Q.Among the elements B, Al, C and Si,

Here's how to determine the highest first ionisation enthalpy and most metallic character among the given elements:

(i) Highest first ionisation enthalpy

Concept: Periodic Trend: First Ionisation Enthalpy

First ionisation enthalpy is the energy required to remove the most loosely bound electron from an isolated gaseous atom. It generally increases across a period due to increasing effective nuclear charge and decreases down a group due to increasing atomic size and shielding.

1. Locate elements: B (Group 13, Period 2), Al (Group 13, Period 3), C (Group 14, Period 2), Si (Group 14, Period 3).
2. Compare within periods: Carbon (Group 14) has a higher effective nuclear charge and smaller atomic radius than Boron (Group 13) in Period 2. Similarly, Silicon (Group 14) has a higher effective nuclear charge and smaller atomic radius than Aluminium (Group 13) in Period 3.
3. Compare within groups: Elements in Period 2 (B, C) have smaller atomic radii and less shielding than their respective counterparts in Period 3 (Al, Si). …

To determine the highest first ionisation enthalpy and the most metallic character among B, Al, C, and Si, we need to understand the periodic trends for these properties. These trends are primarily governed by atomic size and effective nuclear charge ($Z_{eff}$).

Let's first locate these elements in the periodic table:

• Boron (B): Group 13, Period 2
• Aluminium (Al): Group 13, Period 3
• Carbon (C): Group 14, Period 2
• Silicon (Si): Group 14, Period 3

We can arrange them in a block:

Understanding the Concepts

1. First Ionisation Enthalpy ($IE_1$): This is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state.

   • Trend across a period (left to right): $IE_1$ generally increases. As we move across a period, the effective nuclear charge increases, pulling the valence electrons closer to the nucleus. The atomic size decreases, and it becomes harder to remove an electron.
   • Trend down a group (top to bottom): $IE_1$ generally decreases. As we move down a group, the number of electron shells increases, leading to a larger atomic size and increased shielding effect. The outermost electrons are further from the nucleus and experience less attraction, making them easier to remove.

2. Metallic Character: This refers to the tendency of an element to lose electrons and form positive ions (cations). Elements with low ionisation enthalpies and large atomic sizes tend to be more metallic.

   • Trend across a period (left to right): Metallic character generally decreases. Elements on the left side of the periodic table are metals, while those on the right are non-metals. This is because $IE_1$ increases across a period, making it harder to lose electrons.
   • Trend down a group (top to bottom): Metallic character generally increases. As $IE_1$ decreases down a group, it becomes easier for atoms to lose electrons, thus increasing their metallic character.

(i) Which element has the highest first ionisation enthalpy?

1. Compare elements in the same period:

   • In Period 2, Carbon (C) is to the right of Boron (B). Therefore, C has a higher $IE_1$ than B.
   • In Period 3, Silicon (Si) is to the right of Aluminium (Al). Therefore, Si has a higher $IE_1$ than Al.

2. Compare elements in the same group:

   • In Group 13, Boron (B) is above Aluminium (Al). Therefore, B has a higher $IE_1$ than Al.
   • In Group 14, Carbon (C) is above Silicon (Si). Therefore, C has a higher $IE_1$ than Si.

3. Synthesize the comparisons:

   • From step 1, we know C > B and Si > Al.
   • From step 2, we know B > Al and C > Si.
   • Combining these, we can see that Carbon (C) is in the upper right position of our block of elements. It has a higher $IE_1$ than B (due to being further right in the same period) and a higher $IE_1$ than Si (due to being higher in the same group).
   • Therefore, Carbon (C) will have the highest first ionisation enthalpy among B, Al, C, and Si. …