p-block - periodicity + physical properties Flashcards
(43 cards)
p-block element definition
elements which have their valence electrons in p-orbitals
what are the 3 types of element in the p-block?
non-metals, metalloids, metals
how does atomic radii change across a period?
atomic radii tends to decrease across a period due to increasing effective nuclear force
how does atomic radii change down a group?
atomic radii tends to increase down a group because of increasing principle QN meaning more shells, however increase down group is not linear, the change from row 2 to row 3 is significantly larger, this tells us row 2 elements are significantly smaller than row 3
how are atomic radii values for p-block elements obtianed?
based on bonding atoms of that element, so elements that aren’t known to bond will not have these values
electronegativity definition
the ability of an element to attract electrons in a chemical bond
how does electronegativity change across a period?
electronegativity tends to increase across a period as effective nuclear charge increases and atomic radii decreases, leading to an increase in the attraction between nucleus and bonding electrons
how does electronegativity change down a group?
electronegativity tends to decrease down a group as bonding electrons are further from the nucleus as atoms have more electron shells
why do Ne/Ar not have known electronegativities?
they don’t form bonds
how does electronegativity predict element type?
metals X <~2.00
metalloid X ~ 2.00 ~ 2.20
non metal X > ~ 2.20
- this is relatively accurate
homonuclear single bond energy definition
the energy required to homolytically break a single bond between 2 atoms of the same element, represents bond strength
- as homolytic fission this creates 2 radical species
how does homonuclear single bond energy change across a period?
tends to increase across a period as electronegativity increases across a period, so elements attract bonding electrons more strongly so bonds get stronger
- exceptionally group 14 –>15 sees a decrease in bond energy because group 15 elements still have lone pairs when saturated with bonds, which repel and weaken the bond
how does homonuclear single bond energy change down a group?
tends to decrease down a group, except for between N->P, O->S, F->Cl
this is because p-orbitals have n-2 radial nodes, so down a group radial nodes increases as principle QN n increases, reducing in-phase orbital overlap when bonds form, resulting in weaker bonds
- exceptions is because lone pairs on N, O and F weakens their homonuclear single bonds, and as they also have very small atomic radii they have very short bond lengths so lone pair repulsion is greater, compared to row 3
do p-block elements tend to form double bonds?
most p-block elements do not form double bonds
do group 13 elements form double bonds + why?
group 13 elements have only 3 valence electrons, typically held in sp2 orbitals leaving an empty p-orbital which cannot form a π-bond as its empty
do groups 17+18 form double bonds + why?
group 17+18 don’t need to form multiple bonds, due to the octet rule, both achieve an octet either naturally or after just one single bond
are p-block elements more or less likely to form double bonds going down a group?
less likely
as row 2 elements have very small atomic radii their single bonds are shorter, allowing atoms to be close enough to allow effective π-orbital overlap
elements from row 3 down become much larger so π-orbital overlap is poorer - some row 3 elements can be forced to form double/triple bonds
why can P + S form double bonds if row 3 elements?
P + S are the smallest row 3 elements able to form multiple bonds, groups 13 + 17 + 18 aren’t able to at all and the atomic radii of Si is too large so overlap is much poorer, although it ca sometimes be forced
P + S aren’t yet that big and so have decent π-orbital overlap
are double/triple bonds always stronger than single bonds?
yes
why do some elements not naturally form multiple bonds if they are always stronger than single bonds?
this is based on the energy difference for forming a single bond vs multiple bonds
- for N and O forming triple/doble bonds is favourable as their single bonds are much weaker due to lone pair repulsion, and they have good π-orbital overlap as they have short bond lengths, the energy cost of a double bond is < energy cost of 2 single bonds - same for triple
- for P and S, π-orbital overlap isn’t as good and their single bonds are stronger, the energy cost of double bond > energy cost of 2 single bonds, so they prefer to form single bonds
give the structures of the naturally occurring elements O, N, P, S
O=O
N≡N
P-P
S-S
(remember P and S don’t necessarily exist as diatomics)
how does m.p/b.p change in p-block compounds?
in the absence of strong intermolecular interactions, m.p/b.p generally increases with Mr
compounds with ionic/polymeric structures have high m.p/b.p that compounds with discrete molecular structures
compounds that can hydrogen bond also have much higher m.p/b.p
bronsted acid definition
proton donor
bronsted base definition
proton acceptor
