Question

Alkali metals are powerful reducing agents because:
A: these are metals
B: these are monovalent
С: their ionic radii are large
D: their ionization potential is low

Answer 1

‌ Initially one should know what are reducing agents and its basic phenomenon. Then possible reasons for alkali metals are why they act as a reducing agent. The next thing is how the property of the reducing agent is checked (through its affecting factors) and arranged in ascending or descending order. Moreover, which factors deal with the reducing character at first priority.

Complete step by step answer:
Reduction means losing something. Thus, we can guess reducing agent as a substance which loses something. In chemistry losing and gaining is generally termed in electrons. Thus, reducing agents can be termed as the substances which get reduced when oxidised. In simple words we can say they lose electrons easily.
Reducing agents have the atoms which have low electronegativity and a good ability of an atom or a molecule to attract the bonding electrons and the species having very small ionization energies.
Alkali metals are good reducing agents because alkali metals have one valence electron which they lose to attain stability. Alkali metals have the lowest ionization energy in each period and this is because they are having the largest atomic size in their respective period. Low ionization potential means electrons can be easily removed from the atom. This is so because the outermost electron is far from the nucleus. Since electrons are far from the nucleus therefore it is loosely bound and hence removal of these types of electrons requires less energy. Since it has low IP value therefore it can easily get oxidised and hence works as the best reducing agent.
.so the correct answer is option ‘D’.

Note: The reducing property of a reducing agent can be judged by many factors but the first preference must be given to ionisation energy. Also, size can be one of the factors but the option must contain atomic size in place of ionic size.