All matter is made up of atoms. Which sentence correctly describes atoms?

Options

A. All substances are made of the same atoms.
B. An atom is the smallest particle of a nucleus.
C. An atom is the smallest particle of an element.
D. An atom is a substance that has been cut in half.

Explanation

To determine which sentence correctly describes atoms, let's analyze the given options step by step:

1. **Understanding Atoms**: Atoms are the basic building blocks of matter. They are the smallest units of an element that retain the properties of that element.

2. **Evaluating Each Sentence**:
- **Sentence 1**: "All substances are made up of the same atoms."
- This statement is incorrect because different substances are made up of different types of atoms. For example, water (H2O) is made of hydrogen and oxygen atoms, while table salt (NaCl) is made of sodium and chlorine atoms.

- **Sentence 2**: "An atom is the smallest particle of nucleus."
- This statement is also incorrect. An atom is not a particle of the nucleus; rather, it consists of a nucleus (made of protons and neutrons) surrounded by electrons. The nucleus is part of the atom, not the smallest particle of it.

- **Sentence 3**: "An atom is the smallest particle of an element."
- This statement is correct. An atom is indeed the smallest unit of an element that still retains the properties of that element. For example, a single carbon atom is the smallest unit of carbon.

- **Sentence 4**: "An atom is a substance that has been cut in half."
- This statement is incorrect. Atoms cannot be simply cut in half to create smaller atoms; they are indivisible in terms of maintaining their elemental identity. Cutting an atom would result in subatomic particles, which do not represent the original atom.

3. **Conclusion**: After evaluating all four sentences, the only correct description of an atom is found in Sentence 3: "An atom is the smallest particle of an element."

### Final Answer:
The sentence that correctly describes atoms is: **"An atom is the smallest particle of an element."**