Which of the following is the strongest Lewis base?
A) ${ CH }_{ 3 }^{ - }$
B) ${ Ag }^{ + }$
C) ${ NH }_{ 3 }$
D) ${ H }_{ 2 }$

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Hint: Lewis base: It can be defined as a compound that gives an electron pair to a compound that has an insufficiency in electrons to complete its octet. Example: an atom, ion, or molecule with a lone-pair of electrons can thus be able to be a Lewis base.

Complete answer:
A) ${ CH }_{ 3 }^{ - }$: In this, there is an additional electron density present as it is negatively charged, so they can donate their electrons more easily. Hence, acts as a Lewis base.
B) ${ Ag }^{ + }$: It acts as a Lewis acid as it is positively charged, so it will accept the electrons not donate to complete its octet.
C) ${ NH }_{ 3 }$: Since there are lone pairs of electrons present on nitrogen and nitrogen being an electronegative element, so it will donate its lone pair of electrons to anyone. Hence, it acts as a Lewis base.
D) ${ H }_{ 2 }$: It is a Lewis acid because they protonate by accepting an electron pair from non-bonding electrons such as ${ NH }_{ 3 }$, ${ OH }^{ - }$ etc.
Hence, we can say that ${ CH }_{ 3 }^{ - }$ is the stronger Lewis base as compared to ${ NH }_{ 3 }$ because its electronegativity is less as compared to that of nitrogen, so the electron density will have weakly interacted and the electrons can be donated more easily.

Therefore, the correct option is A.

Additional Information Limitations of Lewis theory:
i) This theory can not clarify the strength of acids and bases.
ii) Normally, neutralization reactions are instantaneous (very fast) but Lewis acid-base reaction goes gradually.
iii) All acid-base reactions do not include co-ordinate bond formation.
iv) Hydrogen ion as a catalyst (in some reactions) can not be clarified by this theory.

Note: The possibility to make a mistake is that you may choose option C. As nitrogen has lone pair of electrons, so you may think it will be a strong Lewis base but ${ CH }_{ 3 }^{ - }$ is stronger than ammonia.