lecture 1 recap lecture Flashcards
what elements count as main group elements
elements in the s and p block
what is Zeff
the effective nuclear charge felt by the outermost electron
what rules do we use to predict Zeff
slaters rules
equation for Zeffective
Zeff = Z - shielding constant
where Z is the nuclear charge
Zeff trendssss
across a period ➡️: Zeff increases
down a group⬇️: Zeff decreases
Zeff trends explained
as u go across a period,, the number of protons and electrons increase by1.
e- are in the same or different shells which makes them unale to shield the outermost e- from the increase in nuclear charge.
which orbital is the best at shielding
s>p>d>f
bc its closer to the nuc + has a larger shielding ability.
how does Zeff change as atom radius//size increases
as the atom gets larger,,, the Zeff decreases
theyre inversely proportional
how does Zeff affect atom size
as Zeff increases the atoms size decreases.
due to larger attraction between the outer electron and the nucleus.
what are the 2 different contractions seen in the periodic table used to explain trends seen in the atomic radii
d block contraction
and lanthanide // f block contractions
explain the contractions
atoms below each other may have very similar atomic radii // size.
this is due to added e- being put in the d or f orbital (lots of space) and so they cannot shield the outermost electron from the added nuclear charge
this causes the atom to be much smaller than expected.
what is electronegativity
it is the ability of an atom in a molecule to attract e- towards itself.
what element is the most electronegative
Flourineeee
different definitions for electronegativity
allred rochow
pauling
muliken
muliken electronegativity scale
based on the mean of the sum of IE and EA + constants
pauling electronegativity scale
based on bond strength
heteronuclear bonds are stronger than homonuclear bonds due to the electronegativity difference
and therefore increase in ionic character.
allred rochow electronegativity scale
based on Zeff and covalent radius
Zeff being effective nuclear charge.
allred rochow little arrow diagram thing
as the atom size increases
its electronegactivity decreases
what electronegativity explain
it can explain the e- distribution is a bond
if a bond is polarised itttt
its a polar bond
has an uneven distribution of e-
theres an electronegatvity diffference between the 2 atoms in the molecule.
the more elctronegative moelcule will have a
partial negactive charge
the less electronegative molecule will have a
partial positive charge
when is a bond ionic
when there is a large electronegativity difference
one that is more than 2 units
it forms A+ and B-
no partial +/-
electronegativity has what symbol
X
are the X values the same for the same moelcule
nope
they can change depending on the chemical environment
its O.S,,, bonding partners,, EWDG, EDG
if X can change,, what are the given vales of X
the elements X in an average chemical environment
describe the reaction with NMe3 and BH3
N lone pair attacks the B which has an empty p orbital
NMe3 is now (+) as it has given away its lone pair. HOWEVER,, N is still more elctronegative than C, so it has a partial negative charge.
the B is now (-) as its accepted e-. however B is less electronegaative than H,, and so has a partial positive charge.
describe the bond strengths trends of homonuclear bonds
down a group = weaker bond = less effective overlap of orbitals due to larger atom size
homonuclear bonds that go against the trends
the CNF homonuclear bonds
theyre no small that their lone pairs repel eachother giving a decrease in efficient orbital overlap which weakens the bond.
lone pair lone pair repulsion
are heteronuclear or homonuclear bonds stronger
usually heteronuclear bonds
bc theres a difference in electronegatvity so more ionic character
bond strength is proportional to
(Xa - Xb)^2
the bond is stronger but could be more reactive due to being polarised
is NF3 thermodynamically stable
yessss
the N-F bond is stronger than the homonuclear NN or FF bonds
due to the difference in electronegativity + ionic character.
is NCl3 a stronger or weaker bond than NF3
its a weaker bond as there is no electronegativity difference.
meaning less ionic character.
as the bond isnt polarised due to the same X values.
what do we consider to deduce ionic bond strength
we consider lattice energy
cation + anion + ionic radius
what is lattice energy proportional to
q+ q- /// r+ + r-
VSEPR
when we can predict the shape of a molcule by the number of valence e- pairs it has.
