A simple procedure for writing Lewis structures is given in a previous article entitled “Lewis Structures and the Octet Rule”. Relevant worked examples were given in the following articles: Examples #1, #2, #3 , #4, #5, #6, #7, #8, #9, #10, #11, #12, #13, #14.
Another example for writing Lewis structures following the above procedure is given bellow:
Let us consider the case of the phosphate ion PO4-3 . The phosphate ion combines with various atoms and molecules withing living organisms to form many different compounds essential to life. Some examples of phosphate role in living matter include:
- Giving shape to DNA, which is a blueprint of genetic information contained in every living cell
- Playing a vital role in the way living matter provides energy for biochemical reactions in cells (ATP, ADP, AMP). The compound adenosine phosphate (ATP) stores energy living matter gets from food (and sunlight in plants) and releases it when it is required for cellular activities
- The forming and strengthening of bone and teeth
Step 1: The central atom will be the P atom since it is the less electronegative. Connect the atoms with single bonds:
|Fig. 1: Connecting the P and O atoms with single bonds (step 1)|
Step 2: Calculate the # of electrons in π bonds (multiple bonds) using formula (1) in the article entitled “Lewis Structures and the Octet Rule”.:
Where n in this case is 5 since PO4-3 consists of five atoms.
Where V = (5 + 6 + 6 + 6 +6 ) - (-3) = 32
Therefore, P = 6n + 2 – V = 6 * 5 + 2 – 32 = 0 So there are no π electrons in PO4-3
Step 3 & 4: Therefore, the Lewis structures for PO4-3 are as follows:
|Figure 2: Lewis structures for PO4-3. The P atom can accommodate more than 8 electrons as other third row and below elements do by using their empty d orbitals to accommodate additional electrons.|