bonding and trends Flashcards
(16 cards)
What is the trend in the enthalpy of atomisation?
Enthalpy of atomisation if the enthalpy change when one mole of gaseous atoms is formed from its element in its standard physical state.
Atomisation of 1st row > 2nd row -> bond strengths 1st row > 2nd row.
E(s) -> E(g)
Group 14 bonding
consider each to be sp3 hybridised with abond angle of 109.5, tetrahedral at each atom. Each hybrid atomic orbital bonds with 4 others.
Group 15 bonding
N is a gas at RT, but all other G15 are solids. 3 unpaired electrons leads to 3 bonds, with a lone pair on each atom.
Group 16 bonding
electrons
Each G16 element has 6 valence electrons. ns2 np4. Each structure has two bond and two lone pairs.
Group 17 bonding
7 electrons per atom, therefore the halogen can form one bond and each halogen atom has three lone pairs
Group 18 bonding
forces
G18 elements all exist as monatomic gases under ambient conditions. Only weak interatomic forces (van der Waals), and consequently have low mp an bp. Atomic radius increases down the group.
Atomic radii trends
When descending the group the atomic radii increases, resulting in larger orbitals and consequently poor orbital overlap leading to weaker bond strengths down the group. (The presence of radial nodes interfers with effective overlap as you descend the group). (sigma has good overlap, pi has less effective overlap).
What is valency and hypervalency?
Valency can be defined as the maximum number of univalent atoms (originally hydrogen or chlorine atoms) that may combine with an atom of the element under consideration.
A molecule is described as being hypervalent when it contains one or more main group elements which appears to have more than eight electrons in their valence shells.
What is the inert pair effect?
The inert pair effect is the tendency of the valence s-electrons not to engage in bonding upon descending the group in the p-block. This means that the oxidation state that is two fewer than the group valency becomes most stable for the heavier group 13-16 elements
Whats the difference between Bronsted acids/bases and lewis acids/bases?
Brønsted Theory defines an acid as an H+ donor and a base as an H+ acceptor (Johannes N. Brønsted and Thomas M. Lowry). Lewis theory defines a Lewis acid as a lone pair acceptor and a Lewis base as a lone pair donor (Gilbert Lewis)
What is a frustrated lewis pairs?
Frustrated Lewis pairs (FLPs) are very bulky Lewis acids and bases that cannot not form adducts due to steric hindrance. FLPs have been found to exhibit similar behaviour to certain organometallic compounds and can be used in catalysis.
What is the Valence Shell Electron Pair Repulsion (VSERP)?
A simple way of predicting and rationalising the structures of covalent compounds formed by main group elements. The shape of a polyatomic molecule can be deduced on a qualitative basis by considering the repulsive forces between electron pairs in the valence shell of the central atom.
How many axial and equatorial positions are there?
There are 2 axial positions and 3 equatorial positions.
How does electron repulsion effect the structure of a complex?
axial ligands
The basic shape depends on the number of electron paird surrounding the central atom (ie both bonding and lone pairs).
repulsions > lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair.
The axial ligands are repelled more strongly by the lone pair(s) than by the equatorial bonding pair(s), so that the angles FaxSFax and FaxClFax are actually less than 180°
What is the berry pseudorotation?
The Berry pseudorotation mechanism, is a type of vibration that causes molecules of certain geometries to isomerise through the exchange of ligand positions (axial/equatorial). It most commonly occurs in trigonal bipyramidal molecules, such as PF5.
Tetrahedral - not fluxional
Octahedral - limited fluxionality
Why don’t the first row form hypervalent compounds?
While the second row of the p-block can form hypervalent compounds, the first row do not. How do we explain this without using d-orbitals? The answer is probably due to the small size of the first row atoms which cannot fit more than 4 atoms around them.
