2011/12/20

Simple Method for writing Lewis Structures of the phosphate ion(PO4)-3 -#15

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
Humans and animals get phosphate from the food they eat. Plants get phosphate from the soil. Fertilizer is added to phosphate-deficient soil to replenish this vital chemical species.

Step 1: The central atom will be the P atom since it is the less electronegative. Connect the atoms with single bonds:

lewis electron dot structures for the phosphate ion PO4-3
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:

Easy method for drawing Lewis structures for PO4-3
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.


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