Question

According to Lewis acid-base concept, which is a correct statement? (a) Species in which central atom has completed octet cannot act as acid. (b) All negatively charged ions are acids. (c) Molecule in which the central atom has vacant \(" \mathrm{~d}^{\prime \prime}\) orbitals acts as acid. (d) All positively changed ions are bases.

Short answer

(a) Species in which the central atom has completed octet cannot act as acid. (b) All negatively charged ions are acids. (c) Molecule in which the central atom has vacant d' orbitals acts as acid. (d) All positively charged ions are bases. Answer: (c) Molecule in which the central atom has vacant d' orbitals acts as acid.

Step-by-step solution

Statement Analysis

Let's analyze each statement based on the Lewis acid-base concept. (a) Species in which central atom has completed octet cannot act as acid. By definition, Lewis acids accept electron pairs. If the central atom has already completed its octet of electrons, it cannot accept any more electrons and cannot act as a Lewis acid. This statement, in general, is true. (b) All negatively charged ions are acids. Negatively charged ions have excess electrons and are called Lewis Bases because they donate electron pairs. Lewis acids have a deficiency of electrons and accept electron pairs. This statement is not true. (c) Molecule in which the central atom has vacant \(" \mathrm{~d}^{\prime\prime}\) orbitals acts as acid. Vacant orbitals can accept electron pairs. A molecule with a vacant d-supplementary orbital can accept an electron pair, which means it can act as a Lewis acid. This statement, in general, is true. (d) All positively charged ions are bases. Positively charged ions, also known as cations, have a deficiency of electrons. They can act as Lewis acids because they can accept electron pairs. Due to this, it is not correct to say that all positively charged ions are bases.

Conclusion

Based on the analysis, the correct statement according to the Lewis acid-base concept is: (c) Molecule in which the central atom has vacant \(" \mathrm{~d}^{\prime\prime}\) orbitals acts as acid.

Key concepts

Lewis Acids and Bases

The Lewis acid-base concept is a fundamental theory in chemistry that extends the classical definition of acids and bases. Unlike the traditional definitions that focus on protons, the Lewis concept centers around the transfer of electron pairs. A Lewis acid is a species that can accept an electron pair, whereas a Lewis base is one that can donate an electron pair.

For example, in a reaction between a Lewis acid like BF₃, which lacks a complete octet, and a Lewis base such as NH₃, which has a lone pair of electrons, the lone pair from NH₃ is donated to BF₃ to form a bond. This electron pair donation and acceptance is a core feature of the Lewis acid-base theory and helps to explain a wide range of chemical reactions beyond simple proton transfer.

Electron Pair Acceptance

Electron pair acceptance is a crucial aspect of the Lewis acid-base concept. A Lewis acid accepts an electron pair, filling a void in its valence shell and often leading to the formation of a coordinate covalent bond. This characteristic is central to identifying Lewis acids in chemical reactions.

• Species with incomplete octets are typical Lewis acids because they have room to accommodate more electrons.
• Molecules or ions with a positive charge can also act as Lewis acids, attracting negatively charged electron pairs.
• Certain transition metal complexes with vacant orbitals are prime examples of Lewis acids, as they can readily accept electron pairs from Lewis bases.

In essence, any chemical species that has the potential to accept an electron pair, regardless of its charge, can be classified as a Lewis acid.

Octet Rule in Chemistry

The octet rule is a guiding principle in chemistry that states atoms tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is applicable to the elements of the main groups of the periodic table and plays a pivotal role in predicting the bonding behavior of atoms.

• Atoms with less than eight electrons in their valence shell will tend to accept, share, or donate electrons to fulfill the octet requirement.
• However, in the context of Lewis acid-base chemistry, an atom that has already attained a complete octet is unlikely to act as a Lewis acid, because it does not have the capacity to accept additional electron pairs.

The application of the octet rule helps in understanding the formation of stable molecules and the transfer of electron pairs in Lewis acid-base reactions.

Charged Ions in Acid-Base Theory

In acid-base chemistry, charged ions play significant roles depending on their ability to donate or accept electron pairs. In the Lewis framework, it's not just about hydrogen ions or hydroxide ions; instead, all ions are analyzed based on their electron configuration.

• Negative ions (anions), typically have extra electrons that they can donate, fitting the role of Lewis bases.
• Positive ions (cations), on the other hand, usually lack electrons and may act as Lewis acids by accepting electron pairs from bases.

It's crucial to note, however, that not all negatively charged ions are automatically Lewis acids and not all positively charged ions are bases. The overall context of the molecule or ion, including its electronic structure and the presence of vacant orbitals, determines its behavior as an acid or base in the Lewis sense.