Answer:
Lithium is the most reactive out of them.
Step-by-step explanation:
For this question, we will be looking at the valence electrons each one has. Valence electrons are the electrons that are on the outermost electron shell (the outer circle around each of them).
Lithium has 2 electron shells: the innermost has 2 electrons (this circle can only hold up to 2 electrons), and the outermost circle has 1 electron (this circle can hold up to 8 electrons).
Helium has only 1 electron shell and within it, it has 2 electrons (remember this is the maximum amount this circle can take).
Argon has 3 electron shells: the innermost has 2 electrons (the maximum this circle can have), the middle one has 8 electrons (the maximum this circle can have), and the outermost circle has 8 electrons around it (this circle also holds a maximum of 8 electrons).
Now let's look at the electrons present only on the outer shell of each of them (these are called valence electrons).
Lithium has only 1 electron on its outermost shell but this shell can have up to 8 electrons.
Helium has 2 electrons, the maximum amount that circle can have.
Argon has 8 electrons, the maximum that circle can have.
-Every atom has the goal of wanting to be stable, by completing the maximum amount on their outermost shell by either gaining or losing electrons.
Since Helium and Argon already have the maximum amount of electrons their outermost circle can have, they are already stable and neither want to gain or lose electrons.
Lithium is our only option that has an incomplete outermost electron shell with only 1 out of the 8 electrons it can have.
It is easier for Lithium to give away that 1 electron it has and then have a complete inner-circle after giving out that 1 electron (it would then look like Helium), than to gain 7 electrons and complete the circle.
This is why Lithium is very reactive, because it is constantly looking to find someone to give away their electron to and become stable. Lithium also has a charge of (+1), which means that it gives away one of its electrons.