Hydrogen is included in the reactivity series because it exhibits both metallic and non-metallic properties and can therefore participate in reactions with both metals and non-metals.

Here's why:

* Metallic properties: Hydrogen can lose an electron to form a positive ion (H+), similar to metals. This allows it to react with non-metals to form ionic compounds like HCl (hydrochloric acid).

* Non-metallic properties: Hydrogen can also gain an electron to form a negative ion (H-), behaving like a non-metal. This allows it to react with metals to form ionic compounds like NaH (sodium hydride).

Reactivity in the series:

In the reactivity series, hydrogen acts as a reference point. Metals above hydrogen in the series are more reactive than hydrogen and can displace it from acids. For example, zinc (Zn) is above hydrogen and will react with hydrochloric acid (HCl) to produce hydrogen gas.

Metals below hydrogen are less reactive than hydrogen and cannot displace it from acids. For example, copper (Cu) is below hydrogen and will not react with HCl.

In summary, hydrogen's unique ability to act as both a metal and a non-metal makes it an essential element in the reactivity series, serving as a benchmark for comparing the reactivity of different elements.