A non-metallic element X is a good conductor of electricity in one form and is a bad conductor of electricity in another form. Comment on the thermal conductivity of both forms. Identify the element and mention the above two forms of X.

Explanation

### Step-by-Step Solution:

1. **Identify the Element**: The non-metallic element X that can conduct electricity in one form and not in another is carbon. Carbon exists in different forms known as allotropes.

**Hint**: Think about non-metals that have multiple forms with different properties.

2. **Recognize the Allotropes**: The two allotropes of carbon we are discussing are graphite and diamond.

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