Question

State whether the representative particle in each of the following substances is an atom, a molecule, or a formula unit: (a) uranium, U (b) fluorine, ${\mathrm{F}}_2$ (c) uranium hexafluoride, UF ${}_6$ (d) hydrogen fluoride, HF

Short answer

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

Step-by-step solution

Understanding Elements and Compounds

Identify whether the substance is an element or a compound. Elements are represented by a single chemical symbol, and compounds are represented by a combination of chemical symbols.

Identifying Atoms

An atom is the smallest unit of an element. Single chemical symbols, like uranium (U) in options without any subscript or combination with other elements, represent atoms.

Identifying Molecules

A molecule is a group of two or more atoms chemically bonded together. Molecules consist of two or more of the same or different nonmetals. For example, fluorine, ${\mathrm{F}}_2$, represents a diatomic molecule.

Identifying Formula Units

A formula unit refers to the basic unit of an ionic compound. It represents the lowest whole number ratio of ions in the compound. Compounds like uranium hexafluoride (UF${}_6$) or hydrogen fluoride (HF) contain both a metal or metalloid and a nonmetal, typical of formula units; however, HF is a covalently bonded molecule and not an ionic compound.

Key concepts

Atoms

Atoms are the building blocks of matter and the fundamental unit of an element. They are composed of protons, neutrons, and electrons. Each atom is defined by its atomic number, which is the number of protons in its nucleus.
When we look at chemical substances, those that are represented by single chemical symbols in the periodic table, such as uranium (U), are considered atoms.
Here’s why atoms are important:

• They define the element and its position in the periodic table.
• Atoms can join together to form molecules or compounds by exchanging electrons.
• Atoms determine the chemical properties of an element.

An understanding of atoms is crucial for tracing the origins of any chemical reaction and predicting the behavior of elements in different situations.

Molecules

A molecule is a collection of two or more atoms bonded together, which can be of the same or different elements. Molecular bonds occur through the sharing of electrons, primarily involving nonmetals.
For example, when considering fluorine, ${\mathrm{F}}_2$, it consists of two fluorine atoms sharing electrons, forming a diatomic molecule.
Molecules can be classified as:

• Diatomic Molecules: These have two atoms, either of the same element, like ${\mathrm{F}}_2$, or different elements.
• Polyatomic Molecules: These consist of three or more atoms, which may involve complex bonding structures.

Understanding molecules helps us to grasp how substances interact in nature and the mechanisms behind chemical reactions.

Formula Units

A formula unit is the simplest representation of an ionic compound. It shows the lowest whole number ratio of positively and negatively charged ions in an ionic substance.
Unlike molecules, formula units refer to compounds that involve metals reacting with nonmetals, forming ionic bonds.
Key points about formula units:

• They are applicable to ionic compounds rather than molecular substances.
• Ionic compounds like uranium hexafluoride (UF${}_6$) illustrate formula units, though some compounds like hydrogen fluoride (HF) are covalently bonded and thus molecular.
• They underline the essential ratio concept in balancing and analyzing chemical equations.

By understanding formula units, we can predict the structure and stability of ionic compounds.