For lewis acids and bases, the base is defined as an electron pair donor / acceptor (circle the correct answer).

electron donor.

Explanation:

According to Arrhenius concept, a base is defined as a substance which donates hydroxide ions when dissolved in water and an acid is defined as a substance which donates hydronium ions in water.

According to the Bronsted Lowry conjugate acid-base theory, an acid is defined as a substance which donates protons and a base is defined as a substance which accepts protons.

According to the Lewis concept, an acid is defined as a substance that accepts electron pairs and base is defined as a substance which donates electron pairs.

Here acts as lewis base and act as lewis acid.

Thus lewis base is defined as an electron pair donor.

the answer is:  

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