Structure and Bonding 2 - Bond Polarity and IMFs Flashcards
define electronegativity
the ability of an element to attract the pair of electrons in a covalent bond
if two atoms sharing an electron pair have different electronegative values, where do the electron pair lie, and what does this result in?
the electron pair will lie closer to the more electronegative atom, resulting in the formation of a polar covalent bond
how does electronegativity change across a period?
it increases
why does electronegativity increase across a period?
- nuclear charge increases/atomic radius decreases
- shielding remains fairly constant
- therefore bonding electrons are attracted more strongly
how does electronegativity change down a group?
it decreases
why does electronegativity decrease down a group?
- atomic radius decreases
- shielding increases
- therefore less attraction of outer shell electrons
identify in descending order the three most electronegative elements
fluorine, oxygen and nitrogen, (FON)
why are there no electronegativity values for group 8?
they have full outer shells and so do not form compounds/covalent bonds
describe the electron distribution for molecules containing identical atoms ie. H2 and O2
the shared electron pair will be in the centre of the bond because identical atoms have the same attraction (same electronegativity)
describe the electron distribution for molecules containing non-identical atoms
the shared electron pair lies closer to the more electronegative atom as it exerts a stronger attraction
molecule is said to be polar
describe and explain the bond polarity in a HCl molecule
the shared electron pair lies closer to the Cl atom as it is more electronegative and therefore exerts and stronger attraction. as a result Cl has a slight negative charge and H has a slight positive charge
which symbol is used to denote partial charges in polar molecules?
delta
define permanent dipole
when the unequal sharing of an electron pair, due to substantial differences in electronegativity between two non-identical atoms in a molecule leads to charge separation within that molecule.
why do some molecules with polar bonds not have permanent dipoles?
- due to symmetry
- causes dipoles to cancel one another out eg. CBr4
name the three types of intermolecular forces and list them from weakest to strongest
WEAKEST
van der waal
dipole:dipole
hydrogen
STRONGEST
what are van der waal (vdw) intermolecular forces and what do they result in?
they are temporary induced dipole:dipole attractions
uneven electron distribution within a molecule results in a temporary/instantaneous dipole being formed
this results in the formation of an induced dipole in the neighbouring molecule which is then attracted to the partial charges accordingly
state and describe the 3 factors which affect the magnitude of a vdw IMF
1) number of electrons: more electrons = larger induced
2) atomic radius: smaller radius = smaller induced or not at all
3) surface area of the molecule: bigger surface area = larger vdw
why do dip:dip > vdw in terms of strength?
because permanent dipoles are stronger than temporary ones
when will hydrogen bonding occur?
when hydrogen is covalently bonded to nitrogen, oxygen or fluorine
what angle must a H bond make?
180°
how does the strength of a H bond compare to that of a covalent bond?
H bond is approximately 1/10 that of a covalent bond
how does ice show that hydrogen bonds are present?
when water freezes it expands as H bonded structure is formed. normally, most liquids would decrease in volume upon freezing.
inversely, when melting, ice sees a reduction in volume, when normally solids increase in volume upon melting
draw the bonding in H2O
check against notes
