How many π electrons are there in adenine? State which atoms contribute one electron to the π MOs and which atoms contribute two electrons?

12 π electrons.

Explanation:

The structure of adenine can be seen below.

From the diagram, the total number of electrons in adenine is 70 electrons where 20 of them are core electrons.

Also from this same structure, we will notice that we have a total number of  12 π electrons.

i.e.

Each one of these five atoms described takes part in the π electron. These are N-1, N-3, N-7 which comprise of 2σ bonds, and also each of three N contributes one π electron.

Similarly, N-9 and N-10 contain 3σ bonds; Hence, each of them donates two electrons.

Thus;

5C = 5π electrons

3N = 3π electrons

2N = 4π electrons

     = 12π electrons

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