Which has the largest first ionisation energy ?

Options

A. `Na`
B. `K`
C. `Rb`
D. `Li`

Explanation

To determine which element has the largest first ionization energy among the given options (lithium, sodium, potassium, rubidium), we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is defined as the amount of energy required to remove an electron from an atom in its gaseous state. The first ionization energy specifically refers to the energy needed to remove the outermost electron.

**Hint:** Remember that ionization energy is related to how tightly electrons are held by the nucleus.

### Step 2: Identify the Group of Elements
All the elements mentioned (lithium, sodium, potassium, rubidium) belong to Group 1 of the periodic table, also known as the alkali metals.

**Hint:** Group 1 elements have similar properties, but their ionization energies will vary.

### Step 3: Analyze the Trend in Ionization Energy
In the periodic table, as you move down a group, the size of the atoms increases due to the addition of electron shells. This increase in size means that the outermost electrons are further away from the nucleus and are held less tightly.

**Hint:** Larger atomic size generally leads to lower ionization energy.

### Step 4: Compare the Elements
- **Lithium (Li)**: Smallest atomic size among the given elements.
- **Sodium (Na)**: Larger than lithium.
- **Potassium (K)**: Larger than sodium.
- **Rubidium (Rb)**: Largest among the four.

Since lithium is the smallest atom in this group, it will have the highest ionization energy because its outermost electron is held more tightly by the nucleus compared to the others.

**Hint:** The element with the smallest atomic radius in a group typically has the highest ionization energy.

### Step 5: Conclusion
Based on the trend of decreasing ionization energy down the group, we conclude that lithium (Li) has the largest first ionization energy among the given elements.

**Final Answer:** Lithium (Li) has the largest first ionization energy.