Lithium( atomic number -3), Sodium( atomic number -11)and Potassium (atomic number -19) are in the same group in the periodic table with valence electron 1, but potassium seems to be more reactive than sodium and lithium. Why?

Potassium has an atomic radii that is greater than that of lithium that is why potassium is more reactive than lithium

Explanation:

This is because in lithium the valence electrons are closer to the nucleus therefore the attractive forces between nucleus and valence electrons are very strong.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.