Lewis (electron dot) structures show all the valence electrons of the atoms in the molecule or polyatomic ion. They can be used to predict molecular shape.

Shapes of Molecules 

  • Most atoms form a stable arrangement with eight electrons in their outer shell: the octet rule.
  • There are exceptions to the octet rule:
    Examples
    $\rm BeCl_2, BF_3$: Be and $\rm B$ have small atomic radii and and so have less than an octet.
    $\rm PCl_5, SF_6$:  $\rm P$ and S are both in third period and have $\rm 3d$ orbitals in the valence energy level are available for bonding and so can have more than a stable octet.
  • $\bf VSEPR$ theory: the total number of electron domains determines their geometrical arrangement as they mutually repel; the shape of the molecule depends on the number of bonding pairs within this arrangement.
  • The number of electron domains determines their arrangement and number of bonding pairs determine the shape.
  • The angle is smaller than the symmetrical value due to the increased repulsion from   a non-bong pair of electrons.   
  • The presence of non-boning pairs $\rm (NBP)$ or multiple bonds $\rm (MB)$ effect the bond angle as lone pairs of electrons occupy more space than bonding pairs,
  • The order of repulsion between the electron pairs is:
    $\rm NBP(MB) | NBP(MB)$ $>$ $\rm NBP(MB) | BP > BP | BP$

Methane, ammonia and water have four electron domains in a tetrahedral arrangement around the central atom.
Methane

Ammonia


Water

  • The bond angle is determined by the number of electron domains but is reduced if one of these is non-bonding pair as they have greater repulsion than a bonding pair. 

Resonance Structures

  • Resonance structures occur when there is more than one possible position for a double bond. This result in bond with non-integer bond orders. Ozone for example has a symmetrical structure and the bond order is $1.5$, which is intermediate between a single and double bond.
    Other examples include benzene, $\rm C_6H_6$ and the ethanoate $\rm CH_3CO_2^-$ carbonate $\rm CO_3^{2-}$ and nitrate $\rm NO_3^-$ ions.

Benzene; the carbon -carbon order is $1.5$.

Ethanoate; the carbon-oxygen order is $1.5$.

The carbonate ion. The bond order of the carbon-oxygen bond is $4/3$ and the charge on each $\rm O$ atom is $-2/3$. 

Topic-4-SL-19The nitrate ion. The bond order of the nitrogen-oxyegn bond is $4/3$ and the charge on each $\rm O$ atom is $-1/3$.  

Giant Molecular Strcutures

  • Carbon, silicon, and silicon dioxide form giant covalent molecules.
  • Carbon occurs as allotropes with different bonding within giant molecules – diamond, graphite, fullerene, and graphene.

Diamond


Graphite 


Buckminsterfullerene

which Graphene

Molecular Polarity 

  • The polarity of a molecule depends on:
    • the polarities of its bonds which depends on the difference in electronegativities of the atoms in the bond.
    • its molecular shape – symmetrical geometries can lead to cancellation  between the polar bonds.


The $\rm C = O$ is polar as oxygen is more electronegative than carbon. / The $\rm Be – Cl$ is polar as chlorine is more electronegative than beryllium.

The molecules are however non-polar as the bond polarities act in opposite directions and cancel.


Water has polar $\rm O - H$ bonds and is a polar molecule.