The first ionization enthalpy `(Delta_(i)H)` values of the second period elements `Li, Be` and `C` are respectively `520, 899` and `1086 kJ mol^(-1)`. Predict whther the first `Delta_(i)H` value for `B` will be more close to `690` or `990 kJ mol^(-1)`?

Answer

To determine the first ionization enthalpy (ΔiH) value for boron (B) and predict whether it will be closer to 690 kJ/mol or 990 kJ/mol, we can follow these steps:

### Step 1: Understand Ionization Enthalpy Trends
Ionization enthalpy generally increases across a period in the periodic table due to increasing nuclear charge and decreasing atomic radius. This means that as we move from left to right across a period, the energy required to remove an electron typically increases.

### Step 2: Analyze Given Data
The first ionization enthalpy values for the second period elements are:
- Lithium (Li): 520 kJ/mol
- Beryllium (Be): 899 kJ/mol
- Carbon (C): 1086 kJ/mol

From this data, we can see that the ionization enthalpy increases from Li to Be and then to C.

### Step 3: Position of Boron in the Periodic Table
Boron (B) is located between beryllium (Be) and carbon (C) in the periodic table. Therefore, we can predict that the ionization enthalpy of boron will be higher than that of lithium but lower than that of beryllium.

### Step 4: Consider Electron Configuration
Boron has the electron configuration of 1s² 2s² 2p¹. The presence of a single electron in the 2p subshell means that the effective nuclear charge experienced by this electron is less than that experienced by the fully filled 2s electrons in beryllium. This results in a lower ionization enthalpy for boron compared to beryllium.

### Step 5: Make a Prediction
Since the ionization enthalpy of beryllium is 899 kJ/mol and that of carbon is 1086 kJ/mol, we can infer that the ionization enthalpy for boron will be less than 899 kJ/mol but more than 520 kJ/mol. Given the options of 690 kJ/mol and 990 kJ/mol, it is reasonable to conclude that the first ionization enthalpy of boron will be closer to 690 kJ/mol.

### Conclusion
Therefore, the first ionization enthalpy value for boron (B) will be more close to **690 kJ/mol**.

---