Molecules can be drawn using a variety of different styles. It is important to be able to understand the different styles, as they are likely to show up in the GAMSAT exam.
Lewis structures are useful as they clearly show all atoms and bonds in the molecule. Each bond represents the sharing of two electrons between respective atoms.
Partially condensed structures do not show the bonds between C and H. Atoms are drawn beside each other. Three hydrogens bonded to carbon are shown as a methyl group (CH3).
Condensed structures do not show any single bonds. The structural arrangement of atoms is still shown.
The molecular formula does not provide information about the structural arrangement of atoms. It only shows the number of each type of atom present. Looking at a formula, it can be seen that there can be many arrangements. Many previous GAMSAT questions have asked candidates to determine the number of different arrangements (constitutional isomers) that can be produced from a certain molecular formula.
Bond-line structures are simple and easy to read. They are drawn in a zigzag-like fashion where the endpoints and corners denote a carbon atom. The hydrogen atoms that are bonded to the carbon are not shown. It is assumed there are enough hydrogen atoms so that each carbon atom has 4 bonds. The end points will have 3 hydrogen atoms.
3D Bond-Line Structures
Wedges represent a group coming out of the page (towards you), and a dash represents a group going behind the page (away from you).
Identifying Lone Pairs of Electrons
There are several patterns that must be recognised for oxygen and nitrogen:
- A positive charge on the oxygen atom indicates 3 bonds and one lone pair of electrons; a negative charge on the oxygen atom indicates one bond and three lone pairs. No charge indicates two bonds and two lone pairs.
- A positive charge indicates 4 bonds and no lone pairs of electrons; a negative charge indicates two bonds and two lone pairs. No charge indicates three bonds and one lone pair.
Resonance structures are used to show the spread of positive charge. They show a combination of different structures in a linear fashion:
In terms of the GAMSAT, questions usually arise that require the candidate to identify the most significant resonance structures out of several for a certain compound. There are rules that can be used to determine the significance of resonance structures:
- Structures with minimal charges are more significant than structures with several charges.
- Structures that have a full octet of electrons on their atoms are more significant than those that do not have a full octet.
- If two carbon atoms in a structure have opposite charges, this structure is generally insignificant.
In summary, look for the structure that has minimal charges, a full octet of atoms, and does not show carbon atoms with opposing charges.
The concept of hybrid orbitals tends to confuse many students. For the purpose of the GAMSAT, detailed information is not necessary. Previous GAMSAT questions have asked to choose the compound that contains an sp2 hybridised carbon—easy marks!
The following information is sufficient to tackle questions related to hybrid orbitals:
- An sp3 orbital carbon will be connected to 4 atoms/groups. The angle between atoms/groups is 109.5 degrees.
- An sp2 orbital carbon will be connected to 3 atoms/groups. The angle between atoms/groups is 120 degrees.
- An sp orbital carbon will be connected to 2 atoms/groups. The angle between atoms/groups is 180 degrees.