Which of the following elements differs from the rest in several physical and chemical properties?
(a)Li
(b)Na
(c)Rb
(d)Cs

Options

A. Li
B. Na
C. Rb
D. Cs.

Explanation

To determine which element differs from the rest in several physical and chemical properties among the alkali metals (Li, Na, Rb, Cs), we can analyze the properties of each element:

### Step-by-Step Solution:

1. **Identify the Elements**: The elements in question are Lithium (Li), Sodium (Na), Rubidium (Rb), and Cesium (Cs). These are all alkali metals found in Group 1 of the periodic table.

2. **Understand Alkali Metals**: Alkali metals are known for their similar properties, such as being soft, having low melting and boiling points, and being highly reactive, especially with water.

3. **Examine Lithium**: Lithium is the first element in the alkali metal group. It is known to exhibit some unique properties that set it apart from the other alkali metals.

4. **Compare Physical Properties**:
- **Melting and Boiling Points**: Lithium has a higher melting and boiling point compared to Na, Rb, and Cs.
- **Hardness**: Lithium is harder than the other alkali metals, which are generally soft.

5. **Examine Chemical Properties**:
- **Decomposition of Nitrates**: Lithium nitrate decomposes to form lithium oxide when heated, while the nitrates of other alkali metals decompose to form nitrites.
- **Solubility of Compounds**: Compounds of lithium are partially soluble in water, whereas compounds of sodium, rubidium, and cesium are completely soluble.

6. **Conclusion**: Based on the differences in physical and chemical properties, Lithium (Li) exhibits anomalous behavior compared to Sodium (Na), Rubidium (Rb), and Cesium (Cs). Therefore, the answer is (a) Li.

### Final Answer:
The element that differs from the rest in several physical and chemical properties is **(a) Li** (Lithium).

---