Which of the following has the largest ionisation energy ?

Options

A. Zn
B. Sc
C. Cd
D. Hg

Answer

To determine which of the following elements has the largest ionization energy among scandium, zinc, cadmium, and mercury, we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove the outermost electron from an isolated gaseous atom. It is influenced by two main factors: the effective nuclear charge (Z_eff) and the size of the atom.

### Step 2: Analyze the Given Elements
The elements given are:
- Scandium (Sc)
- Zinc (Zn)
- Cadmium (Cd)
- Mercury (Hg)

### Step 3: Compare the Elements
1. **Scandium to Zinc**:
- As we move from scandium to zinc across the periodic table, the atomic size decreases due to an increase in effective nuclear charge. Therefore, the ionization energy increases from scandium to zinc.

2. **Zinc, Cadmium, and Mercury**:
- These elements are in the same group (Group 12). Generally, as we move down a group, the atomic size increases, leading to a decrease in ionization energy. However, the transition metals experience a phenomenon known as the lanthanide contraction, which affects their atomic sizes.
- Cadmium (Cd) and mercury (Hg) are heavier elements, but due to lanthanide contraction, the size of cadmium is nearly the same as that of mercury. However, mercury has a higher atomic number, which means it has a greater effective nuclear charge (Z_eff).

### Step 4: Conclusion
Since mercury has the highest effective nuclear charge among the given elements, it will also have the largest ionization energy. Therefore, the element with the largest ionization energy is:

**Answer: Mercury (Hg)**

---