A low electronegativity for metals and a high electronegativity for nonmetals.
An element must have similar chemical properties and valency as the element it is replacing in order to replace it in a compound. This ensures that the chemical reaction remains balanced and the new compound formed is stable.
CARBON
Electrons are the subatomic particles that make an element reactive. Elements react with one another by gaining, losing, or sharing electrons to attain a more stable electron configuration.
Boron is a reactive element, but it is not as reactive as some other elements like alkali metals. It can react with oxygen, water, and acids under certain conditions. Overall, it is considered a moderately reactive element.
In a single replacement reaction, the element that is most active is the one that is higher up on the reactivity series than the element it is trying to replace. The more active element will displace the less active element in the reaction.
two or more different elements
The key to "happiness" for an atom is a full outer electron shell. (The outer electron shell is called the valence shell.) There are two conditions that cause a shell not to be full. Either it has only an electron or two (or three) in the outer electron shell or it's short an electron or two in that outer shell. The direct answer to the question is that if an element is chemically active, its outer electron shell is incomplete or is not full.
When one element replaces another element in a compound during a chemical reaction
It is nitrogen and it makes more than 78% of the air.
This question is meaningless without more information. What is meant by 'active'? If you mean reactive, then they are similar, both being alkali metals with 1 valence electron.
Nitrogen
helium