Aexists in the same physical state as that of the reaction it catalyzes.

A Homogeneous Catalyst exists in the same physical state as that of the reaction it catalyzes.

Explanation:
                     Catalysts are classifies as follow,
                                                      1. Homogeneous Catalyst
                                                      2. Heterogeneous Catalyst

Homogeneous Catalyst are those which are in same phase as that of reactants are.

Example:

         Acetic Acid + Ethanol + H₂SO₄  →  Ethyl Acetate + H₂O + H₂SO₄

In this reaction catalyst and reactants all are in same phase i.e. Liquids.

Heterogeneous Catalyst are those which are in different phase as that of reactants are.

Example:

                                 Ethene + H₂ + Ni  →  Ethane + Ni

In this reaction ethene and hydrogen are gases while Nickle catalyst is in solid state. 

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.