N2 - Triple
SiO2 - Double
Explanation:
The best and easiest way to solve this (until you get to organic chemistry and memorize everything) is to create lewis structures for all of the molecules.
First, we must take each molecule and add its constituents atom's valence electrons together. The number of valence electrons correspond to the number of the row.
Example: SiO2
4 + 6*2 = 16
This means that the total number of electrons in our lewis structure will be equal to 16.
Second, we need to put the LEAST electronegative atom in the center. You can find an electronegativity table in your textbook or online. That would be silicon in this case, with a value of only 1.8 to oxygen's 3.5.
Third, we can start by simply putting a single bond between every atom pair to showcase the chemical bond between them.
So that's something like:
OSiO
Fourth, we now must follow the ever-pressing octet rule, meaning that the atom must have 8 electrons to be constitute a valid structure. This has some exceptions, but none of them apply to any of these molecules, so I'm sure you'll learn those soon enough. The one I will mention is hydrogen because it's everywhere --- it's never going to be in the center of a structure, even though it may be the least electronegative. It will also always have only one bond to any given atom.
Each bond counts for two electrons, so in the structure above, there are four. We show things called "lone pairs" by placing sets of two dots on the atoms so the atom can satisfy the octet rule. Of course, they can only have up to eight electrons total.
Using only single bonds, this is the closest we can get to satisfying the octet rule:
:::O:Si:O:::
You can count the electrons and see that there are 20 here, but we can only have 16. This is where we need double and triple bonds to play a role. It isn't too complicated, though.
Fifth, we will see that a double bond is essentially two atoms sharing electrons. This will help to bring down the number of electrons to 16 by doing it to both sides, in this case. This is what our new molecule structure now looks like:
::OSiO::
A double bond counts for 4 electrons from each atom it is connected to, so we can add these up to see that each atom has 8 electrons and that they also happen to all sum up to 16, the number we wanted.
This method will work for all lewis structures so you can use your understanding to do these questions on your own. (Except those structures that do not follow the octet rule.)
