the number of orbital pairs of electrons in an atom with an electron
configuration of 1s2 2s2 2p4 is:
a. 1  b. 3  d. 4

Hello teatea, 
 
There are a total of 3 electron pairs. The total number of electrons
in the atom can be found by adding the 2 in the 1s2, 2 in the 2s2, and
the 4 in the 2p4. These represent the electrons in the atom. There are
a total of 8 electrons. Each orbital contains 2 electrons, thus 8
electrons divided by 2 means there are a total of 4 orbital pairs. 
(The answer is (d) 4.)
 
This is further explained at the following site:  
http://van.hep.uiuc.edu/van/qa/section/New_and_Exciting_Physics/Inside_the_Atom/943245785.htm
 
Please let me know if you require a further explanation before you
rate this answer.
 
Thanks, 
chis-ga