Problem 19 - Chemistry: Atoms First

Question

How are the molecules in oxygen gas, the molecules in hydrogen gas, and water molecules similar? How do they differ?

Step-by-Step Solution

Answer

Oxygen gas (O2) and hydrogen gas (H2) are diatomic molecules formed from the same atom types, while water (H2O) is a compound of different atoms. O2 and H2 are nonpolar and gases at room temperature, whereas H2O is a polar molecule and a liquid.

Step 1: Understanding Molecular Composition

The first step is to understand the molecular composition of each substance. Oxygen gas (O2) consists of two oxygen atoms bonded together. Hydrogen gas (H2) consists of two hydrogen atoms bonded together. Water (H2O) consists of two hydrogen atoms and one oxygen atom bonded together.

Step 2: Identifying Similarities

Identify similarities among the molecules. All three are molecules made up of non-metal atoms and are held together by covalent bonds. Furthermore, they are all diatomic molecules (O2 and H2) or formed from diatomic molecules (in the case of H2O).

Step 3: Identifying Differences

Next, pinpoint the differences. Oxygen and hydrogen gases are composed of the same type of atoms bonded together, while water is a compound composed of two different types of atoms. Oxygen and hydrogen are both gases at room temperature, while water is a liquid. Additionally, water is a polar molecule due to the difference in electronegativity between hydrogen and oxygen atoms, while O2 and H2 are nonpolar.

Key Concepts

Diatomic Molecules

In the study of chemistry, certain elements are known as diatomic molecules, which means they consist of two atoms. These atoms are not necessarily identical, but in the context of oxygen (O₂) and hydrogen (H₂) gases, they are. Oxygen gas is made of two oxygen atoms bonded together, and similarly, hydrogen gas consists of two hydrogen atoms. The diatomic nature of these gases is significant as it is the most stable form of these elements under standard conditions.

Think of diatomic molecules as a pair of dancers holding hands—they move together as a unit. This unity is due to the strong covalent bonds, which we'll discuss later. It's crucial for students to understand that while both O₂ and H₂ are diatomic, they differ from water molecules, H₂O, which is a combination of atoms from two different elements forming not a diatomic molecule, but a compound.

Covalent Bonds

Focusing on the bonds that bring elements together in molecules, we're delving into covalent bonds. These bonds are like the glue that holds atoms in a molecule tightly together. Covalent bonds form when two atoms share one or more pairs of electrons. Oxygen gas (O₂), hydrogen gas (H₂), and water (H₂O) all share the characteristic of being held together by covalent bonds.

Imagine two friends who are sharing toys equally; similarly, in a covalent bond, the electrons are shared between atoms, allowing each atom to achieve a stable electron configuration. This sharing of electrons is what gives molecules their shape and stability. Diatomic molecules, in particular, often share electrons in a simple and straightforward manner, leading to the formation of identical or nearly identical covalent bonds, as seen in O₂ and H₂.

Polar and Nonpolar Molecules

When it comes to molecular polarity, we're considering whether a molecule has a positive and negative side, much like a magnet. Polar molecules, like water (H₂O), have an uneven distribution of electrons, leading to one side of the molecule having a slight negative charge and the other a slight positive charge. This is because the oxygen atom in water has a higher electronegativity, meaning it pulls the shared electrons closer to itself, compared to the hydrogen atoms.

In contrast, nonpolar molecules, like oxygen gas (O₂) and hydrogen gas (H₂), have an even distribution of electrons. This happens when atoms with similar electronegativity share electrons, resulting in a molecule that doesn’t have distinct poles. Nonpolar molecules don’t mix well with polar molecules because they lack this uneven charge that can create an attraction to polar molecules. Understanding the difference between polar and nonpolar molecules helps explain why substances like oil (nonpolar) and water (polar) do not mix.