Question

In terms of orbitals and electron arrangements, what must be present for a molecule or an ion to act as a Lewis acid? What must be present for a molecule or an ion to act as a Lewis base?

Short answer

In order for a molecule or ion to act as a Lewis acid, it must have an empty orbital and a central atom with an incomplete octet or positive charge, allowing it to accept electrons. For a molecule or ion to act as a Lewis base, it must have a lone pair of electrons available for donation and an electronegative central atom that can efficiently donate the electron pair to form a coordinate covalent bond.

Step-by-step solution

Definition of Lewis Acid and Lewis Base

A Lewis acid is a molecule or ion that can accept a lone pair of electrons, while a Lewis base is a molecule or ion that can donate a lone pair of electrons.

Properties of a Lewis Acid

For a molecule or an ion to act as a Lewis acid, it should have the following properties: 1. It should have an empty orbital to accept the lone pair of electrons. This is generally associated with electron-deficient species, such as boron trifluoride (BF3) or aluminum chloride (AlCl3). 2. It should possess a central atom with an incomplete octet or a positive charge that can accept electrons, thus stabilizing the system.

Properties of a Lewis Base

For a molecule or an ion to act as a Lewis base, it should have the following properties: 1. It should have a lone pair of electrons that are readily available to be donated. This is generally associated with electron-rich species, such as ammonia (NH3) or water (H2O). 2. It should have an electronegative central atom that can efficiently hold onto the electron pair and donate it accordingly to form a coordinate covalent bond. In summary, a Lewis acid should have an empty orbital and an ability to accept electrons, while a Lewis base should have a lone pair of electrons and an ability to donate them.