Which of the following phrases would be incorrect to use ?

Options

A. A molecule of an element
B. An atom of an element
C. A molecule of a compound
D. An atom of a compound

Answer

To determine which phrase is incorrect, we need to analyze each option carefully. Let's break it down step by step:

### Step 1: Analyze the first option - "A molecule of an element"
- **Explanation**: A molecule of an element refers to two or more atoms of the same element bonded together. For example, hydrogen exists as H₂, which is a molecule made up of two hydrogen atoms.
- **Conclusion**: This phrase is correct.

### Step 2: Analyze the second option - "An atom of an element"
- **Explanation**: An atom of an element refers to a single atom of that element. For instance, a single hydrogen atom is represented as H. This is a valid phrase.
- **Conclusion**: This phrase is correct.

### Step 3: Analyze the third option - "A molecule of a compound"
- **Explanation**: A molecule of a compound refers to a specific number of atoms of different elements bonded together. For example, NaCl (sodium chloride) is a compound, and its molecule consists of one sodium atom and one chlorine atom.
- **Conclusion**: This phrase is correct.

### Step 4: Analyze the fourth option - "An atom of a compound"
- **Explanation**: This phrase suggests that a compound can exist as a single atom, which is incorrect. Compounds are made up of molecules, and molecules consist of atoms. Therefore, you cannot refer to a compound as an atom.
- **Conclusion**: This phrase is incorrect.

### Final Answer:
The incorrect phrase is "An atom of a compound."

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