Which of the following elements will have the lowest first ionisation energy?

Options

A. `Mg`
B. `Rb`
C. `Li`
D. `Ca`

Explanation

To determine which of the given elements has the lowest first ionization energy, we can follow these steps:

### Step 1: Identify the Elements
The elements mentioned in the question are lithium (Li), magnesium (Mg), rubidium (Rb), and calcium (Ca).

### Step 2: Understand Ionization Energy
Ionization energy is the energy required to remove an electron from an atom in the gaseous state. Generally, ionization energy trends can be summarized as follows:
- **Increases across a period (left to right)**: As you move from left to right across a period in the periodic table, the ionization energy increases due to increasing nuclear charge, which holds the electrons more tightly.
- **Decreases down a group (top to bottom)**: As you move down a group, the ionization energy decreases because the outermost electrons are farther from the nucleus and are shielded by inner electrons, making them easier to remove.

### Step 3: Grouping the Elements
- **Lithium (Li)** and **Rubidium (Rb)** are alkali metals (Group 1).
- **Magnesium (Mg)** and **Calcium (Ca)** are alkaline earth metals (Group 2).

### Step 4: Compare the Elements
- **Lithium (Li)**: Located at the top of Group 1, it has a relatively higher ionization energy compared to other alkali metals.
- **Rubidium (Rb)**: Located lower in Group 1, it has a lower ionization energy than lithium because it is further down the group.
- **Magnesium (Mg)**: Located in Group 2, it has a higher ionization energy than lithium.
- **Calcium (Ca)**: Located below magnesium in Group 2, it has a lower ionization energy than magnesium but is still higher than rubidium.

### Step 5: Conclusion
Since rubidium (Rb) is the lowest in Group 1, it will have the lowest first ionization energy among the given elements. Therefore, the answer is **Rubidium (Rb)**.

### Final Answer
Rubidium (Rb) has the lowest first ionization energy.

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