Molecular Structure

Question 1

The Octet Rule (Guidelines?)

Answer 1

The octet rule is not without exceptions. Known examples
are:
* Molecules in which the central atom is surrounded by
MORE than 8 electrons
- common for elements in period 3 onwards which have
empty d-orbitals. E.g. SF6
* Molecules in which the central atom is surrounded by
FEWER than 8 electrons - mostly group 2 and 13 ( Be, B, Al
E.g. BF3)
* Compounds with an ODD number of valence electrons (eg.
NO).

Question 1

Resonance Theory

Answer 1

• Chemical structures are not static. In reality “real” structure lies somewhere between what we draw.

Question 2

Coordinate Covalent Bond or Dative Bond

Answer 2

• So far, we’ve seen examples where each atom contributes
  at least one electron to the covalent bond. It possible,
  however, that BOTH electrons in a particular covalent bond
  originates from only ONE atom. This is called a Coordinate
  Covalent Bond or a Dative Bond.

Question 3

Valence Shell Electron Pair Repulsion (VSEPR) Theory

Answer 3

applied (to the Lewis structures) in order to obtain the
molecular shape (3-D arrangement of nuclei joined by the
bonding groups)

Question 4

VSEPR theory aims

Answer 4

to minimise electron repulsions by
arranging each group of valence electrons (i.e. double bond
is considered as a single ‘group’ even though there are 4 e-)
around the central atom as far as possible from the others

Question 5

NOTE!!

Answer 5

Only electrons around the CENTRAL ATOM influence molecular shape

Question 6

Molecular Polarity

Answer 6

• Molecular shape has a strong influence on
  molecular polarity.
  Influences:
• Melting points
• Boiling points
• Solubility
• Chemical reactivity
• Biological function
• Molecular shape is crucial in determining polarity
  180° bond angle: Nett effect is no dipole moment = nonpolar.

Question 7

RECALL - Covalent Bonding:

Answer 7

A covalent bond involves the sharing of electrons between two atoms such that there is a balance between all attractive and repulsive forces between the atoms.

Question 8

Valence Bond Theory

Answer 8

states that a covalent bond forms when the orbitals of two atoms overlap and a pair of electrons occupy the overlap region (between the nuclei).
* The interactions of atomic orbitals to form a new “hybrid”
orbitals.
* Two overlapping wave functions in phase.
* Maximum of 2e- in the overlap and with opposite spins
(exclusion principle).
* The greater the overlap, the stronger/more stable the bond.