Diamonds are mostly composed of what element?

Diamonds are composed almost entirely of the element carbon, often with some other impurities in them, such as nitrogen (i.e., pure diamonds are entirely carbon). The element carbon comes in three different natural forms, or allotropes: diamond, graphite, and amorphous.

Explanation:

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.