Atoms in Lewis structures may be joined by multiple bonds involving two or three electron pairs.
Oxoanions are an important group of ions that have one or more double bonds to the central atom from outer oxygens.
Both the shape of an oxoanion and the number of doubly-bonded oxygens can be derived from its Lewis structure.
The example shows the procedure for drawing the Lewis structure for the oxoanion HCO2– and using this structure to determine the shape of this ion.
Oxoanions are an important group of ions that have one or more double bonds to the central atom from outer oxygens.
Both the shape of an oxoanion and the number of doubly-bonded oxygens can be derived from its Lewis structure.
The example shows the procedure for drawing the Lewis structure for the oxoanion HCO2– and using this structure to determine the shape of this ion.
This same procedure can be used for other oxoanions where the central atom is from row 2 of the periodic tablle.
These elements can be surrounded by a maximum of four electron pairs.
First: Determine total number of available valence electrons. | ||
 | Group 14 C(4 × 1) | 4 |
| 2O Group 16 (6 × 2) | 12 | |
| H (1) | 1 | |
| Adjust for charge | 1 | |
| valence electrons available | 18 | |
Second: Subtract electrons used in bonds and as non-bonding pairs at outer atoms. | ||
| electrons used in bonds: electrons used to complete octets: electrons remaining: | –6 –12 0 | |
Third: When all electrons are accounted for, check for octet at central atom. | ||
 | Electrons at central atom: Electrons needed to complete octet | 6 2 |
Fourth: Complete octet at central atom by moving nonbonding electron pair(s) from oxygen to between central atom and O as bonds | ||
 to give final structure  | ||
| Fifth: Deduce the shape of the ion after counting the number of electron pair domains at the central atom. | ||
| Number of bonding (single or double) domains plus the number of non-bonding electron pair domains. | Electron pair domains at C: | 3 |
| Position of outer atoms when the electron pair domains are as far apart as possible. | Shape of HCO2– | trigonal planar |