Understanding Lewis Acids & Bases: HCl, H2SO4, H3PO4, CO2, HCN

[utkarsh009]

hi all! I am not able to understand what's a Lewis acid and a Lewis base. tell me if these are Lewis acids or Lewis bases and tell me the reason. HCl, H2SO4, H3PO4, CO2, HCN.
my approach: first 3 I'm not able to decide. 4th one I thought to be Lewis base as 2lone pairs of electrons left in each oxygen atom. but I found somewhere written that it's an acid. 5th one is base as 1pair of electrons left in nitrogen atom. please help. also tell me the criteria to check if a compound is electron deficient or has excess electrons. also how to find the central atom.

 

[Nino MMP]

A Lewis acid is an electron-pair acceptor, while a Lewis base is an electron-pair donor. HCl: Lewis Acid - HCl is an electron deficient molecule and the central atom is hydrogen. The molecule has an empty orbital, which it can use to form a covalent bond with an electron pair from a Lewis base. H2SO4: Lewis Acid - H2SO4 is an electron deficient molecule and the central atom is sulfur. The molecule has an empty orbital, which it can use to form a covalent bond with an electron pair from a Lewis base. H3PO4: Lewis Acid - H3PO4 is an electron deficient molecule and the central atom is phosphorus. The molecule has an empty orbital, which it can use to form a covalent bond with an electron pair from a Lewis base. CO2: Lewis Acid - CO2 is an electron deficient molecule and the central atom is carbon. The molecule has an empty orbital, which it can use to form a covalent bond with an electron pair from a Lewis base. HCN: Lewis Base - HCN is an electron rich molecule and the central atom is nitrogen. The molecule has an extra pair of electrons which it can donate to form a covalent bond with an electron deficient species (Lewis acid). The criteria for determining if a compound is electron deficient or electron rich is to look at its Lewis structure and count the number of valence electrons on the central atom. If there are more electrons than the octet rule predicts, then it is electron rich; if there are fewer electrons than the octet rule predicts, then it is electron deficient. To find the central atom, look for the atom that is connected to the most other atoms in the molecule.