Which of the following statements are incorrect ?
I. Dichromate ion is a divalent and positive ion.
II. Barium ion is trivalent and positive.
III. Solid sulphur is a polyatomic molecule.
IV. Ammonium ion is divalent and positive.

Options

A. `I` and `II` only
B. `III` and `IV` only
C. `I, II` and `III` only
D. `I, II` and `IV` only

Explanation

To determine which statements are incorrect, let's analyze each statement one by one.

### Step 1: Analyze Statement I
**Statement I:** Dichromate ion is a divalent and positive ion.

- The dichromate ion is represented as \( \text{Cr}_2\text{O}_7^{2-} \).
- It has a charge of -2, which means it is a negative ion (anion).
- Therefore, this statement is **incorrect**.

### Step 2: Analyze Statement II
**Statement II:** Barium ion is trivalent and positive.

- The barium ion is represented as \( \text{Ba}^{2+} \).
- It has a charge of +2, which means it is a divalent positive ion, not trivalent.
- Therefore, this statement is **incorrect**.

### Step 3: Analyze Statement III
**Statement III:** Solid sulphur is a polyatomic molecule.

- Solid sulfur exists in the form of \( \text{S}_8 \), which is a molecule consisting of 8 sulfur atoms.
- Since it consists of multiple atoms, it is indeed a polyatomic molecule.
- Therefore, this statement is **correct**.

### Step 4: Analyze Statement IV
**Statement IV:** Ammonium ion is divalent and positive.

- The ammonium ion is represented as \( \text{NH}_4^+ \).
- It has a charge of +1, which means it is a monovalent positive ion, not divalent.
- Therefore, this statement is **incorrect**.

### Conclusion
The incorrect statements are:
- Statement I: Dichromate ion is a divalent and positive ion. (Incorrect)
- Statement II: Barium ion is trivalent and positive. (Incorrect)
- Statement IV: Ammonium ion is divalent and positive. (Incorrect)

### Final Answer
The incorrect statements are I, II, and IV.

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