Which one of the following elements has lowest ionisation energy:
`H, Mg, Cs, B`
Report the Atomic number of that element.

Answer

To determine which element has the lowest ionization energy among hydrogen (H), magnesium (Mg), cesium (Cs), and boron (B), we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove an electron from an atom in its gaseous state. Generally, ionization energy decreases as you move down a group in the periodic table and increases as you move from left to right across a period.

### Step 2: Locate the Elements in the Periodic Table
- **Hydrogen (H)**: Atomic number 1, located at the top of Group 1.
- **Boron (B)**: Atomic number 5, located in Group 13.
- **Magnesium (Mg)**: Atomic number 12, located in Group 2.
- **Cesium (Cs)**: Atomic number 55, located at the bottom of Group 1.

### Step 3: Analyze the Trends
- **Hydrogen** has a small atomic size, which means it has a relatively high ionization energy compared to larger atoms.
- **Boron** is in the p-block and has a higher ionization energy than both magnesium and cesium due to its position in the periodic table.
- **Magnesium** is in Group 2 and has a higher ionization energy than cesium because it is smaller in size compared to cesium.
- **Cesium**, being in Group 1 and located at the bottom of the group, has the largest atomic size among the given elements. As atomic size increases, the outermost electron is further from the nucleus and is held less tightly, resulting in a lower ionization energy.

### Step 4: Conclusion
Based on the trends in ionization energy:
- Cesium (Cs) has the lowest ionization energy among the four elements listed.

### Step 5: Report the Atomic Number
The atomic number of cesium (Cs) is **55**.

### Final Answer
The element with the lowest ionization energy is cesium (Cs), and its atomic number is **55**.

---