Draw the Lewis dot structures for the ammonium ion and the hydroxide ion and predict their molecular shapes.
The Correct Answer and Explanation is:
Lewis Dot Structures and Molecular Shapes
1. Ammonium Ion (NH₄⁺)
Lewis Dot Structure:
- Nitrogen (N) has 5 valence electrons.
- Each hydrogen (H) has 1 valence electron × 4 = 4 electrons.
- Total = 5 + 4 = 9 electrons.
- Since it is a positive ion (NH₄⁺), subtract 1 electron: 9 – 1 = 8 valence electrons.
Structure:
Nitrogen is in the center, bonded to four hydrogen atoms with single bonds. All 8 valence electrons are used to form four N–H bonds (2 electrons per bond). No lone pairs remain on nitrogen.
markdownCopyEdit H
|
H – N – H
|
H
Each bond contains a shared electron pair (2 electrons). The charge is shown in brackets as [NH₄]⁺.
Molecular Shape:
The ammonium ion has four bonding pairs and no lone pairs on the central nitrogen atom. According to VSEPR theory, this gives it a tetrahedral shape with bond angles of about 109.5°.
2. Hydroxide Ion (OH⁻)
Lewis Dot Structure:
- Oxygen (O) has 6 valence electrons.
- Hydrogen (H) has 1 valence electron.
- Total = 6 + 1 = 7 electrons.
- Since it is a negative ion (OH⁻), add 1 electron: 7 + 1 = 8 valence electrons.
Structure:
Oxygen is bonded to hydrogen with a single bond. The remaining 6 electrons go to three lone pairs on oxygen.
markdownCopyEditH – O⁻
..
..
Oxygen has one single bond and three lone pairs.
Molecular Shape:
The hydroxide ion has one bonding pair and three lone pairs on oxygen. According to VSEPR theory, the electron geometry is tetrahedral, but the molecular shape is bent or linear-like due to only one bonded atom. In practice, with only two atoms, its shape is considered linear, but with notable bond angle reduction due to lone pair repulsion.
Summary
- NH₄⁺: Tetrahedral shape, 109.5° angles.
- OH⁻: Linear shape due to two atoms, but bent influence from lone pairs on oxygen.
