Question

State whether the representative particle in each of the following substances is an atom, a formula unit, or a molecule: (a) propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (b) platinum, \(\mathrm{Pt}\) (c) hematite, \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) (d) sulfur, \(S_{8}\)

Short answer

(a) Molecule, (b) Atom, (c) Formula unit, (d) Molecule.

Step-by-step solution

Introduction to Representative Particles

The representative particle of a substance is the smallest unit that retains the chemical identity of the substance. It can be an atom, a molecule, or a formula unit, depending on the type of substance.

Analyzing Propane (C_{3}H_{8})

Propane (C_{3}H_{8}) is a molecular compound made up of carbon and hydrogen atoms bonded together. The smallest unit retaining propane's chemical identity is a molecule. Therefore, the representative particle is a molecule.

Analyzing Platinum (Pt)

Platinum (Pt) is an elemental substance consisting of one type of atom. In this case, the representative particle is an atom, as there is no molecular or compound structure involved.

Analyzing Hematite (Fe_{2}O_{3})

Hematite (Fe_{2}O_{3}) is an ionic compound consisting of iron and oxygen ions arranged in a crystal lattice. The smallest unit that retains the chemical identity of hematite is the formula unit, typically used for ionic compounds.

Analyzing Sulfur (S_{8})

Sulfur (S_{8}) is a molecular element where eight sulfur atoms are covalently bonded to form a molecule. Thus, the representative particle of sulfur is a molecule.

Key concepts

Atoms

Atoms are the basic units of matter and the defining structure of elements. Think of them as individual building blocks that compose everything around us. Atoms consist of a nucleus made of protons and neutrons, surrounded by electrons that orbit in various energy levels. These particles are charged: protons are positive, electrons are negative, and neutrons are neutral.

When we refer to a "pure element" like platinum (\( \mathrm{Pt} \)), we're talking about a substance made entirely of one type of atom. Each atom of platinum is identical, featuring the same number of protons in their nuclei. In this case, platinum does not form molecules or complex ionic structures. Therefore, its representative particle is simply the atom itself. Anytime you encounter an individual element, especially metals or noble gases, consider that each atom stands alone as its own distinct particle.

Molecules

Molecules are groups of two or more atoms chemically bonded together. These atoms can be of the same element, as in oxygen molecules (\( O_{2} \)) or different elements forming compounds, as in water (\( H_{2}O \)). Molecules are often produced through covalent bonds, where atoms share electrons to fill their outer shells and achieve stability.

Molecular compounds, like propane (\( \mathrm{C}_{3} \mathrm{H}_{8} \)) and sulfur (\( S_{8} \)), showcase these characteristics well. Propane is composed of carbon and hydrogen atoms. These atoms are arranged in a specific geometric structure that defines propane as a molecule. Similarly, sulfur is an interesting case as it exists as \( S_{8} \), meaning eight sulfur atoms are bonded in a circular formation. In both cases, the molecule is the smallest unit retaining the identity of the substance. So, in substances where atoms are bonded in specific arrangements, the representative particle is typically a molecule.

Formula Units

Formula units are specific to ionic compounds which consist of positive and negative ions. These compounds do not form discrete molecules but rather exist as lattice structures that extend in three-dimensional space.

For instance, hematite (\( \mathrm{Fe}_{2} \mathrm{O}_{3} \)) is an ionic compound made of iron (\( \mathrm{Fe}^{3+} \)) and oxygen (\( \mathrm{O}^{2-} \)) ions. These ions attract and organize in a repeated pattern, forming a crystal lattice.

Because the ions interact in such a complex structure, it's impractical to think of a molecule bearing this compound's identity. Therefore, we use a "formula unit" as the smallest quantity preserving the composition and proportion of its constituent ions. Hence, for ionic compounds like hematite, a formula unit effectively represents the compound as a whole.