Which element has:
a] two shells, each completely filled with electrons?
b] the electron configuration 2, 8, 2? 
c] three shells, with four electrons in the valence shell? 
d] two shells, with three electrons in its valence shell? 
e] twice as many electrons in its second shell as in its first shell?

Explanation

Using the fact that the period number denotes the number of shells while the number of electrons in the valence shell is given by the group number, we can find the required elements as follows:
a) Last noble gas element in the `2^(nd)` period - Neon (Ne).
b) `2^(nd)` group element in the 3 period - Magnesium (Mg).
c] `3^(rd)` period, `14^(th)`  group element - Silicon (Si).
d] `2^(nd)` period, `13^(th)` group element - Boron (B).
(e ) First shell can contain a maximum of 2 electrons. So, the required element should have an electronic configuration - 2,4. This would be the `2^(nd)` period, `14^(th)`  group element - Carbon (C).