To solve the question, "If the magnetic moment of the atom of a substance is zero, the substance is called," we need to analyze the properties of different types of magnetic materials.
### Step-by-Step Solution:
1. **Understanding Magnetic Properties**:
- The magnetic moment of an atom refers to its ability to generate a magnetic field. If the magnetic moment is zero, it indicates that the atom does not exhibit any magnetic behavior.
2. **Types of Magnetic Materials**:
- There are four main types of magnetic materials: diamagnetic, paramagnetic, antiferromagnetic, and ferromagnetic. We need to determine which of these has a magnetic moment of zero.
3. **Analyzing Diamagnetic Materials**:
- Diamagnetic materials have no unpaired electrons, which means their magnetic moments cancel out. When placed in an external magnetic field, they do not get polarized in any direction. Therefore, their net magnetic moment is zero.
4. **Analyzing Paramagnetic Materials**:
- Paramagnetic materials have unpaired electrons, which means they have a non-zero magnetic moment. When placed in an external magnetic field, their magnetic domains align with the field, indicating a non-zero magnetic moment.
5. **Analyzing Antiferromagnetic Materials**:
- Antiferromagnetic materials also have unpaired electrons, but the magnetic moments of adjacent atoms align in opposite directions, leading to a net magnetic moment of zero in bulk materials. However, the individual atoms still have non-zero magnetic moments.
6. **Analyzing Ferromagnetic Materials**:
- Ferromagnetic materials have unpaired electrons and their magnetic moments align parallel to each other, resulting in a strong net magnetic moment. Thus, they do not have a zero magnetic moment.
7. **Conclusion**:
- Since only diamagnetic materials have a magnetic moment of zero and do not exhibit any polarization in an external magnetic field, we conclude that the substance in question is a **diamagnetic material**.
### Final Answer:
The substance is called **diamagnetic**.
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