Question

State whether the representative particle in each of the following substances is an atom, a formula unit, or a molecule: (a) xenon, Xe (b) phosphorus, \(\mathrm{P}_{4}\) (c) xenon trioxide, \(\mathrm{XeO}_{3}\) (d) magnetite, \(\mathrm{Fe}_{3} \mathrm{O}_{4}\)

Short answer

(a) atom; (b) molecule; (c) molecule; (d) formula unit.

Step-by-step solution

Understanding Representative Particles

Before we identify the representative particles, we need to understand that in chemistry, substances are represented by atoms, molecules, or formula units. An atom is the smallest unit of an element, a molecule is a group of atoms bonded together representing the smallest unit of a chemical compound, and a formula unit is the empirical formula of an ionic compound representing the smallest ratio of ions.

Identifying Representative Particles of Elements

For elements that exist as single atoms, like noble gases, the representative particle is an atom. For elements that exist as multi-atomic entities, like phosphorus ( P_4 ), the representative particle is a molecule.

Identifying Xenon as an Atom

(a) Since xenon, Xe , is a noble gas and exists as individual atoms, the representative particle is an atom.

Identifying Phosphorus as a Molecule

(b) Phosphorus is represented as P_4 , which is a molecular entity made of four phosphorus atoms joined together. Therefore, the representative particle is a molecule.

Identifying Xenon Trioxide as a Molecule

(c) Xenon trioxide, XeO_3 , is a covalent compound formed by xenon and oxygen atoms. The smallest particle maintaining the properties of xenon trioxide is a molecule, so the representative particle is a molecule.

Identifying Magnetite as a Formula Unit

(d) Magnetite, Fe_3O_4 , is an ionic compound made of iron and oxygen. For ionic compounds, the representative particle is a formula unit representing the smallest whole-number ratio of ions, so the representative particle of magnetite is a formula unit.

Key concepts

Understanding Atoms as Representative Particles

Atoms are the fundamental building blocks of matter and represent the smallest unit of an element that retains the chemical properties of that element. Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons. In the context of representative particles:

• An atom is used as the representative particle for elements that exist naturally in their simplest forms as individual units.
• Noble gases, such as xenon (Xe), are prime examples where the representative particle is an atom.
• These elements are stable enough to exist independently without forming bonds under normal conditions.

Understanding atoms as representative particles helps in recognizing elements like xenon, where each atom stands alone as a particle of the element.

The Role of Molecules in Chemical Compounds

Molecules are entities comprising two or more atoms bonded together, representing the smallest physical unit of a chemical compound that maintains its chemical properties. When identifying molecules as representative particles:

• It refers to substances that are made or exist primarily as covalently bonded entities.
• Substances like phosphorus (\( \mathrm{P}_4 \)) and xenon trioxide (\( \mathrm{XeO}_3 \)) are considered molecules because they exist as specific groups of atoms bonded covalently.
• The molecule is the simplest unit that retains all the chemical characteristics of the compound.

By focusing on how atoms bond to form molecules, one can better understand and identify molecular substances.

Formula Units in Ionic Compounds

Formula units refer to the simplest ratio of the ions in an ionic compound, representing its simple whole-number stoichiometric composition. When dealing with formula units as representative particles:

• They are used for ionic compounds where individual molecules do not exist. Instead, these compounds form extensive lattices of ions.
• Magnetite (\( \mathrm{Fe}_3\mathrm{O}_4 \)) is a classic example where the representative particle is a formula unit, indicating the smallest ratio of iron ions to oxygen ions in the structure.
• Ionic compounds do not form discrete molecules but rather large networks held together by ionic bonds.

Understanding formula units is crucial for appreciating the structure of ionic compounds and their representation in chemical formulas.