Why do elements in the same period have different physical and chemical properties? What about elements in the same group?

Answer

### Step-by-Step Solution

1. **Understanding Groups and Periods:**
- Elements in the periodic table are organized into groups (columns) and periods (rows).
- Groups contain elements with similar properties, while periods contain elements with varying properties.

2. **Elements in the Same Group:**
- Elements in the same group have the same number of valence electrons.
- For example, Group 1 elements (Lithium, Sodium, Potassium, etc.) all have one electron in their outermost shell.
- This similarity in valence electrons leads to similar physical and chemical properties. For instance, they all tend to lose one electron easily, making them highly reactive metals.

3. **Example of Group 1:**
- Let's look at Sodium (Na) and Potassium (K):
- Sodium: Electronic configuration is 1s² 2s² 2p⁶ 3s¹.
- Potassium: Electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
- Both have one valence electron, hence they exhibit similar properties like reactivity and metallic character.

4. **Elements in the Same Period:**
- Elements in the same period have the same number of electron shells but different numbers of valence electrons.
- For example, in Period 4, we can compare Potassium (K) and Calcium (Ca):
- Potassium: Electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹.
- Calcium: Electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
- Here, both elements have their outermost shell as the 4th shell, but Potassium has one valence electron while Calcium has two.

5. **Different Properties in the Same Period:**
- The difference in the number of valence electrons results in different chemical behaviors.
- For instance, Potassium is more reactive than Calcium due to its single valence electron, while Calcium can form different compounds due to having two valence electrons.

6. **Conclusion:**
- In summary, elements in the same group share similar properties due to having the same number of valence electrons, while elements in the same period differ in their properties because they have different numbers of valence electrons despite having the same number of electron shells.