Caesium is the lightest alkali metal.

Explanation

To determine whether the statement "Caesium is the lightest alkali metal" is true or false, we can follow these steps:

### Step 1: Understand the Properties of Alkali Metals
Alkali metals are found in Group 1 of the periodic table, and they include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

### Step 2: Identify the Trend in Atomic Mass and Volume
As we move down the group from lithium to cesium, both the atomic mass and atomic volume of the alkali metals increase.

### Step 3: Analyze Atomic Density
Atomic density is calculated using the formula:
\[ \text{Atomic Density} = \frac{\text{Atomic Mass}}{\text{Atomic Volume}} \]
While atomic mass increases, atomic volume also increases, but the increase in atomic mass is greater than the increase in atomic volume for alkali metals.

### Step 4: Compare the Density of Alkali Metals
Among the alkali metals, cesium has the highest atomic mass and also a significant atomic volume. Therefore, cesium has the highest atomic density compared to the other alkali metals.

### Step 5: Conclusion
Since cesium has the highest atomic density, it is considered the heaviest alkali metal, not the lightest. Thus, the statement "Caesium is the lightest alkali metal" is false.

### Final Answer
The statement is **false**; cesium is not the lightest alkali metal but rather the heaviest.

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