding pairs
- 2 lone pairs
- bond angle 104.5
- e.g. H2O
Describe a tetrahedral shaped molecule and give an example.
- 4 bonding pairs
- bond angle 109.5
- e.g. CH4
Describe a trigonal bypyramidal shaped molecule and give an example.
- 5 bonding pairs
- bond angle 90 (on normal wedges) and 120 between (solid and dotted wedges)
- e.g. PCl5
Describe an octahedral shaped molecule and give an example.
- 6 bonding pairs
- bond angle 90
- e.g. SF6
How do you work out the shape of a molecule (4 steps) ?
- Write number of electrons on outer shell of central atom
- Write the number of atoms bonded to the central atom
- Work out number of lone pairs —> 1/2 (step 1- step 2)
- Find shape and bond angle with number of bonding pairs and lone pairs of electrons
What are the 5 steps used to explain the shape of a molecule?
- State the number of bonding and lone pairs of electrons
- State: ‘electron pairs repel and try to get as far apart as possible to a position of minimal repulsion’
- If there are NO lone pairs, state: ‘the bonded pairs repel equally’
- If there are lone pairs, state that: ‘the lone pairs repel more than bonded pairs’
- State the actual shape and bond angle.
Define BOND ANGLE
the angle between two bonds in a molecule
Define BONDED PAIR
A pair of electrons shared between two atoms to make a covalent bond
Define DIPOLE
A separation in electrical charge so that one end of a polar molecule has a slightly positive charge and the other end has a slightly negative charge
Define ELECTRONEGATIVITY
The ability of an atom to attract electrons in a covalent bond
Define COVALENT BOND
the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
Define INTERMOLECULAR FORCE
An attractive force between molecules (e.g. London forces, permanent dipole-dipole interactions, hydrogen bonding)
Define HYDROGEN BOND
A strong dipole-dipole attraction between an electron- deficient hydrogen atom on one molecule and a lone pair of electrons (N,O or F) on a different molecule