Problem 6 - Introductory Chemistry Atoms First

Question

Why is water \(\mathrm{H}_{2} \mathrm{O}\) and not \(\mathrm{H}_{3} \mathrm{O}\) or something else? Justify your answer with dot diagrams.

Step-by-Step Solution

Answer

Water has the chemical formula H2O because it consists of two hydrogen atoms and one oxygen atom bonded together in a stable molecule. The Lewis structure for H2O demonstrates that each hydrogen atom shares one electron with the oxygen atom, satisfying the duet rule, and the oxygen atom has 8 valence electrons, satisfying the octet rule. A hypothetical H3O molecule would not follow these rules, making it unstable. Therefore, water is H2O and not H3O or something else.

Step 1: Understanding the Lewis structures

To justify the chemical formula of water, we will use Lewis structures, which help us visually understand the bonding between atoms in a molecule. In these diagrams, the valence electrons (outer shell electrons) are represented by dots around the element symbol.

Step 2: Determining the number of valence electrons for hydrogen and oxygen

In order to draw the Lewis structure for water, we must first identify the number of valence electrons for each atom. Each hydrogen atom has 1 valence electron, while each oxygen atom has 6 valence electrons. This will help us determine how the atoms bond together.

Step 3: Drawing the Lewis structure for H2O

When creating a Lewis structure, we want to ensure that each atom achieves the desired number of valence electrons. Hydrogen requires 2 electrons to achieve a full outer shell, while oxygen requires 8 electrons. In the H2O molecule, the oxygen atom is in the center, with one hydrogen atom on each side. The oxygen atom forms 2 single covalent bonds, one with each hydrogen atom. These single bonds consist of 2 electrons each, one from the hydrogen atom and one from the oxygen atom. So, the final Lewis structure for H2O is: O / \ H H Here, the oxygen atom has 8 valence electrons (2 in each bond and 4 nonbonding electrons), satisfying the octet rule, and each hydrogen atom has 2 valence electrons, satisfying the duet rule.

Step 4: Discussing why water is H2O and NOT H3O or something else

If we were to consider an H3O molecule, an oxygen atom would need to form covalent bonds with 3 hydrogen atoms. However, oxygen only has 2 available bonding sites (it has 2 unpaired electrons from its 6 valence electrons). Forming another bond with a hydrogen atom would mean that oxygen would not follow the octet rule, as it would have 10 valence electrons. Therefore, a stable H3O molecule cannot exist. In conclusion, the Lewis structure for water, H2O, demonstrates the stable bonding of two hydrogen atoms and one oxygen atom. This stability is achieved by satisfying the octet rule for oxygen and the duet rule for hydrogen. As the bonding in H3O or any other chemical formula would not follow these rules, water is H2O and not H3O or something else.

Key Concepts

Valence Electrons

Valence electrons play a crucial role in chemical bonding and are essential for understanding Lewis structures. These electrons are the electrons located in the outermost shell of an atom. They are responsible for forming bonds with other atoms to create molecules. For instance, in water (H₂O), hydrogen atoms possess 1 valence electron each, while the oxygen atom has 6 valence electrons.

When drawing a Lewis structure, valence electrons are represented as dots surrounding the chemical symbol of the element.

• Hydrogen: 1 dot
• Oxygen: 6 dots

These dots visually indicate potential bonding opportunities between atoms. Knowing the number of valence electrons allows us to predict how atoms will interact and form various molecules by sharing or transferring these valence electrons with each other.

Octet Rule

The octet rule is a simple guideline used in chemistry to ensure atoms achieve stability in their valence shell, similar to the configuration of a noble gas. According to this rule, atoms are most stable when they have eight valence electrons.

In the case of water, the oxygen atom strives to fulfill the octet rule by obtaining a total of 8 valence electrons. It already has 6, so it shares two additional electrons—one from each hydrogen atom—through covalent bonds.

• Oxygen ends up with 2 pairs of nonbonding electrons and 2 bonding pairs, making a total of 8 electrons around it.

This fulfillment of the octet rule is one reason why water exists as H₂O rather than something like H₃O, as an extra hydrogen would add more electrons and disrupt this stable configuration.

Covalent Bonds

Covalent bonds form when two atoms share one or more pairs of valence electrons to achieve stability. For water, these bonds are particularly important.

Each hydrogen atom shares its one valence electron with the oxygen atom, and in return, oxygen shares one of its valence electrons with each hydrogen atom.

• For hydrogen, sharing means achieving a full outer shell of 2 electrons (duet rule).
• For oxygen, sharing allows it to complete its octet, fulfilling its need for stability.

The resulting covalent bonds in a water molecule are depicted in a simple Lewis structure, where the oxygen atom is in the center, bonding with each hydrogen atom on either side. This strong interaction explains the stable formation of the water molecule, H₂O, over other less stable arrangements like H₃O.