The element with lowest ionization energy among the following is :

Options

A. Li
B. N
C. K
D. Rb

Explanation

To determine which element has the lowest ionization energy among a given set of elements, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Ionization Energy**:
- Ionization energy is defined as the energy required to remove an electron from an isolated gaseous atom. The lower the ionization energy, the easier it is to remove an electron.

2. **Identify the Elements**:
- The question states that the elements in consideration are alkali metals. Alkali metals belong to Group 1 of the periodic table, which includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs).

3. **Analyze the Trend in Ionization Energy**:
- Ionization energy decreases as you move down a group in the periodic table. This is due to the increase in atomic size; as the atomic radius increases, the outermost electron is farther from the nucleus. Consequently, the attraction between the nucleus and the outermost electron decreases, making it easier to remove the electron.

4. **Compare the Elements**:
- For the alkali metals:
- Lithium (Li) has the smallest atomic radius and thus the highest ionization energy.
- Sodium (Na) has a larger atomic radius than lithium, so it has a lower ionization energy than lithium but higher than potassium.
- Potassium (K) has a larger atomic radius than sodium, leading to a lower ionization energy than sodium.
- Rubidium (Rb) is larger than potassium, resulting in an even lower ionization energy.
- Cesium (Cs) has the largest atomic radius among these elements, thus it has the lowest ionization energy.

5. **Conclusion**:
- Among the alkali metals, rubidium (Rb) has the lowest ionization energy compared to lithium, sodium, and potassium. Therefore, if rubidium is included in the options, it would be the correct answer. If cesium (Cs) is included, it would actually have the lowest ionization energy.

### Final Answer:
- The element with the lowest ionization energy among the given alkali metals is **Rubidium (Rb)**.