Which does not form double salt-

Options

A. `Li_(2)SO_(4)`
B. `Na_(2)SO_(4)`
C. `K_(2)SO_(4)`
D. `Rb_(2)SO_(4)`

Explanation

To determine which compound does not form a double salt among the given options (Li2SO4, Na2SO4, K2SO4, Rb2SO4), we can analyze the properties of these compounds:

### Step-by-Step Solution:

1. **Understanding Double Salts**:
- Double salts are formed when two different salts crystallize together in a fixed ratio. They usually consist of a combination of a cation from one salt and an anion from another.

2. **Identifying the Compounds**:
- The compounds given are:
- A: Li2SO4 (Lithium sulfate)
- B: Na2SO4 (Sodium sulfate)
- C: K2SO4 (Potassium sulfate)
- D: Rb2SO4 (Rubidium sulfate)

3. **Analyzing Alkali Metal Sulfates**:
- All these compounds are sulfates of alkali metals. The ability to form double salts depends on the size of the cations involved.

4. **Cation Size and Solubility**:
- As we move down the group in the periodic table, the size of the alkali metal cations increases (Li+ < Na+ < K+ < Rb+).
- Smaller cations like Li+ can stabilize the sulfate ion (SO4^2-) better due to their charge density, but they also have a higher lattice energy which makes it difficult for them to accommodate another salt in a double salt formation.

5. **Conclusion on Li2SO4**:
- Lithium sulfate (Li2SO4) is less likely to form double salts compared to the other sulfates (Na2SO4, K2SO4, Rb2SO4) because of the small size of the lithium ion, which leads to a strong lattice energy that does not favor the formation of double salts.

6. **Final Answer**:
- Therefore, the compound that does not form a double salt is **Li2SO4** (Option A).