An acid is a substance (molecule or ion) which can accept a pair of electrons.
A base is a substance (molecule or ion) which can donate a pair of electrons.
Species that can be Lewis bases
(i) Neutral species (substances) having at least one lone pair of electrons
Ammonia, amines, alcohols
(ii)Negatively charged species (anions)
chloride ion, cyanide ion, hydroxide ion etc.
Species that can be Lewis acids
(i)Molecules in which the central atom has incomplete octet
BF3, AlCl3, FeCl3
(ii) Simple cations.
Some cations like Na+, K+, Ca²+ have very little tendency ot accept electrons.
But cations like H+, Ag+ etc. hve a greater electron accepting tendency and therefore act as Lewis acids.
(iii) Molecules in which the central atom has empty d-orbitals.
Molecules such as SiF4, SnCl4, and PF5 have their central atoms with empy d- orbitals. They can expand their outer octet by accepting electrons from other substances.
(iv) Molecules in which atoms dissimilar electronegativities are joined by multiple bonds.
In these molecules, electrons one atoms are pulled towards it by the other atom. Due to this, under the influence of the attacking Lewis base, the pair of electrons in the π-bond or one of the π-bonds(if more than one multiple bond is there like CO2) will be shifted toward the oxygen atom and the pair donated by the Lewis base is accepted.
All Bronsted bases are also Lewis bases.
But all Bronsted acids are not Lewis acids. For instance, HCL, H2SO4 can give a proton but cannot accept a pair of electrons.