The alkaline earth metals Ba,Sr,Ca and Mg may be arranged in the order of their decreasing first ionisation potential as

Options

A. Mg,Ca,Sr,Ba
B. Ca,Sr,Ba,Mg
C. Sr,Ba,Mg,Ca
D. Ba,Mg,Ca,Sr

Explanation

To determine the order of decreasing first ionization potential (I.P.) for the alkaline earth metals barium (Ba), strontium (Sr), calcium (Ca), and magnesium (Mg), we need to consider the general trends in ionization energy across the periodic table.

### Step-by-Step Solution:

1. **Understand Ionization Potential (I.P.):**
- Ionization potential is the energy required to remove an electron from an isolated gaseous atom. Higher I.P. means it is harder to remove an electron.

2. **Identify the Group and Period:**
- Barium (Ba), Strontium (Sr), Calcium (Ca), and Magnesium (Mg) are all alkaline earth metals found in Group 2 of the periodic table.

3. **Trends in Ionization Energy:**
- Ionization energy generally decreases down a group in the periodic table. This is due to the increase in atomic size and the shielding effect, which makes it easier to remove the outermost electron.

4. **List the Elements by Atomic Number:**
- Magnesium (Mg) - Atomic number 12
- Calcium (Ca) - Atomic number 20
- Strontium (Sr) - Atomic number 38
- Barium (Ba) - Atomic number 56

5. **Apply the Trend:**
- Since ionization energy decreases down the group, we can arrange the elements in order of decreasing ionization potential:
- Magnesium (Mg) has the highest I.P.
- Calcium (Ca) is next.
- Strontium (Sr) follows.
- Barium (Ba) has the lowest I.P.

6. **Final Order:**
- The order of decreasing first ionization potential is:
- Mg > Ca > Sr > Ba

### Final Answer:
The alkaline earth metals Ba, Sr, Ca, and Mg may be arranged in the order of their decreasing first ionization potential as:
**Mg > Ca > Sr > Ba**