Molecular Shape, Electronegativity Flashcards
What are the different molecular shapes?
Linear Trigonal Tetrahedral Trigonal pyramidal Bent Trigonal Bipyramidal Octahedral
What are the features of a linear molecule?
2 atoms attached to the central atom = has 2 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 180
What are the features of a trigonal molecule?
3 atoms attached to the central atom - has 3 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 120
What are the features of a tetrahedral molecule?
4 atoms attached to the centra atom - has 4 electron domains
no non-bonding electrons (no lone pairs)
bond angle - 109.5 = atom are as far apart as possible
What are the features of a trigonal pyramidal molecule?
3 atoms attached to the central atom and 1 lone pair = 4 electrons domains
bond angle - 107
- lone pair exerts extra repulsion on the 3 bonding pairs
- lone pair contracts the bond angle
- decreases bond angle from 109.5 to 107
What are the features of a bent molecule?
2 atoms attached to the central atom and 2 lone pair = 4 electrons domains
bond angle - 104.4
- lone pair exerts extra repulsion on the 2 bonding pairs
- lone pair contracts the bond angle
- decreases bond angle
What are the features of a trigonal bipyramidal molecule?
5 atoms attached to the central atom - 5 electron domains
bond angle - 90, 120
2 axial positions and 3 equatorial positions
What are the features of a octahedral molecule?
6 atoms attached to the central atom - 6 electron domains
bond angle - 90
What is VSEPR theory?
Valence Shell Electron Pair Repulsion
- count the valence electrons (group number)
- complete the octet around the central atom = make double, triple bonds if there aren’t enough electrons to give an octet
- calculate formal charges
formal charges = valence electrons - electrons in the atom
- sum up the number of electron domains
- find the geometry and bond angle
What are electrons domains?
each bond is an electrons domain
lone pairs count for one electrons domain
What is electronegativity?
tendency of an atom to attract a bonding pair of electrons within a covalent bond
How does electronegativity change across the periodic table?
increases from the left to right across a period
- has more electrons in the orbitals and more protons in the nucleus. this causes the nucleus to contract as attractive force increases
decreases down a group
- valence electrons are in shells further away from the nucleus so there is weaker attractive force from the nucleus
no trend in transition metals
What are the three classifications of intramolecular bonding regarding electronegativity difference?
non-polar covalent
- bonds whom occur between atoms with little or no electronegativity difference
- less than 0.5
polar covalent
- bonds which occur between atoms with definite electronegativity difference
- between 0.5 and 2.0
ionic
- bonds which occur between atoms with large electronegativity difference where electron transfer can occur
- greater than 2.0
What are intermolecular forces?
intermolecular forces are weaker than intramolecular forces
intra - forces within the compound/ molecules (holds molecule together)
inter - forces between molecules
intermolecular forces
- determines the physical properties of molecules = fusion, vaporisation
- boiling point can be used to reflect the strength of intermolecular forces = the higher the bp, the stronger the intermolecular forces
Why don’t elements other than fluorine in group 7 form hydrogen bonds when in the hydrogen halide form?
electronegativity decreases down group 7
- fluorine is the most electronegative
- H-X bond for the other halides is less polar therefore cannot form hydrogen bonds
- lone pairs for the other halides are in higher energy levels. this makes the lone pairs bigger so they don’t have an intensely concentrated negative charge for hydrogens to be attracted to
What is hydrogen bonding?
formed when there are functional groups with N, S or O present and there is a H atom linked to it
hydrogen bonds are directional
- H bonds are stronger when all the atoms are in a straight line
requires H atoms directly bonded to an electronegative atom and an electron acceptor (atom with a lone pair or negative charge) to occur
Why does ice float?
liquid water molecules - bonds are continually forming and breaking
ice water molecules - forms 4 H bonds = ordered open arrangement
- O can make 2 bond and H can make 1 bond each
What are London Dispersion Forces?
London dispersion forces are the weakest intermolecular forces in neutral molecules
- applies to covalent molecules with no dipole moment
- result of the motion of electrons
electrons distribution may become uneven at any point resulting in an instantaneous dipole. this can induce a dipole in neighbouring atoms. these atoms are then weakly attracted to each other
- temporary, small and local instantaneous dipole
What is the trend in intermolecular forces of different alkanes?
the larger the alkane, the larger the induced temporary dipole-dipole force
- because they have more electrons and are more polarisable
