Question

For a species to act as a Brönsted base, an atom in the species must possess a lone pair of electrons. Explain why this is so.

Short answer

A Brönsted base must have a lone pair to accept a proton by forming a bond.

Step-by-step solution

Understanding Brönsted Base

A Brönsted base is defined as a substance that can accept a proton ( ext{H}^+). This means that the base must have the capability to form a bond with a proton.

Role of Lone Pair

A lone pair of electrons on an atom is a pair of valence electrons that are not shared with another atom. These electrons are available to form a new bond, allowing the atom to accept a proton.

Mechanism of Proton Acceptance

When a species acts as a Brönsted base, it uses its lone pair of electrons to form a coordinate covalent bond with the proton. The lone pair provides the necessary electron density to hold the proton.

Conclusion

Without a lone pair of electrons, an atom cannot participate in accepting a proton, as it would lack the requisite electron pair needed to stabilize the new bond formation with ext{H}^+.

Key concepts

Lone Pair of Electrons

A lone pair of electrons refers to two valence electrons that occupy the same orbital but do not partake in bonding with other atoms. These electrons are often found in the outermost shell of an atom. This makes them significant in chemical reactions. Lone pairs are crucial in determining the chemical reactivity and the ability of a molecule to act as a Brönsted base, which is a substance capable of accepting a proton (H\(^+\)).

The presence of a lone pair allows an atom to provide the necessary electron density to form new bonds. In the context of Brönsted bases, the lone pair is used to connect with a proton. This connection leads to the formation of a new bond called a coordinate covalent bond.

• Lone pairs do not participate in covalent bonding, so they are available for reactions.
• Lone pairs help in stabilizing the new bond formed when a proton is accepted.

Understanding the role of lone pairs is vital for grasping how a substance can function as a Brönsted base.

Proton Acceptance

The concept of proton acceptance is central to the definition of a Brönsted base. A Brönsted base must possess the ability to accept a proton. This means it must have an electron pair available to bond with the protons. This electron pair typically comes from a lone pair on an atom within the base.

When an atom accepts a proton, it uses its lone pair to grab the proton, effectively sharing its electrons. By accepting the proton, the atom can stabilize the positive charge the proton carries. This stabilization is critical because protons are positive hydrogen ions (H\(^+\)) and need electron density to neutralize the charge.

• The acceptance of a proton is not a simple attraction; it involves the formation of a new bond.
• This interaction can only happen if the base has an unclaimed electron pair—specifically a lone pair.

Therefore, understanding proton acceptance involves recognizing how lone pairs are utilized in forming new bonds.

Coordinate Covalent Bond

A coordinate covalent bond, also known as a dative bond, is a special type of covalent bond. In this bond, both electrons come from the same atom. This is unlike a standard covalent bond, where each atom contributes one electron to the bond pair. In the case of a Brönsted base, the lone pair of electrons from the base is solely responsible for bonding with the proton.

When a lone pair forms a coordinate covalent bond with a proton (H\(^+\)), the proton becomes effectively part of the base. This bond formation is crucial in many chemical reactions and processes.

• Coordinate covalent bonds are pivotal in acid-base chemistry and enzyme interactions.
• They provide stability and structure in complex molecules.

Knowing about coordinate covalent bonds helps explain how atoms and molecules interact at a detailed level during reactions, particularly for substances acting as Brönsted bases.