How many groups and periods are there in the Modern Periodic Table? How do the atomic size and metallic character of elements vary as we move:
(a) down a group and (b) from left to right in a period

Explanation

### Step-by-Step Solution

1. **Identify the Number of Groups and Periods in the Modern Periodic Table**:
- The Modern Periodic Table is divided into **18 groups** (vertical columns) and **7 periods** (horizontal rows).

2. **Understand the Variation of Atomic Size and Metallic Character**:
- **(a) Down a Group**:
- As we move down a group in the periodic table, the number of energy levels (shells) increases.
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