* Highly electronegative: These atoms have a strong attraction for electrons, making them eager to gain electrons and form bonds. Examples include fluorine (F) and oxygen (O).

* Have a single unpaired electron in their outer shell: This makes them highly likely to react with other atoms to complete their outer shell and achieve stability. Examples include hydrogen (H) and the halogens (F, Cl, Br, I).

* Have a low ionization energy: These atoms readily lose electrons and become cations. Examples include alkali metals (Li, Na, K, Rb, Cs).

Here's a breakdown of the most reactive elements and their characteristics:

* Group 1 (Alkali Metals): These elements have only one valence electron, making them highly reactive. They readily lose this electron to form positive ions (cations) and readily react with water and other elements.

* Group 2 (Alkaline Earth Metals): These elements have two valence electrons and are less reactive than alkali metals, but still highly reactive. They form positive ions and react with water, though typically not as vigorously as alkali metals.

* Group 17 (Halogens): These elements have seven valence electrons, needing one more to complete their outer shell. They are highly reactive and readily gain an electron to form negative ions (anions).

* Group 18 (Noble Gases): These elements have a full outer shell of electrons, making them very stable and unreactive. They are rarely found in compounds.

In general, reactivity increases from right to left and from top to bottom on the periodic table. This is because elements on the left have fewer valence electrons and more easily lose them, while elements at the top have fewer electron shells, making it easier for them to gain or lose electrons.

Keep in mind: Reactivity is a complex concept that can be influenced by various factors. While the above provides a general overview, specific reactions and the reactivity of individual elements can vary widely.