2.6 Shapes of Molecules and Forces Flashcards
What is the valence shell repulsion theory ?
-Valence electrons exist in pairs.
-Electron pairs move away from each other to minimise electrostatic repulsion.
-Electron pairs are either bonding or lone pairs.
What is the strongest bond - bond repulsion in order ?
Lone pair - Lone pair
Lone pair - Bonding pair
Bonding pair - Bonding pair
Linear shape
-2 electron pairs (2bp).
-180 bond angle.
E.g. BeCl2
Trigonal Planar
-3 electron pairs (3bp).
-120 bond angle.
E.g. BF3
Tetrahedral
-4 electron pairs (4bp).
-109.5 bond angle.
E.g. CH4
Pyramidal
-4 electron pair (3bp / 1lp).
-107.0 bond angle.
E.g. NH3
Non-linear
-4 electron pairs (2bp / 2lp).
-104.5 bond angle.
E.g. H20
Octahedral
-6 electron pairs (6bp).
-90 bond angle.
E.g. SF6
What are multiple bonds classed as ?
Multiple bonds, close together, are classed as one area of electron bonding region.
What is electronegativity ?
The ability of an atom to attract the shared electron pair in a covalent bond.
What are 2 factors that affect electronegativity ?
-Size of the atom.
-Nuclear charges are different.
What are Pauling values ?
Used to compare electronegativity of the atoms of different elements. Large value indicates the atoms are very electronegative.
-Covalent = o difference
-Polar covalent = less than 1.8 difference.
-Ionic = more than 1.8 difference.
What is a non polar bond ?
Bonded electron pair is shared equally between bonded atoms.
What is a polar bond ?
Bonded electron pair is shared unequally between bonded atoms. Atoms will have different electronegativity values resulting in a polar covalent bond.
What are polar molecules ?
-Contain polar bonds.
-Not symmetrical.
-Dipoles do not cancel but act in different directions.
What are intermolecular forces ?
Intermolecular forces are weak forces of attraction between molecules.
London Forces
Temporary dipole - induced dipole.
Variation in external electrical field strength creates a temporary dipole. There is a shift in electron density. This causes a dipole in a neighbouring molecule.
What is the typical bond enthalpy for London forces ?
10 KJ mol-1
What is the permanent dipole-dipole interaction ?
Molecules with permanent dipoles interact. Creates a permanent dipole-dipole interaction between molecules.
What is a typical bond enthalpy of permanent dipole-dipole interaction ?
25 KJ mol-1
What is the melting/boiling point of simple molecular substances ?
Low melting and boiling point due to weak intermolecular forces between molecules.
Can non polar simple molecular substances dissolve in polar solvents ?
No because non-polar substance only has London forces, whilst polar solvent has London forces and permanent dipole - dipole interactions.
Can non polar simple molecular substances dissolve in non polar solvents ?
Yes because they will create London faces between each other as they have London forces between similar molecules.
Can polar simple molecular substances dissolve in non polar solvents ?
No because non-polar substance only has London forces but polar substance has permanent dipole-dipole interactions.
Can polar simple molecular substances dissolve in polar solvents ?
Yes because all have permanent dipole-dipole interactions so form the dipole-dipole attractions between each other.
What is the electrical conductivity of simple molecular
substance ?
Doesn’t conduct as they are not charged
What are hydrogen bonds ?
Attraction between an electron deficient hydrogen atoms on one molecule and a lone pair of electrons on an electronegative atom of a neighbouring molecule.
How many hydrogen bonds can form to one water molecule ?
Water can form 4 hydrogen bonds.
What is the anomalous property of water in terms of boiling point ?
Boiling point of H2O is relatively high due to hydrogen bonds that require more energy to break, increasing boiling point.
What is the anomalous property of water in terms of density ?
Hydrogen bonds fix further apart from one another, which creates more space between molecules so is less dense. The volume in a solid is bigger than the volume in a liquid, so ice is less dense.
What is the anomalous property of water in terms of surface tension ?
Hydrogen bonds hold water molecules together more than without bonds, which creates surface tension.
