Two or more compounds having the same molecular formula but different arrangement of atoms are called isomers and the phenomenon is called isomerism.

Because of different arrangement of atoms, isomers differ in one or more physical or chemical properties. Isomers can be broadly classified into two major categories:

(A) Structural isomers (B) Stereoisomers.

These are further sub-divided as shown below:

In coordination compounds, the hydrate isomers differ

Both 1 and 2

The correct answer is (3) Both 1 and 2.

Hydrate isomers are isomers of coordination compounds that differ in the number of water molecules of hydration and the number of water molecules present as ligands. The water molecules of hydration are not part of the coordination sphere, but they can be exchanged with other water molecules or with other ligands. This results in isomers that have the same chemical formula but different physical properties.

The coordination number of the metal atom is the number of ligands that are directly bonded to the metal atom. This number does not change in hydrate isomers, so option (4) is incorrect.

Here are some examples of hydrate isomers:

\([Cr(H_2O)_6]Cl_3\) and \([Cr(H_2O)_5Cl]Cl_2H_2O\)

\([Co(H_2O)_6]Cl_2\) and \([Co(H_2O)_4Cl_2]Cl.2H_2O\)