Draw Lewis structures that show how electron pairs move and bonds form and break in the following reaction, and identify the Lewis acid and Lewis base.

Draw Lewis structures that show how electron pairs move and bonds form and break in the following reaction, and identify the Lewis acid and Lewis base.

The Correct Answer and Explanation is:

Reaction Example:
NH3+BF3→F3B–NH3\text{NH}_3 + \text{BF}_3 \rightarrow \text{F}_3\text{B–NH}_3NH3​+BF3​→F3​B–NH3​

Step-by-step Lewis Structures with Electron Movement

1. Draw Lewis Structure for NH₃ (Ammonia):

Ammonia has:

• Nitrogen (N) in the center with 5 valence electrons.
• Three hydrogen (H) atoms bonded to N using single bonds.
• One lone pair of electrons remaining on the nitrogen atom.

Structure:

mathematicaCopyEdit   H
   |
H–N:      (one lone pair on N)
   |
   H

2. Draw Lewis Structure for BF₃ (Boron trifluoride):

Boron (B) has 3 valence electrons, each bonded to a fluorine (F) atom:

rCopyEdit   F
   |
F–B–F

Note: Boron has only six electrons around it, making it electron-deficient, and thus it seeks an electron pair to become stable.

3. Electron Pair Movement:

The lone pair on nitrogen in NH₃ donates to the vacant p-orbital on boron in BF₃.

Arrow notation:

mathematicaCopyEdit   H
   |
H–N:  →  B      (Arrow shows the lone pair from N attacking B)
   |
   H

4. Resulting Complex:

rCopyEdit   H
   |
H–N→B–F
   |   |
   H   F
       |
       F

A coordinate covalent bond is formed between nitrogen and boron.

Lewis Acid and Lewis Base Identification:

• Lewis base: NH₃ (donates an electron pair)
• Lewis acid: BF₃ (accepts an electron pair)

Explanation

In Lewis theory, a Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is a substance that donates a pair of electrons. This definition is broader than the Brønsted–Lowry acid-base theory because it includes reactions without hydrogen ion exchange.

In the reaction between ammonia (NH₃) and boron trifluoride (BF₃), we see a textbook example of a Lewis acid-base reaction. Nitrogen in ammonia has one lone pair of electrons. On the other hand, boron in BF₃ is electron-deficient. Although each fluorine atom contributes electrons to boron via single bonds, boron only ends up with six electrons in its valence shell. This makes BF₃ a strong Lewis acid, seeking an electron pair to reach an octet configuration.

When NH₃ and BF₃ come into contact, nitrogen’s lone pair is attracted to the electron-deficient boron atom. The lone pair from nitrogen is donated to boron, forming a new bond. This bond is called a coordinate covalent bond, where both electrons in the bond originate from the same atom—in this case, nitrogen.

The result is a stable complex, F₃B–NH₃, in which boron now satisfies the octet rule. The nitrogen atom, despite forming four bonds, does not violate the octet rule because the lone pair was used in bonding.

This reaction illustrates fundamental concepts in acid-base chemistry, showing how electron flow, not proton transfer, defines acidity and basicity under Lewis theory.