Problem 9 - Anatomy and Physiology of Animals

Question

Which of the following is a molecule, but not a compound? a. \(\mathrm{H}_{2} \mathrm{O}\) b. \({ }^{2} \mathrm{H}\) c. \(\mathrm{H}_{2}\) d. \(\mathrm{H}^{+}\)

Step-by-Step Solution

Answer

The answer is c: \(\mathrm{H}_{2}\).

Step 1: Define a Molecule and a Compound

A molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction. A compound is a molecule that contains at least two different types of elements.

Step 2: Analyze Each Option

Evaluate each given option:- Option a: \(\mathrm{H}_{2}\mathrm{O}\) (water) is made of two hydrogen and one oxygen atom. It is a compound because it has more than one type of element.- Option b: \({}^{2}\mathrm{H}\) is a form of hydrogen, also known as deuterium. It's not a molecule but rather an isotope. - Option c: \(\mathrm{H}_{2}\) is made of two hydrogen atoms. It's a molecule because it contains bonded atoms, but it is not a compound as it consists of only one type of element.- Option d: \(\mathrm{H}^{+}\) is a hydrogen ion, not a molecule.

Step 3: Identify the Correct Answer

From the analysis, \(\mathrm{H}_{2}\) is a molecule because it is formed by two atoms bonded together, yet it is not a compound because it contains only one element, hydrogen.

Key Concepts

Chemical Compound

A chemical compound is a substance formed when two or more different elements are chemically bonded together. This bond creates a stable unit that has unique properties distinct from the individual elements that compose it. For example, water (\(\mathrm{H}_2\mathrm{O}\)) is a chemical compound because it consists of hydrogen and oxygen elements bonded together.

• Compounds always have fixed ratios of elements, meaning that a water molecule always contains two hydrogen atoms for every oxygen atom.
• These bonds can be covalent, ionic, or metallic depending on how the electrons are shared or transferred between the atoms.

Chemical compounds have their own specific chemical formulas that represent the types and numbers of atoms involved. This makes them fundamental to the study and understanding of chemistry.

Elements

Elements constitute the basic building blocks of all matter. Each element is a pure substance that consists of only one type of atom. For example, a gold bar is made up purely of gold atoms.

There are currently 118 known elements, each with unique properties and characteristics. They are organized in the periodic table, which helps to categorize them based on shared properties and atomic structure.

• Metals, like iron and copper, are typically shiny and good conductors of electricity.
• Nonmetals, such as carbon and oxygen, are generally poor conductors of electricity.
• Metalloids have mixed properties that can resemble both metals and nonmetals.

Understanding elements is essential for studying how they combine to form the more complex structures of molecules and compounds.

Isotope

Isotopes are variants of the same chemical element that have identical numbers of protons but different numbers of neutrons in their nuclei. This difference in neutron count gives isotopes distinct atomic masses.

An element can have several isotopes, and these variations can lead to different nuclear properties.

• For instance, hydrogen has three main isotopes: protium (\({}_1^1\mathrm{H}\)), deuterium (\({}_1^2\mathrm{H}\)), and tritium (\({}_1^3\mathrm{H}\)).
• Each isotope of an element demonstrates slightly different behavior, which can be useful in areas such as radiometric dating, medicine, and nuclear power.

Despite differing neutron numbers, isotopes of the same element exhibit nearly identical chemical behavior due to their unchanged electron configuration.

Hydrogen Molecule

A hydrogen molecule, represented as \(\mathrm{H}_2\), consists of two hydrogen atoms bonded together. Despite being a molecule, it is not considered a chemical compound.

• This is because a compound requires at least two different types of elements, and \(\mathrm{H}_2\) contains only hydrogen.
• The bond holding the hydrogen atoms together is known as a covalent bond, sharing electron pairs to achieve stability.

Hydrogen molecules are fundamental in both chemical and biological processes.

• They are the simplest molecules, having significant roles in fields such as chemistry and energy production.
• Hydrogen itself is the most abundant element in the universe, mostly in diatomic form, contributing to numerous reactions including those in the stars and industrial applications such as fuel cells.