Lec 1-5 Mike Flashcards
What increases as you go across the periodic table?
- Zeff
- IE
- Ea
- Electronegativity
What decreases as you go down the groups?
- Zeff
- IE
- Ea
- Electronegativity
What decreases as you go across the period?
- Size
What increases as you go down a group?
- Size
The P Block (main group elements) properties:
- Boron/carbon/silicon/nitrogen/phosphorous are…
semi/non metals
As you go down the group in the p block there is more…
metallic character and larger radii
- allows for higher coordination numbers
- due to decrease in Zeff
Most p block elements can adopt more than one…
oxidation states
Properties of boron
- Compact, good overlap, semi metal
- only non metallic element in group 13
- forms covalent bonds
- has one less valence electron than number of valance orbitals
Properties of carbon
- form strong bond as good orbital overlap
- bond is a 2e- wave function
- lowered energy state
- non metallic structure
- forms covalent compounds with other non-metals and structures with a high ionic character with electropositive metals
- Caternation
- double bonds and triple bonds more stable for C than other 14 elements
Properties of Silicon
- 3p orbital overlap rather than 2p
- acts as semiconductor as diamond like structure
- more diffuse
- overlap less efficient
- still 2e- wave function
- but weaker bond
- so more reactive
In groups 16 and 17 are anions or cations formed?
- anions
As you go down P block what properties do you see?
- increasing s-p gap
- orbitals more diffuse - weaker bonding
- less favourable hybridization of S etc
- everything goes up in energy as wave function feels Zeff more than p orbital of same principle quantum number.
- highest possible oxidation state of -2 becomes preferred down the group, due to ns2 not engaging in chemical bonding
Group 13 properties
- 3 valence electrons = ns2np1
- Maximum oxidation state = +3
- aluminium most abundant group 13 element
- increase in metallic (ionic) character as you go down group
Why do Al and Ga have similar atomic radius and 1st IE?
- due to transition elements before gallium.
- Zeff increases across so elements decrease in size across
- decreasing radius due to filling of d orbitals is enough to offset what you would expect
EX3 compound properties
- trigonal planar
- this leaves you with a valence orbital (p2) which is unfilled therefore available to accept e- density in p orbital
- group 13 elements have 3e-
- so in EX3 there is an incomplete octect (only 6e-)
- empty 2pz orbital is low in energy –> electron deficient (liable to pick up e- from elsewhere)
- vacant orbital leads to lewis acid behaviour
Group 13 Lewis Acids and Bases
LA - if empty orbital available then can accept e- density and so is a lewis acid
- can combine via donation of e- density from LB –> LA vacant orbital
What is a lewis acid?
Lewis acids - species capable of accepting a pair of e-s acceptor (e.g. BX3 or AlCl3, transition metals)
What is a lewis base?
Lewis Base - species with a pair of e-s available for donation e.g. H2O, NH3, F- and other halogens
Lewis Acid + Lewis Base
BX3 + NH3 –>
- Lewis Pair Adduct
- products final bonding wave function has more nitrogen character than B
- polarised bond
Boron Hydride properties
BH3
- almost molecular like in structure, forms strong bonds to itself in good orbital overlap
- Trigonal planar
- It dimerises as only has 6e- to form B2H6 (diborane)
Key things to remember in chemistry
1) Zeff
2) Valance electron configuration
3) How bonds will form and bond strength
4) Is it thermodynamically favourable or is there an unfeasibly high Ea?
BX3 (boron) Compounds properties
- partially overcomes lack of electrons by forming pPIE - pPIE bonds
- these occur between the halogen p orbitals and the empty 2pz orbital on boron
- Can interact in PIE type interactions due to extra e- density as well as sigma interactions
What is the relative order of ease of electron acceptance by BX3?
BF3 < BCl3 < BBr3 < BI3
- increased ability to accept e- density
BN compounds
- BN unit is isoelectric to CC (similar structures to carbon)
- Boron nitride has a structure like graphite (2d) or like diamond (3d)
- Borazine (B3N3H6) has 6e- in a delocalised ring structure - like benzene