Question

Which of the following is a lewis base?
A.$BC{L}_3$
B.$C_2{H}_2$
C.$HCL$
D.$A{g}^{+}$

Answer 1

Hint:Bases are defined in many terms. Substances which donate hydroxide ions are called bases, substances which donate electrons are called bases. The strong bases are alkali metals and alkaline earth metals.

Complete step by step answer:
Now, according to Lewis, the concept of acids and bases were stated as bases were the substances which can donate electrons or a pair of them, and acid were the substances which can accept an electron or a pair of electrons.
To identify a substance as base or acid, we need to see which substance has more electron density. The substance which has more electron density will be a good base as it can donate electrons easily. The substance which has less electron density will be a good acid as they can accept electrons easily.
Now in the above question, we are provided with four options that are boron trichloride, hydrochloric acid, ethene and silver ion.
In boron, tri chloride boron is attached with three chlorine atoms, which are highly electronegative and therefore it is not a lewis base as it will not donate an electron.
In ethene molecules, the carbon atoms have a double bond between them. And therefore it has a Pi-bond which is electron-rich, the carbon atom in the influence of any electronegative atom will break the Pi-bond and donate an electron pair, hence it is a lewis base.
Hydrochloric acid is a strong acid and hence therefore it cannot be a Lewis base.
While in silver ions it has already donated an electron and gained a positive charge, now it will be harder to extract one more electron from it and hence it is not a Lewis base.
Therefore, the Lewis base in the above question is ethene.
So correct answer is (b).

Note:
Though correct Lewis concept. Also had some limitations. Lewis concepts include all the coordination compounds which may not be true for all of them. Also, the relative strength of acid and bases have not been described by him.