"Electrophilic atoms are the electron-deficient atoms in molecules or ions.

The common types of electron-deficient atoms are

a. Atoms that lack an octet of valence electrons in one or more resonance forms

b.  Formally neutral atoms bearing partial positive charge but no lone pairs

Lone pairs refer to the pairs of valence electrons that are not shared with other atoms.

Nucleophilic atoms are the electron-rich atoms in molecules or ions.  

The most common types of electron-rich atoms are

a.  Atoms bearing one or more lone pairs

b. Formally neutral atoms bearing partial negative charges

The acidity of the conjugate acid expresses the basicity of chemical species.

Lewis acids are electron-pair acceptor while Lewis bases Lewis bases are any bases which are electron pair donors. "