Final answer:
Hydrogen is not listed among other metals in the activity series because it is a unique nonmetal with low electronegativity and does not fit the characteristics of an alkali metal. It is included in the activity series as a point of comparison for metals reacting with acids.
Step-by-step explanation:
Hydrogen cannot be listed amongst the other metals in the activity series because it is not an alkali metal despite its placement in group 1 of the periodic table. Hydrogen is a unique element with chemical properties that resemble those of nonmetals, and has a relatively low electronegativity. It forms various types of compounds such as ionic and covalent hydrides, depending on the electronegativity of the other element involved.
While hydrogen does appear in the activity series, it is included for comparison as a reference point to determine if metals can displace hydrogen from acids to generate hydrogen gas. Metals that are more active than hydrogen will react with acids to produce hydrogen gas and metal salts, whereas metals less active than hydrogen, such as copper, silver, mercury, and gold, do not.