An element with atomic number 55 belongs to which block of periodic table?

Question

Accepted Answer

The element with atomic number 55 is cesium (Cs). To determine which block of the periodic table cesium belongs to, we need to consider its electron configuration and the structure of the periodic table.

1. **Explanation for Option 1 (s-block):**
   - The s-block of the periodic table includes elements where the outermost electrons are in an s orbital. For cesium, the electron configuration is [Xe] 6s¹. This indicates that the outermost electron is in the 6s orbital.
   - Elements in the s-block have their last electron entering an s orbital. Since cesium's last electron enters the 6s orbital, it is classified as an s-block element.
   - The s-block includes groups 1 and 2 of the periodic table, which are the alkali metals and alkaline earth metals, respectively. Cesium is an alkali metal, which further confirms its placement in the s-block.

2. **Explanation for why other options are incorrect:**

- **Option 2 (p-block):**
     - The p-block consists of elements where the last electron enters a p orbital. These elements are found in groups 13 to 18 of the periodic table. Since cesium's last electron enters an s orbital, not a p orbital, it cannot be part of the p-block.

- **Option 3 (d-block):**
     - The d-block contains transition metals, where the last electron enters a d orbital. These elements are located in groups 3 to 12. Cesium is not a transition metal, and its electron configuration does not involve a d orbital, so it is not part of the d-block.

- **Option 4 (f-block):**
     - The f-block includes the lanthanides and actinides, where the last electron enters an f orbital. These elements are typically shown below the main body of the periodic table. Cesium's electron configuration does not involve an f orbital, so it is not part of the f-block.

In conclusion, cesium, with atomic number 55, is correctly classified as an s-block element due to its electron configuration, which places its outermost electron in an s orbital. This aligns with the characteristics of the s-block, confirming that Option 1 is the correct answer.