Alkali metals are strong reducing agents because they can release their valence electrons very easily. When consider about first ionization enthalpy in a period, alkali metals has the lowest values.
All alkali metals easily remove its last shell electron to make +1 ion.
This strong reducing behaviour of alkali metals is also indicated by their very large negative values of reduction potentials in electrochemistry series. Lithium has the highest negative reduction potential value in the electrochemistry series.
Alkali earth metals exist as compounds because they are very reactive. They combine with other elements to form compounds. Alkali earth metals do not exist as free elements.
All of alkali metals are better reducing agents than hydrogen. Therefore alkali metals can liberate H2 by reacting with compounds which include acidic hydrogen atoms such as HCl, H2SO4, alcohols, acetylene. Also alkali metals can liberate H2 from water by forming strong hydroxides.
Reducing ability increases along the group. It means that, reducing ability of potassium is higher than sodium.
See the electro-chemistry series and look its top elements. You can see in the top, there are lithium, potassium and sodium as alkali metals.
Atomic radius of alkali metals increases when going down the group. Therefore attraction of only electron in the last shell towards the nucleus decreses. Then ability of releasing that electron increses when going down the group. That increases the reducing ability.
Electrons in the outer shells of group 2 metals are strongly attracted towards the nucleus compared to group 1 metals. Therefore, releasing outer shells electrons is much more difficult for group 2 metals. Therefore, reducing power of group 2 metals is much weaker than group 1 metals.
Related Topics to Reducing ability of alkali metals