What is common in graphite and diamond:

Explanation

**Step-by-Step Solution:**

1. **Identify the Elements**: Both graphite and diamond are forms of carbon. They are composed entirely of carbon atoms.

2. **Define Allotropes**: Graphite and diamond are classified as allotropes of carbon. Allotropes are different structural forms of the same element, where the atoms are bonded together in different ways.

3. **Common Atomic Weight**: Since both substances are made of carbon, they share the same atomic weight, which is approximately 12 amu (atomic mass units).

4. **Different Physical Properties**: While both are allotropes of carbon, they exhibit different physical properties:
- **Density**: Graphite is less dense than diamond.
- **Crystal Structure**: Graphite has a layered structure, while diamond has a tetrahedral structure.
- **Conductivity**: Graphite conducts electricity due to the mobility of electrons, whereas diamond is an electrical insulator.
- **Melting and Boiling Points**: They also differ in their melting and boiling points.

5. **Conclusion**: The commonality between graphite and diamond is that they are both allotropes of carbon and share the same atomic weight, but they differ in their physical properties.

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