Question

a. Draw the Lewis structure for the water molecule. b. Draw Lewis structures for the hydrogen ion and the hydroxide ion. c. Write a chemical reaction that relates all three structures from parts a and b.

Short answer

The Lewis structure of water shows O bonded to two H atoms with two lone pairs. The hydrogen ion (H⁺) has no electrons, and the hydroxide ion (OH⁻) has O bonded to H with three lone pairs. The related reaction is H₂O ↔ H⁺ + OH⁻.

Step-by-step solution

Understanding Lewis Structure

Lewis structures are diagrams that represent the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. To draw a Lewis structure, we start by counting all the valence electrons in the molecule.

Drawing Water's Lewis Structure

Water (H₂O) consists of two hydrogen atoms and one oxygen atom. Oxygen has 6 valence electrons, and each hydrogen has 1 valence electron, totaling 8 valence electrons. In the Lewis structure, the oxygen atom is in the center with two hydrogen atoms bonded to it. Oxygen will have two lone pairs of electrons.

Drawing the Hydrogen Ion's Lewis Structure

A hydrogen ion (H⁺) is formed when a hydrogen atom loses an electron, leaving it with no electrons. Thus, the Lewis structure of H⁺ is simply 'H' without any dots around it as it has no electrons.

Drawing the Hydroxide Ion's Lewis Structure

The hydroxide ion (OH⁻) consists of an oxygen atom bonded to a hydrogen atom. Oxygen has 6 valence electrons, the hydrogen contributes 1, and there is an additional electron due to the negative charge, totaling 8 electrons. The structure has a single bond between O and H and three lone pairs around the oxygen.

Writing the Chemical Reaction

The chemical reaction that shows the relationship between H₂O, H⁺, and OH⁻ is the self-ionization of water: \[ \text{H}_2\text{O (l)} \rightleftharpoons \text{H}^+ (aq) + \text{OH}^- (aq) \] This shows water dissociating into a hydrogen ion and a hydroxide ion.

Key concepts

Valence Electrons

Valence electrons are the electrons that occupy the outermost shell of an atom. These electrons play a crucial role in chemical bonding. They determine how atoms will interact with one another.
In the Lewis structure, these valence electrons are shown as dots around the atomic symbols. Each dot represents one valence electron.
For example, in a water molecule (H₂O), the oxygen atom has six valence electrons, while each hydrogen atom has one valence electron. This means we have a total of 8 valence electrons to arrange.

• The electrons are used in forming bonds, which are represented by lines between atoms.
• In molecules, atoms strive to complete their outer shell with 8 electrons, following the octet rule.
• In the case of water, oxygen gets additional electrons through bonding with hydrogen.

Thus, understanding valence electrons is key to predicting how atoms bond in a molecule.

Hydrogen Ion

A hydrogen ion ( H⁺ ) is a positively charged ion formed when a hydrogen atom loses an electron. This leaves just a proton, as the hydrogen atom initially has only one electron.
The absence of any electrons gives the hydrogen ion its positive charge.

• The hydrogen ion is simply denoted as H, and in the Lewis structure, it doesn’t have any dots around it.
• It’s a very reactive ion and often wants to pair quickly with other electrons or ions.

In the context of water chemistry, H⁺ often pairs with other molecules, such as oxygen-containing ions or water itself.
When dissolved in water, it makes the solution acidic, which is an essential concept in acid-base chemistry.

Hydroxide Ion

The hydroxide ion ( OH⁻ ) is a negatively charged ion consisting of one oxygen and one hydrogen atom. An extra electron gives this ion its negative charge.
This means there are eight electrons to arrange in its Lewis structure.

• In OH⁻ , one pair of electrons form a bond between the oxygen and hydrogen atoms.
• The remaining six electrons exist as three lone pairs around the oxygen.
• The extra electron explains the negative charge on the hydroxide ion.

The hydroxide ion is basic. It is fundamental in both biological systems and industrial processes.
In water chemistry, OH⁻ balances the H⁺ ions to maintain pH balance.

Chemical Reaction

A chemical reaction involves the rearrangement of atomic structure to form new substances. In the context of water, a critical reaction is its self-ionization.
For water, the chemical equation is: \[ \text{H}_2\text{O (l)} \rightleftharpoons \text{H}^+ (aq) + \text{OH}^- (aq) \] This represents the process where water molecules dissociate into hydrogen ions and hydroxide ions.

• The reaction is in equilibrium, which means it can go forwards or backward.
• This self-ionization is a dynamic process, crucial in maintaining the balance of H⁺ and OH⁻ in water.
• It is also fundamental in understanding the concept of pH—a measure of acidity or basicity.

Chemical reactions, like the one for water, provide the basis for understanding behaviors in various chemical systems.