Question

Of the following elements, which is the LEAST reactive? A. C B. H C. Li D. He

Answer 1

The least reactive elements are the noble gases since they have a full valence shell and thus require a lot of energy in order to form bonds.
For metals, reactivity increases as you move down a group and as you move leftward within a period.
The trend within a period is due to the increase in ionization energy caused by the decrease in electrons needed to be lost in order for an element achieve a full valence electron shell. The trend within a group is due to the valence shell being further away from the nucleus, resulting in less attractive forces and thus less energy needed for reaction.

For nonmetals, the trends are reversed. Reactivity as you move up a group and as you move rightward within a period.
As you move up a group, the element increases in electromagnetism and thus increasing its reactivity. As you move across a period, there is less electrons needed to be received in order for the element to achieve a full valence shell
hope this helps hun

Answer 2

Definitely D: Helium

Step-by-step explanation:

The Electronegativity or pauling is its reactivity, aka the more likely it is to steal an electron. so if it doesnt have ANY reactivity like helium, there is no chance it will steal one. therefore its helium.