Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In comparison to boron, berylium has: (a) lesser nuclear charge and lesser first ionisation enthalpy. (b) greater nuclear charge and lesser first ionisation enthalpy. (c) greater nulear charge and greater first ionisation enthalpy. (d) lesser nuclear charge and greater first ionisation enthalpy.

Step-by-Step Solution

Answer

(d) lesser nuclear charge and greater first ionisation enthalpy.

Step 1: Understanding the Elements

Beryllium (Be) and Boron (B) are elements with atomic numbers 4 and 5, respectively. Beryllium has a lower atomic number, which means it has fewer protons compared to Boron.

Step 2: Analyzing Nuclear Charge

Nuclear charge is determined by the number of protons in the nucleus. Since Beryllium has 4 protons and Boron has 5, Beryllium has a lesser nuclear charge compared to Boron.

Step 3: First Ionisation Enthalpy Consideration

First ionisation enthalpy is the energy required to remove the outermost electron. Beryllium, being an s-block element with a completely filled orbital, has a higher ionisation enthalpy compared to Boron, which is a p-block element. The presence of a partially filled p-orbital in Boron makes it easier to remove its outer electron.

Step 4: Comparing Both Properties

Combining the findings, Beryllium has a lesser nuclear charge and greater first ionisation enthalpy compared to Boron.

Key Concepts

Nuclear Charge

Every atom has a positively charged nucleus, and the number of protons within that nucleus determines the nuclear charge. More protons mean a greater nuclear charge.
Since protons carry a positive charge, the nuclear charge is directly proportional to the number of protons. In atomic comparison, Beryllium (with 4 protons) has a lesser nuclear charge compared to Boron (with 5 protons).

• The fewer the number of protons, the lesser the nuclear charge.
• Nuclear charge affects an element's attraction towards electrons, specifically in its outer shell.
• This property is foundational in understanding periodic trends and will influence other properties like ionization enthalpy.

It's crucial to consider nuclear charge as it defines the innate electrical pull an atom can exert on its electrons and adjacent interactions with other atoms.

Ionisation Enthalpy

Ionisation enthalpy is a measure of the energy needed to remove an electron from an atom, specifically the most loosely bound electron. This becomes a fundamental metric for understanding how readily an atom gives up electrons, reflecting its reactivity and chemistry.
For Beryllium and Boron, Beryllium shows higher ionisation enthalpy. It requires more energy to remove an electron from Beryllium than from Boron.

• Beryllium's electrons reside in a completely filled 2s-orbital. The stability of a full orbital increases the ionisation enthalpy.
• Boron, having its outer electron in a partially filled 2p-orbital, has less stable electron configurations making it easier for the electron to be removed. Thus, Boron displays lower ionisation enthalpy.

Understanding ionisation enthalpy not only helps in gauging the reactivity but is crucial for predicting chemical behavior across the periodic table.

s-block and p-block elements

Elements in the periodic table are grouped into blocks according to their electron configurations, which define their placement and chemical properties. Within these, the s-block and p-block elements exhibit distinct differences.
Beryllium is an s-block element. It means its outermost electrons are in an s orbital. S-block elements often show stark differences in properties among their rows, like ionisation enthalpy, as seen with Beryllium.

• These elements have their last electron entering the s sub-level.
• S-block elements, like Beryllium, often have higher ionisation energies owing to their compact electron configurations.

Boron represents the p-block with its partially filled p-orbitals. This categorization affects its chemistry and properties:

• P-block elements are defined with their last electron going into the p orbital.
• They typically display greater chemical diversity and often have varied oxidation states.

Understanding these blocks is fundamental in predicting the periodic trends and behaviors for each set of elements.