Group 16,17,18 Flashcards
use of polonium
chemical weapons (highly radioactive)
astatine
highly radioactive
helium
2nd most abundant element - used in NMR (super conducting magnet)
group 16 - general properties
= chalcogens
-same periodic trend as in group 13-15
-non-metals = O,S
-semi-conductors = Se, Te
-others = metallic
last group to contain true metal = polonium
what is catenation?
ability to form chains
which elements is catenation important for?
S, Se + Te
group 16 - oxidation states
max. = +6 (important for S)
max. for oxygen = +2
most common = -2,-1,0
allotropes of oxygen
O2 and O3
ozone
strong oxidising agent
-each O has 6 valence e-
-central O = sp2 hybridised
-1 l.p. in sp2 orbital
-1 l.p. in p orbital
-each terminal O involved in 1 bond (1 unpaired e- in each)
-total of 4e- in p orbitals
what do electrode potentials show?
how effective an oxidising agent something is
ozone - bond lengths
shorter than O-O in H2O2 but longer than O=O in O2
allotropes of S
S has more allotropic forms than any other compound + has more compounds with single bonds
trend in allotropes down group
decreased tendency to form multiple bonds
decreased tendency to catenate (related to bond energies)
trend in acidity down group
bonds get weaker down group (poorer overlap + increased mismatched)
reduced stability
increased acidity
trend in sp mixing down group
energy difference between s+p orbitals increases
less s-p mixing
bonds become more p-like in character
H-M-H gets closer to 90°
hydrogen peroxide
v. pale blue liquid
liquid at rtp
more dense and viscous than water
oxn state = -1
can be both a strong oxidising agent (acidic conditions) + reducing agent (basic conditions)
prone to disproportionation - promoted by storing in glass
halides of oxygen
only consider fluorides
highly reactive
[O2F2]
-similar structure to H2O2
-prepared by passing electrical discharge through F2 and O2
-solid
-decomposes at rtp over days
-extremely powerful oxidative fluorinating agent (hard to control)
NpF4 + O2F2 -> NpF6 + O2
sulfur halides
S2F2 - v. unstable and highly reactive
SF4 - acts as both lewis acid and lewis base
[why?]
6e- from sulfur and 4e- from fluorine (10e- altogether - hypervalent)
expect to act as Lewis acid
10e- to 12e- = expansion of octet
F is oxidising enough to allow S to get group oxidation state to +6
should be reactive to H2O but due to high kinetic stability, H2O can’t get past 6 S-F bonds to react
high activation barrier to overcome despite favourable reaction thermodynamically
SF6 -> SF4/SF2
F-F = weak compared to Cl-Cl
no strong driving force for decomposition
basic oxides
involves metal oxides
react with acids to produce salt + H2O
acidic oxides
involves non-metals
reacts with acids to produce salt + H2O
amphoteric oxides
reacts with both acids and bases to produce salt + H2O
formed by metalloids and some metals that lie at border of metal/non-metal behaviour
neutral oxides
react with neither acid or base
important oxides of S + properties
SO2 + SO3
high Zeff
elements = smaller
behave like elements in the first row
why do we have O2 and S8?
O2 = 2 p-orbitals
S8 = 3 p-orbitals (more diffuse ∴ weaker π component - more favourable to have σ-bonds)
O=O - strong π component - x2 2p overlap; higher electron density; not v. diffuse
S=S - weak π component - x3 2p overlap; v. diffuse
SO2 - 2p+3p overlap; 3p contracted due to high Zeff of S; allows π-bonding to occur
preparation of sulfur oxides
S + O2 -> SO2 (burn in air)
2SO2 + O2 -> 2SO3 (use of catalyst)
what type of oxides are SO2 + SO3?
[acidic]
SO2 -> H2SO3
SO3 -> H2SO4
sulfuric acid
prepared by Contact process
4FeS2 + 11O2 -> 8SO2 + 2Fe2O3
SO2 purified
2SO2 + O2 + catalyst -> 2Fe2O3
2SO3 dissolved in conc. H2SO4 to give oleum (H2SO4 . SO3)
SO3 reactivity with water
doesn’t react - too violent; no solution (mist instead)
F2
pale yellow gas
v. corrosive
most reactive element known
unique properties of F
strongest oxidising agent
ΔHe- attachment out of order - smaller; most electron repulsion
weak F-F bond - kinetically reactive
hydride complexes show unusual H-bonding effects
trend in pKa down halogen group
decrease down group - acidity increases as bond strength decreases
expect HF to be most acidic as it’s most electronegative but it’s not
[reasons]
- HF bond has partial ionic character (charges slightly separated)
- [H3O+.F-] = ion pair
stick together due to strength of H bonding
don’t generate lots of H3O+ - decreases acidity - F- + HF ⇌ [HF2]-
lower conc. of HF present in initial solution
fluorine - trend in boiling point
high - due to H bonding (less than water)
[reasons]
- H-F has unidirection H-bonding
- H bonding retained in gas phase - heat to 60°C + still have some H bonding
interhalogen compounds
4 families - XY, YX3, XY5, XY7
prep = direct combination of elements
highly reactive (ΔG < 0 - exergonic)
X-F = more stable than X-Cl
IF = most stable (due to polar contribution)
charged interhalogens also known
F always -1 oxidation state
biggest atom = central
stabilisation required non-coordinating (weakly basic) anions
halogen oxides
can form acidic species
increased number of 0, acid becomes stronger
oxidation state becomes more +ve
oxygen stabilises conjugate base
group 18
= noble gases
all monoatomic gases
helium = 2nd most abundant element in the universe - demand > supply
unique property of helium
when cooled to 2.2K, transforms to He(II) = superfluid (has 0 viscosity)
is the I.E. of the noble gases expected to be high or low?
high - due to stable octet, highest possible Zeff
