Describe the trend of atomic size along a period
and a group in the periodic table.

Explanation

To describe the trend of atomic size along a period and a group in the periodic table, we can break it down into two main parts: the trend along a period and the trend along a group.

### Step 1: Atomic Size Trend Along a Period
1. **Definition of a Period**: A period is a horizontal row in the periodic table.
2. **Atomic Number Increase**: As we move from left to right across a period, the atomic number of the elements increases. For example, moving from lithium (Li) to fluorine (F).
3. **Nuclear Charge Increase**: With the increase in atomic number, the nuclear charge (the positive charge of the nucleus) also increases.
4. **Number of Electron Shells**: The number of electron shells remains the same across a period. For instance, both lithium and fluorine have electrons in the same principal energy level.
5. **Effect on Atomic Size**: Due to the increased nuclear charge, the electrons are pulled closer to the nucleus, resulting in a decrease in atomic size. Therefore, atomic size decreases from left to right across a period.
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