The element with atomic number `55` belongs to which block ?

Options

A. s-block
B. p-block
C. d-block
D. f-block

Explanation

To determine which block the element with atomic number 55 belongs to, we can follow these steps:

### Step 1: Identify the Element
The element with atomic number 55 is cesium (Cs).

### Step 2: Determine the Electron Configuration
The electron configuration of cesium is given as:
\[ \text{Xe} 6s^1 \]
This indicates that the last electron is being added to the 6s subshell.

### Step 3: Identify the Block
Elements are classified into different blocks based on the subshell that is being filled with the last electron:
- **s-block**: Last electron in s subshell (groups 1 and 2)
- **p-block**: Last electron in p subshell (groups 13 to 18)
- **d-block**: Last electron in d subshell (transition metals)
- **f-block**: Last electron in f subshell (lanthanides and actinides)

Since the last electron of cesium is in the 6s subshell, cesium belongs to the **s-block**.

### Step 4: Classify the Element
Cesium is also classified as an alkali metal, which are the elements in group 1 of the periodic table.

### Final Answer
The element with atomic number 55 (cesium) belongs to the **s-block**.

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