Question

State whether the representative particle in each of the following substances is a formula unit or a molecule: (a) methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) (b) cobaltous chloride, \(\mathrm{CoCl}_{2}\) (c) acetone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (d) stannous carbonate, \(\mathrm{SnCO}_{3}\)

Short answer

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

Step-by-step solution

Understanding the Terms

First, let's clarify the terms: a 'molecule' is the smallest unit of a covalent compound which can exist independently and consists of one or more atoms bonded covalently. A 'formula unit' refers to the simplest ratio of ions in an ionic compound.

Analyze Methanol (CH3OH)

Methanol is composed of non-metal elements carbon, hydrogen, and oxygen bound together covalently. Since it is made of non-metals, methanol is a molecule.

Analyze Cobaltous Chloride (CoCl2)

Cobaltous chloride is composed of the metal cobalt and the non-metal chlorine. This combination indicates an ionic compound, making cobaltous chloride a formula unit.

Analyze Acetone (CH3COCH3)

Acetone is made of carbon, hydrogen, and oxygen, all of which are non-metals bonded covalently. Therefore, acetone is a molecule.

Analyze Stannous Carbonate (SnCO3)

Stannous carbonate includes the metal tin combined with a polyatomic ion (carbonate), indicating an ionic compound. Thus, stannous carbonate is represented by a formula unit.

Key concepts

Molecule

A molecule is a fundamental concept in chemistry. It represents the smallest unit of a covalent compound that retains all the chemical properties of that compound. Molecules consist of two or more atoms that are covalently bonded. This means the atoms share electrons to achieve stability. These can be atoms of the same element or different elements. Some key points about molecules include:

• Molecules can exist independently.
• They are typically composed of non-metal elements.
• Examples include water (H_{2}O), methane (CH_{4}), and carbon dioxide (CO_{2}).

Understanding molecules is vital because they make up most of the substances we interact with in daily life, from the air we breathe to the food we eat.

Formula Unit

In ionic compounds, the term 'formula unit' is used instead of the term 'molecule.' A formula unit refers to the simplest ratio of ions in an ionic compound. While a molecule is a distinct entity, a formula unit is more of an abstract concept that describes the composition of an ionic compound holistically. Key characteristics include:

• Formula units are used for ionic compounds, which involve metals and non-metals.
• They help to understand the proportion of different ions in an ionic structure.
• Examples of substances described by formula units include sodium chloride (NaCl) and magnesium oxide (MgO).

Recognizing the difference between molecules and formula units is essential for classifying substances correctly and understanding their structures.

Ionic Compound

Ionic compounds play a crucial role in chemistry and everyday life. These compounds are formed when metals and non-metals combine. Metals, which lose electrons, become positively charged (cations), while non-metals gain electrons, becoming negatively charged (anions). This creates electrical attraction between the ions. Characteristics of ionic compounds include:

• They have high melting and boiling points due to strong ionic bonds.
• Ionic compounds typically form crystalline solids.
• They are often soluble in water and conduct electricity when dissolved or melted.

Some typical ionic compounds include sodium chloride (NaCl) and calcium chloride (CaCl_{2}). Understanding ionic compounds helps in comprehending how many common substances are formed and function.

Covalent Compound

Covalent compounds are an essential class of chemical compounds where atoms share electrons to form molecules. This sharing of electrons is called covalent bonding. A covalent bond usually forms between non-metal atoms. As a result, covalent compounds can exist as individual molecules with specific shapes and sizes. Here's what you need to know:

• Covalent compounds can be gases, liquids, or solids at room temperature.
• They generally have lower melting and boiling points compared to ionic compounds.
• Covalent compounds do not conduct electricity, as they lack charged particles.

Some examples of covalent compounds include methane (CH_{4}) and carbon dioxide (CO_{2}). A clear understanding of covalent compounds and how they differ from ionic compounds reinforces a foundational chemical knowledge.