` Na_(2)SeO_(4)` and `Na_(2)SO_(4)` are:

Options

A. Isomorphous
B. Polymorphs
C. Alloys
D. Ferromagnetic

Answer

To determine the relationship between `Na2SeO4` (Sodium Selenate) and `Na2SO4` (Sodium Sulphate), we can analyze their structures and properties step by step.

### Step 1: Identify the Compounds
- `Na2SO4` is Sodium Sulphate, which contains sulfur as the central atom.
- `Na2SeO4` is Sodium Selenate, which contains selenium as the central atom.

**Hint:** Look at the central atom in both compounds to understand their similarities.

### Step 2: Compare the Central Atoms
- Both sulfur (S) and selenium (Se) are in the same group (Group 16) of the periodic table. This means they have similar chemical properties and valencies.

**Hint:** Remember that elements in the same group often exhibit similar behaviors and bonding characteristics.

### Step 3: Analyze the Geometry
- Since both compounds have the same valency and similar atomic sizes, the geometry around the central atoms (S and Se) will be similar. This results in both compounds having a similar arrangement of atoms.

**Hint:** Consider how the arrangement of atoms affects the overall structure of the compounds.

### Step 4: Determine Crystal Structure
- Because both compounds have similar geometries and arrangements, they will form similar crystal structures. Compounds that have the same crystal structure but different chemical compositions are referred to as isomorphous.

**Hint:** Isomorphism occurs when different compounds can crystallize in the same structure.

### Step 5: Evaluate Other Options
- **Polymorphous:** This term refers to different forms of the same compound, which is not applicable here.
- **Alloys:** These are mixtures of metals, and since we only have sodium in these compounds, this option is not applicable.
- **Ferromagnetic:** For a substance to be ferromagnetic, it must have unpaired electrons. Both sulfur and selenium in these compounds have a +6 oxidation state, indicating a stable electronic configuration with no unpaired electrons.

**Hint:** Assess the definitions of polymorphous, alloys, and ferromagnetic to eliminate incorrect options.

### Conclusion
Based on the analysis, `Na2SeO4` and `Na2SO4` are **isomorphous** because they have similar crystal structures due to the presence of central atoms from the same group in the periodic table.

**Final Answer:** `Na2SeO4` and `Na2SO4` are isomorphous.