which type of bonding involves sharing valence electrons, but the valence electrons aren't confined to a specific region between atoms and instead exist throughout the crystal structure of the material?

metallic bonding

Explanation:

Metallic bonding is bonding where electrons exist as "sea of electrons" meaning they can go anywhere in the structure and are not localized in specific place like covalent bonds are sometimes.

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.