The first shell can hold a maximum of two electrons. how does this explains the valence of hydrogen?

Hydrogen is the first element in the periodic table, and it has one proton. This means that it has one electron. 
This electron is in hydrogen's first shell, and hydrogen exhibits a valency of +1 but also may exhibit -1 just as easily. This is because the shell needs only one electron to be complete, or it may lose the electron and the hydrogen atom becomes a proton.

Heat of fusion= heat/mass  heat of fusion = 12.03kj/36 g= 0.3342 kj/g or 334.2 j/g