Gen Chem Q

[topdent1]

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If 2 atoms are bonded together, one with six valence electrons, the other with 4, how many electrons must they share for both to achieve a full octet?

The answer is 6 but I don't know why.

 

[Mstoothlady2012]

Total electrons --> 6+4 =10
Now you have to combine them in such a way that each atom fulfills the octet rule. In order to do that both atoms will have to form 3 covalent bonds (6 electrons) between them. Remaining 4 will be divided among two atoms; 2 for each. Hence now each atom has 8 electrons; 1 lone pair; and 6 shared electrons.

 

[DDSguyLA]

WELL said.
Add the total Valence electrons. Start with the bonds give as much then try to obey octet rule on each atom... a little mix and match will do the job.

 

[smile101]

A more systematic way I learned in my gchm class is here:

Suppose the two elements are X and Y

Total valence e: 10
so put X-Y
Now, # available: 8 (bc you used 2 to form a single bond between the two)
# required to fill the octet: 12 (6/ element)
This means that you need 4 extra electrons than available (12-8=4), so put 2 extra bonds, which results in a triple bond between X and Y.
These are all the bonds you will have, and you will put one extra pair of electrons on each one to use all the 10 electrons.

Wish I had paper and pen to explain this, but oh well.
Hope this helps!!