Choose the correct statement(s):

Options

A. `BeCO_(3)` is kept in the atmosphere of `CO_(2)` since, it is least thermally stable
B. Be dissolves in alkali forming `[Be(OH)_(4)]^(2-)`
C. `BeF_(4)` forms complex ion with NaF in which Be goes with cation
D. `BeF_(4)` forms complex ion with NaF in which Be goes with anion

Explanation

To analyze the statements regarding beryllium (Be) and its compounds, let's break down the information provided in the video transcript and evaluate the correctness of the statements step by step.

### Step 1: Understanding Beryllium's Behavior
Beryllium is known for its unique properties among the alkaline earth metals. It has a strong tendency to form covalent compounds due to its small size and high charge density.

**Hint:** Consider the properties of beryllium compared to other alkaline earth metals.

### Step 2: Beryllium in CO2 Atmosphere
The transcript mentions that beryllium (Be) is kept in an atmosphere of CO2 because it is least thermally stable. This statement can be misleading. Beryllium does not react readily with CO2, and it is actually more stable than other alkaline earth metals in certain conditions.

**Hint:** Think about the reactivity of beryllium with carbon dioxide and its thermal stability.

### Step 3: Beryllium and Alkali
The statement about beryllium dissolving in alkali to form a negative ion is incorrect. Beryllium does not typically form negative ions in alkali solutions. Instead, it can form beryllates when reacting with alkali metals.

**Hint:** Review the common reactions of beryllium with alkali and the types of ions formed.

### Step 4: Complex Ion Formation
The transcript mentions that BeF2 can react with NaF to form Na2BeF4. This is a correct statement. Beryllium fluoride can indeed form complex ions with fluoride ions.

**Hint:** Look into the formation of complex ions and how they are represented in chemical equations.

### Step 5: Beryllium and Boron Compounds
The statement regarding boron (B) forming complexes with NaF is also correct. Beryllium and boron can form different types of complexes, but the details in the transcript may be confusing.

**Hint:** Differentiate between the types of complexes formed by beryllium and boron.

### Conclusion
After analyzing the statements, we can conclude that:
1. The statement about beryllium being least thermally stable is incorrect.
2. The statement about beryllium dissolving in alkali to form a negative ion is incorrect.
3. The statement regarding the formation of Na2BeF4 is correct.
4. The statement about boron forming complexes is correct, but the specifics may need clarification.

### Final Answer
The incorrect statements are:
- Beryllium is least thermally stable.
- Beryllium dissolves in alkali to form a negative ion.