The decreasing order `IE` for elements `Li, Be, C B` is `"______"` .

Explanation

To determine the decreasing order of ionization energy (IE) for the elements Lithium (Li), Beryllium (Be), Boron (B), and Carbon (C), we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove an electron from an atom in its gaseous state. Generally, ionization energy increases across a period (from left to right) in the periodic table due to increasing nuclear charge, which holds the electrons more tightly.

### Step 2: Write the Electronic Configurations
- **Lithium (Li)**: 1s² 2s¹
- **Beryllium (Be)**: 1s² 2s²
- **Boron (B)**: 1s² 2s² 2p¹
- **Carbon (C)**: 1s² 2s² 2p²

### Step 3: Analyze the Trends
- As we move from Li to Be, the ionization energy increases because Be has a full 2s subshell, making it more stable and requiring more energy to remove an electron.
- Moving from Be to B, the ionization energy decreases slightly because B has one more electron in the 2p subshell, which is at a higher energy level and is less tightly held than the 2s electrons.
- From B to C, the ionization energy increases again because C has a greater nuclear charge and a more stable configuration compared to B.

### Step 4: Arrange in Decreasing Order
Based on the analysis:
1. **Beryllium (Be)** has the highest ionization energy due to its stable configuration.
2. **Carbon (C)** follows, as it has a higher nuclear charge than Boron.
3. **Boron (B)** comes next, as it has a lower ionization energy than Carbon.
4. **Lithium (Li)** has the lowest ionization energy among these elements.

Thus, the decreasing order of ionization energy for the elements Li, Be, C, and B is:
**Be > C > B > Li**

### Final Answer
The decreasing order of ionization energy for the elements Li, Be, C, and B is: **Be > C > B > Li**

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