Explain
a. The basic of sumilarities and differences between metallic and ionic crystals.
b. Ionic solids are hard and brittle.

Explanation

### Step-by-Step Solution:

#### Part a: Similarities and Differences between Metallic and Ionic Crystals

1. **Definition of Crystals**:
- **Ionic Crystals**: These are formed by the electrostatic attraction between positively and negatively charged ions.
- **Metallic Crystals**: These consist of positively charged metal ions surrounded by a sea of delocalized electrons.

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