Referring to Periodic Table
Elements naturally react when they don't have full valence shells. Noble Gases have a full valence shell making them very nonreactive. But elements with very little valence electrons (like Hydrogen, Lithium, everything to the far left) tend to react with elements with almost full valence shells (like Chlorine, Oxygen, everything to the far right except for the final column).
Additionally, when electrons are farther from the nucleus i.e. there are more shells and the element is lower on the table, the element is also more reactive because the electrons are attracted by the nucleus less.
TLDR: the lower down the more reactive, the further right the less reactive.
It depends on "first ionization potential". Noble gases have full shells, so it takes a high potential to remove or add an electron. There is a diagonal line, B-C, Si-P, Ge-As, Sb-Te, and Po-At. Elements to the left of the line are metals, meaning they lose an electron easily, elements to the right of the line are non-metals, so it's easy to add an electron, and elements on the line are semiconductors.