Which of the following statement is correct ?

Options

A. Ionisation energiest of elements decrease along the period.
B. Ionisation energies of the IIA group elements are less than that of the corresponding III A group elements.
C. Ionisation energies of group 15 elements are less than that of the corresponding group 16 elements.
D. Ionisation energy of Ga is greater than Al.

Explanation

To determine which statement is correct regarding ionization energies, we will analyze each statement step-by-step.

### Step 1: Analyze the first statement
**Statement:** Ionization energies of the elements decrease along the period.

**Analysis:**
- As we move from left to right in a period of the periodic table, the atomic size decreases due to an increase in nuclear charge (more protons in the nucleus).
- A smaller atomic size means that the outermost electrons are held more tightly by the nucleus.
- Therefore, the ionization energy (the energy required to remove an electron) actually **increases** along a period, not decreases.

**Conclusion:** This statement is **incorrect**.

### Step 2: Analyze the second statement
**Statement:** Ionization energy of second group elements are less than that of corresponding third group elements.

**Analysis:**
- The second group elements (alkaline earth metals) have the outer electronic configuration of ns².
- The third group elements (boron family) have the outer electronic configuration of ns²np¹.
- The presence of a fully filled s subshell (ns²) in the second group provides a stable configuration, while the third group has a less stable configuration due to the presence of an additional p electron.
- Therefore, the ionization energy of second group elements is actually **greater** than that of the third group elements.

**Conclusion:** This statement is **incorrect**.

### Step 3: Analyze the third statement
**Statement:** Ionization energy of group 15 elements are less than that of group 16 elements.

**Analysis:**
- Group 15 elements (nitrogen family) have the outer electronic configuration of ns²np³, which is a half-filled configuration, providing stability.
- Group 16 elements (oxygen family) have the outer electronic configuration of ns²np⁴, which is less stable than the half-filled configuration of group 15.
- Therefore, the ionization energy of group 15 elements is actually **greater** than that of group 16 elements.

**Conclusion:** This statement is **incorrect**.

### Step 4: Analyze the fourth statement
**Statement:** Ionization energy of gallium is greater than aluminum.

**Analysis:**
- Both gallium and aluminum belong to the same group (group 13).
- Gallium has a d orbital that is being filled, which leads to a greater nuclear charge and a smaller atomic size compared to aluminum.
- The smaller size of gallium means that its outermost electrons are held more tightly by the nucleus, resulting in a higher ionization energy compared to aluminum.

**Conclusion:** This statement is **correct**.

### Final Conclusion
The only correct statement among the given options is that the ionization energy of gallium is greater than that of aluminum.

---