Question

For each of the questions, four choices have been provided. Select the correct alternative. A species \(\mathrm{H}_{3} \mathrm{O}^{+}\) pyramidal in shape. The number of lone pairs of electrons is (a) 0 (b) 1 (c) 2 (d) 3

Short answer

Answer: There is 1 lone pair of electrons in the \(\mathrm{H}_{3} \mathrm{O}^{+}\) species.

Step-by-step solution

Find the total number of valence electrons

Oxygen has 6 valence electrons, and hydrogen has 1 valence electron, making a total of 3 valence electrons from the hydrogens. Since the species has a positive charge of +1, we will subtract one electron from the total. Total valence electrons = 6 (oxygen) + 3 (hydrogen) - 1 (positive charge) = 8

Calculate the number of electron pairs

Divide the total number of valence electrons by 2 to find the number of electron pairs: Number of electron pairs = 8 / 2 = 4

Determine the molecular geometry

The \(\mathrm{H}_{3} \mathrm{O}^{+}\) species has a pyramidal shape. In the pyramidal geometry, the central atom (oxygen) forms three covalent bonds with the hydrogen atoms, and there is one lone pair on the central atom (oxygen).

Identify the number of lone pairs of electrons

The molecular geometry shows that there is one lone pair of electrons on the central atom (oxygen) in the \(\mathrm{H}_{3} \mathrm{O}^{+}\) species. So, the correct answer is: (b) 1

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom, playing a critical role in chemical bonding. They are involved in forming bonds and determine an atom's reactivity and bonding capacity. Oxygen, a key element in many molecules, has 6 valence electrons. Hydrogen, on the other hand, is much simpler and only has 1 valence electron. For the \(H_3O^+\) species, we calculate the total valence electrons by adding those from the oxygen atom and the three hydrogen atoms. These add up to 9. However, due to the positive charge of the species, we subtract one electron, leaving us with a total of 8 valence electrons.
Understanding the count of valence electrons is crucial. It sets the stage for predicting the arrangement of electrons, which directly influences molecular geometry and properties.

Electron Pairs

Once the total number of valence electrons is determined, the next step is to find the number of electron pairs. Electrons tend to pair up in bonds or as lone pairs. We calculate the number of electron pairs by dividing the total valence electrons by 2.
For \(H_3O^+\), with its 8 valence electrons, we have 4 electron pairs. These pairs are crucial as they can show us how electrons are arranged around the central atom, which, in this case, is oxygen.

• Covalent bonds form between atoms by sharing these electron pairs.
• Electron pairs help in maintaining the structure and stability of the molecule.

This division into pairs offers insight into potential lone pairs, which significantly influence the shape of the molecule.

Lone Pairs

Lone pairs are the pairs of valence electrons not shared between atoms and do not contribute to bonding. Instead, they remain attached to a single atom. For the molecule \(H_3O^+\), we found that there are 4 electron pairs in total with three hydrogen atoms bonded to oxygen, leaving one electron pair as a lone pair.
Lone pairs are essential because they occupy space around the central atom, affecting the shape of the molecule. Here’s why they matter:

• Lone pairs push bonded atoms closer together due to electron repulsion.
• They influence the overall geometry by reducing the bond angles.

In \(H_3O^+\), the presence of one lone pair gives it a distinctive pyramidal shape.

Pyramidal Shape

A pyramidal shape in molecular geometry means the central atom is at the top of a pyramid with bonded atoms forming the base. The shape and arrangement are influenced heavily by lone pairs. When it comes to \(H_3O^+\), the presence of three bonded hydrogen atoms around the oxygen and one lone pair results in a pyramidal structure.
Key characteristics of this shape include:

• The lone pair is at the apex of the shape, giving it a 3-dimensional geometric form.
• The bond angles are influenced by the lone pair's spatial requirements, usually less than the typical tetrahedral angle of 109.5°.

This peculiar shape affects the molecular interaction, reactivity, and physical properties of the molecule. The pyramidal shape of \(H_3O^+\) is therefore crucial to understanding its chemical behavior.