Lecture Seven Flashcards
Explain the VSEPR theory.
The Valance Shell Electron Pair Repulsion theory is a model used to predict the shape of molecules containing p-block atoms. The basic premise is that: Each group of valance electrons around a central atom is located as far away from the others as possible in order to minimise repulsions. The lowest energy geometries are:
How do lone pairs and multiple bonds repspectively effect the molecular and parental geometries?
A lone pair of electrons will influence where the bonding atoms are positioned, due to their repulsive nature.
A multiple bond is treated as a sinlge group of valance electrons.
Electron repulsions involving multiple bonds are greater than those involving single bonds.
Explain molecular polarity.
Most chemical bonds are polar, meaning thar one end is slightly negitive and the other is slightly positive.
A molecule with this type of asymmetrical distribution of electron density is said to have a dipol moment, symbolised by the greek letter mu, μ.
As a general rule, the greater the difference in electronegitivity, Δχ, the larger the μ.
The greater the Δχ, the more the molecule can be said to be ionic. The lower the Δχ, the more the molecule can be said to be covalent.
How can the dipol moment of an alactron be measured?
By placing a sample in an electric field.
In the absense of an applied field, the molecules are orientated randomly throughout the volume of the device.
When an electrical potential is applied across the plates, the molecule aligns spontaneously.
The positive ends of the molecules point towards the nevitive plates, and visa versa.
What are bond lengths and how to the effect a molecules behaviour?
The bond length of a covalent bond is the nuclear deparation distance at which the molecule is more stable. Several traneds emerge when comparing bond lengths of similar molecules:
1) As the atomic radii increases, bond lengths increases.
2) As bond polarity increases, bond length decreases, since partial charges generate electrostatic attraction that pulls the atoms closer together.
3) Bond order - placing additional electrons between atoms decreases internuclear repulsion, leading to an increase in the net attraction, thereby allowing the atoms to come closer together.
What are bond energies and how do they effect the behaviour of molecules.
Bond energyies are defined as the amount of energy that must be supplied to break a particular chemical bond.
Bond energies, like bond lengths, vary in ways that can be traced to atomic properties, and there are three consistent trends in bond energies:
1) Bond energies increase as more electrons are shared between teh atoms. Shared electrons are a kind of ‘molecular glue,’ so sharing more electrons strengthens the bond.
2) Bond energies increase as the electronegativity difference (Δχ) between bonded atoms increases. Polar bonds gain stability from the electrostatic attraction bertween the negaitve and positive partical charges around the bonded atoms.
3) Bond energies decrease as bonds become longer. As atoms become larger, the electron density of a bond is spread over a wider region. this decreases the net attraction between the electrons and the nuclei.
