Question

State whether the representative particle in each of the following substances is an atom, a molecule, or a formula unit: (a) plutonium, $\mathrm{Pu}$ (b) oxygen, ${\mathrm{O}}_2$ (c) plutonium(III) oxide, ${\mathrm{Pu}}_2{\mathrm{O}}_3$ (d) hydrogen peroxide, ${\mathrm{H}}_2{\mathrm{O}}_2$

Short answer

a) Atom b) Molecule c) Formula Unit d) Molecule

Step-by-step solution

Identify Particles of Elements

For pure elements, the representative particle is typically an atom. Therefore, for plutonium (466), the representative particle is an atom.

Molecular Substances

A molecule is a representative particle for molecular substances composed of two or more atoms bonded together chemically. Oxygen (
f466466) is a diatomic molecule, so its particle is a molecule.

Identify Formula Units in Ionic Compounds

A formula unit is the representative particle for ionic compounds, which consist of a metal and a non-metal. Plutonium(III) oxide (
f466466466) is an ionic compound, so its representative particle is a formula unit.

Molecular Substances (Continued)

For molecular compounds like hydrogen peroxide (
f466466466466), the representative particle is a molecule, as it consists of covalently bonded atoms.

Key concepts

Atoms

Atoms are the basic units of chemical elements. They represent the smallest component of a substance that retains the chemical properties of that element. Each atom consists of a nucleus surrounded by electrons. Atoms are the building blocks of matter and can exist on their own or in groups to form more complex particles.
When we talk about representative particles in a pure element, typically, the particle is an atom. For example, plutonium (Pu) is a pure element, and its representative particle is an atom. This is because plutonium consists solely of plutonium atoms without any chemical bonding between them.
Atoms as individual entities are crucial in chemical reactions. They can combine through various types of bonds to form molecules or ionic compounds. When understanding chemical equations and reactions, identifying the number and types of atoms involved is essential.

Molecules

Molecules are formed when two or more atoms bond together chemically. They are the smallest unit of a chemical compound that can participate in a chemical reaction. Molecules can be simple, like hydrogen gas ($${\mathrm{H}}_2$$), or complex, like DNA.
A molecule is the representative particle for molecular substances. For example, oxygen ($${\mathrm{O}}_2$$) is a diatomic molecule. This means it naturally exists as a pair of oxygen atoms bonded together. Similarly, hydrogen peroxide ($${\mathrm{H}}_2{\mathrm{O}}_2$$) is a molecular compound with two hydrogen atoms and two oxygen atoms covalently bonded.

• Molecules are essential in studying chemical reactions because they often determine the properties of a substance.
• They help us understand how different substances interact at the atomic level.

Recognizing molecules as representative particles helps students conceptualize how atoms join to create the substances we study in chemistry.

Formula Units

Formula units describe the representative particles in ionic compounds. These particles are not molecules, because they do not form discrete units like molecular compounds. Instead, ionic compounds form a three-dimensional lattice structure of ions.
For example, plutonium(III) oxide ($${\mathrm{Pu}}_2{\mathrm{O}}_3$$) is an ionic compound. Here, plutonium and oxygen ions are arranged in a repeating pattern that balances the overall charge to form a stable compound. The formula unit represents the simplest whole number ratio of ions present.
In ionic compounds:

• Metals typically lose electrons to form positive ions, or cations.
• Non-metals typically gain electrons to form negative ions, or anions.

Understanding formula units helps us analyze the composition and properties of ionic substances, explaining how different ions interact to form electrically neutral compounds. This knowledge is essential for predicting compound behavior and properties.