Question

Two lone pairs of electrons and two bond pairs are present in (1) \(\mathrm{NH}_{3}\) (2) BF \(_{3}\) (3) \(\mathrm{CO}_{3}^{2}\) (4) \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

\(\text{{H}}_2\text{{O}}\).

Step-by-step solution

- Identify the lone pairs and bond pairs in \(\text{{NH}}_{3}\)

In an \(\text{{NH}}_{3}\) (ammonia) molecule, nitrogen forms 3 single bonds with hydrogen atoms and has 1 lone pair of electrons.

- Identify the lone pairs and bond pairs in BF \(_{3}\)

In a BF \(_{3}\) (boron trifluoride) molecule, boron forms 3 single bonds with fluorine atoms and has no lone pairs of electrons.

- Identify the lone pairs and bond pairs in \(\text{{CO}}_{3}^2-\)

In a \(\text{{CO}}_{3}^2-\) (carbonate ion), carbon forms 3 bonds with oxygen atoms within a resonance structure and has no lone pairs of electrons. Each oxygen has lone pairs but carbon itself does not.

- Identify the lone pairs and bond pairs in \(\text{{H}}_{2}\text{{O}}\)

In an \(\text{{H}}_{2}\text{{O}}\) (water) molecule, oxygen forms 2 single bonds with hydrogen atoms and has 2 lone pairs of electrons.

- Compare all options

Comparing all options: \(\text{{NH}}_{3}\) has 1 lone pair and 3 bond pairs, \(\text{{BF}}_{3}\) has 0 lone pairs and 3 bond pairs, \(\text{{CO}}_{3}^2-\) has 0 lone pairs on carbon, and \(\text{{H}}_{2}\text{{O}}\) has 2 lone pairs and 2 bond pairs.

- Choose the correct answer

Based on the analysis, \(\text{{H}}_{2}\text{{O}}\) has 2 lone pairs and 2 bond pairs.

Key concepts

Lone Pairs

Lone pairs refer to pairs of valence electrons that are not shared with other atoms. These electrons are localized on a single atom. For example, in an \(\text{{H}}_{2}\text{{O}}\) molecule, oxygen has two lone pairs of electrons. These lone pairs are important as they influence the shape of the molecule.

Lone pairs can repel other electron pairs (both bonding and lone pairs), which causes the molecule to adopt a shape that minimizes repulsions. When drawing Lewis structures, lone pairs are usually represented as dots next to the atom.
Identify lone pairs for atoms:

• Use the atom's valence electron count.
• Tally the electrons used in bonds.
• Subtract the bonded electrons from the total valence electrons to find lone pairs.

Bond Pairs

Bond pairs are pairs of valence electrons that are shared between atoms in a molecule, creating a covalent bond. In contrast to lone pairs, bond pairs are shared and provide structural stability to the molecule.

For instance, in an \(\text{{NH}}_{3}\) molecule, each nitrogen atom forms three bond pairs with hydrogen atoms. Each bond pair involves one electron from nitrogen and one from a hydrogen atom.
Characteristics of bond pairs:

• They determine the basic structure of the molecule.
• Bond pairs are counted for each bond - single, double, or triple.

Understanding bond pairs helps in predicting how molecules interact and bond with each other.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is a principle used to predict the shape of individual molecules. According to this theory, electron pairs around a central atom will position themselves as far apart as possible to minimize repulsion.

Key Points of VSEPR Theory:

• Lone pairs have stronger repulsive forces compared to bond pairs.
• The shape of a molecule depends on the number of bond pairs and lone pairs around the central atom.

Applying VSEPR Theory:

• Determine the number of lone pairs and bond pairs around the central atom.
• Use electron pair geometry to predict molecular shape.
• Adjust for lone pairs which can cause deviations from ideal geometries.

For example, in \(\text{{H}}_{2}\text{{O}}\), there are 2 bond pairs and 2 lone pairs, leading to a bent shape rather than a linear one due to lone pair repulsion.
VSEPR Theory simplifies predicting molecular structures by focusing on electron pair repulsions.