Helium will be inert. Lithium is the most reactive element, followed by chlorine.
Explanation:
The elements tend to resemble the closest noble gases in terms of their electronic configuration of the last layer, that is, having eight electrons in the last layer to be stable. Then, the reactivity of an element measures the tendency to combine with others to generate the mentioned stability.
Within a period, the groups located on the left of the periodic table are more reactive than groups on the right. This is because it is easier to lose the electron of the last layer possessed by the elements located on the left than more than one electron in the rest of the elements. When we reach a certain group the trend is reversed since it will be easier to gain the missing electrons to resemble the nearest noble gas. Therefore, in a period the reactivity of metals increases the further to the left in the period (less electrons to remove). The reactivity of nonmetals increases as the period progresses since it is the case in which it is easier to acquire electrons to acquire stability, and increases to advance in a period since the less electrons they need to acquire, the more reactants they are.
Within a group, as we descend into it, the electrons of the last layer are farther from the nucleus and, therefore, it will be easier to remove them. In the case of nonmetals, it will be more difficult to take electrons. Then the reactivity of the metals increases when advancing in a group (greater tendency to lose electrons) and the reactivity of the nonmetals increases the higher in the group (greater tendency to take electrons).
Finally, you have noble or inert gases. Its inert name is due to the lack of reactivity with other elements. This is due to its electronic configuration, because its outermost layer or valence layer is always complete, without the need to share, transfer or receive electrons forming bonds. That is, its outer layer is so stable that the element tends not to react with others except in very specific cases.
Taking into account everything mentioned above, it is possible to say that helium is a noble gas, so it will be inert. Lithium is the most reactive element, followed by chlorine.
