Molecular compounds, or covalents, are formed through covalent bonds, those in which we do not have the involvement of metals (with the exception of Beryllium) in their formation. So, in this type of bond, all atoms need to receive electrons and, therefore, they will always share them.
THE structural formula is one of the ways used to represent the sharing of electrons between the atoms of molecular compounds. So that we can carry out the assembling a structural formula, it is necessary that first of all we know what is the need for each of the elements involved in the compound according to the octet theory. The table below shows the need for each element involved according to its family:
From the needs expressed above, we should know that by build a structural formula, we only have the following tools as resources:
• single bond: − (sharing of two electrons);
• double bond: = (sharing of four electrons);
• triple bond: ≡ (six electron sharing);
Knowing the needs and tools, assembling the structural formula of a molecular compound can be performed from the following step by step:
a) Diatomic molecule (formed by only two atoms):
Just write an atom on the left and an atom on the right and place the bond that suits the needs of both. See some examples:
- O2
As oxygen belongs to the VIA family, it needs two electrons. Therefore, the bond that fits best between them is the double bond.
- HCl
Since H and Cl need only one electron, the bond that best fits between them is simple.
b) Molecules that have more than two atoms
In molecular compounds that have more than two atoms, we must place in the center of the molecule the atom that needs the greatest number of bonds (priority) or the least electronegative atom. The others must be at the four poles (north, south, east and west). When placing the connection between each of them, we must prioritize the atoms that are at the poles. The central atom will always be stabilized following this order of assembly. See some examples:
- H2O
As oxygen needs more bonds (two, as it belongs to the VIA family), it will be placed at the center of the molecule, and hydrogens, at the poles. As the need for each H is only one link for each one of them, we will use a simple link. Since oxygen will make two single bonds, it will be stable.
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- NH3
As nitrogen needs more bonds (three, as it belongs to the VA family), it will be placed at the center of the molecule, and hydrogens, at the poles. As the need for each hydrogen is only one bond for each of them, we will use a single bond. Since oxygen will make three single bonds, it will be stable.
- CH4
As carbon needs more bonds (four, as it is from the VIA family), it will be placed at the center of the molecule, and hydrogens, at the poles. As the need for each hydrogen is only one bond for each of them, we will use a single bond. Since carbon will make four single bonds, it will be stable.
- CO2
As carbon needs more bonds (4, as it belongs to the IVA family), it will be placed at the center of the molecule, and oxygen at the poles. Since the need for each O is two bonds for each of them, we will use a double bond. Since oxygen will make two double bonds, it will be stable.
Observation: When an atom of the molecule, during the assembly of the structural formula, is stable and another one still needs of two electrons, we can use a different tool, which is called the coordinate covalent bond dative. This type of bond can only be used under this condition (one atom stable and another one requiring two electrons). See some examples:
- CO
Since we only have two atoms, let's place one on the left and one on the right. Oxygen needs two bonds, so we have to use a double bond.
However, when using the pair, oxygen is stable and carbon still needs two more electrons. Therefore, we can use the dative covalent bond, which is represented by an arrow that always goes from the stable atom to the non-stable atom.
- O3
As we have three atoms, one of the oxygens must be in the center of the molecule and the other two, in the poles. The rule studied always asks that, in these cases, we first put bonds on the atoms at the poles. However, here we can only add a single double, as all oxygens only make two bonds.
The oxygen in the center is stable, while the oxygen in the left still needs two electrons. For that reason, we can use a dative link to stabilize it.
By Me. Diogo Lopes Dias
