Question

Why is it incorrect to use the term "molecule of \(\mathrm{NaCl}\)" but correct to use the term "molecule of \(\mathrm{H}_{2} \mathrm{O}^{\prime \prime}\) ? Is the term "molecule of diamond" correct? Explain.

Short answer

The term "molecule of NaCl" is incorrect because NaCl or sodium chloride is an ionic compound, not a molecule. It is formed by the electrostatic attraction between the sodium ion and the chloride ion, leading to a crystal lattice structure, so it should be referred to as a "formula unit of NaCl". In contrast, "molecule of H2O" is correct as water is a molecule formed by the covalent bonding of two hydrogen atoms and one oxygen atom. As for the term "molecule of diamond", it is technically incorrect. A diamond is a form of pure carbon, where each carbon atom is covalently bonded to four other carbon atoms in a 3-dimensional structure, known as a macromolecule. Since there is no fixed number of atoms in the unit, and the entire structure is a single covalently bonded unit, it is more appropriate to describe it as a "crystalline structure" or a "macromolecule" of carbon.

Step-by-step solution

Understand Molecules and Ionic compounds

Molecules are formed by the covalent bonding of two or more non-metal atoms. In covalent bonding, atoms share electrons with each other to complete their outer electron shells. Ionic Compounds, on the other hand, are formed by the electrostatic attraction between positive ions (cations) and negative ions (anions) resulting in the formation of a crystal lattice structure. In this type of bonding, metallic atoms transfer electrons to non-metallic atoms, becoming positively charged ions. The non-metallic atoms gain electrons and become negatively charged ions.

Explain why "molecule of NaCl" is incorrect

NaCl (sodium chloride) is an ionic compound, not a molecule, as it is formed by the electrostatic attraction between the sodium ion (Na+) and the chloride ion (Cl-). It forms a crystal lattice structure where each ion is surrounded by six oppositely charged ions. Therefore, it is incorrect to refer to it as a molecule of NaCl. Instead, it should be referred to as a formula unit of NaCl.

Explain why "molecule of H2O" is correct

Water (H2O) is a molecule formed by the covalent bonding of two hydrogen atoms and one oxygen atom. The oxygen atom shares a pair of electrons with each hydrogen atom. Since the atoms are bonded together by sharing electrons, it is a molecule. Thus, it is correct to use the term "molecule of H2O."

Determine if "molecule of diamond" is correct

A diamond is a form of pure carbon, where each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This forms a giant covalent structure, also known as a macromolecule, that extends indefinitely in three dimensions. Since there is no fixed number of atoms in the unit, and the entire structure is a single covalently bonded unit, it is technically incorrect to refer to it as a "molecule of diamond." Instead, it is more appropriate to describe it as a "crystalline structure" or a "macromolecule" of carbon.

Key concepts

Ionic Compounds

Ionic compounds are fascinating structures where metals and non-metals come together. They form due to a significant difference in electronegativity between the atoms involved. When a metal atom meets a non-metal atom, the metal tends to lose one or more electrons, becoming a positively charged ion, or cation. The non-metal, in turn, gains these electrons to become a negatively charged ion, or anion.

These oppositely charged ions are strongly attracted to one another by electrostatic forces. This attraction results in the formation of a crystal lattice structure, which is a repeating pattern of ions. Each ion is surrounded by ions of the opposite charge, creating a stable structure.

• Examples include sodium chloride ( NaCl ), magnesium oxide ( MgO ), and calcium fluoride ( CaF_2 ).
• Are typically solid at room temperature and have high melting and boiling points.

Understanding ionic compounds is key to understanding the incorrect usage of terms like "molecule of NaCl," since they do not form distinct molecules.

Covalent Bonding

Covalent bonding is the glue that holds atoms together by sharing electrons. This type of bonding usually occurs between non-metal atoms. Unlike ionic bonds, which involve the transfer of electrons, covalent bonds involve shared electrons to achieve full outer shells, usually completing the octet rule.

Atoms bond covalently to increase stability and decrease potential energy. The shared pair of electrons travels in the orbitals of both bonded atoms. This sharing creates a powerful connection, forming molecules. This is why water ( H_2O ) is a molecule, as it consists of hydrogen and oxygen atoms sharing electrons.

• Examples of covalent bonds include water ( H_2O ), methane ( CH_4 ), and carbon dioxide ( CO_2 ).
• Covalent compounds can exist in all three physical states: solid, liquid, or gas.

Covalent bonds are central to molecular chemistry, offering insights into why terms like "molecule of diamond" can be misleading.

Crystal Lattice Structure

When discussing ionic compounds, the crystal lattice structure is a crucial concept to understand. This term refers to the highly ordered, three-dimensional arrangement of ions within a solid. Each ion in a crystal lattice is surrounded by ions of the opposite charge, forming a stable and rigid structure.

In substances like sodium chloride ( NaCl ), each sodium ion is surrounded by six chloride ions, and vice versa. This extensive lattice is held together by strong ionic bonds, giving ionic compounds high melting and boiling points. The crystal lattice structure is also why ionic compounds tend to be hard and brittle.

• Crystal lattices result in distinctive properties such as high stability and density.
• The lattice maximizes the attractive forces while minimizing repulsive forces within the solid.

The concept also highlights why "molecule of NaCl" is an incorrect term, as NaCl's formation does not result in discrete molecules but in a lattice.

Molecules

Molecules are the smallest units of a compound that retain the chemical properties of that compound. They consist of two or more atoms bonded covalently—this means the atoms share electrons. Molecules can be as simple as diatomic oxygen ( O_2 ) or as complex as a protein or DNA.

In molecular substances, the distinct number of atoms form specific discrete structures. For instance, each molecule of water ( H_2O ) is composed of two hydrogen atoms and one oxygen atom, bonded covalently. Molecules can exist independently and can interact with other molecules in various ways, like hydrogen bonding or Van der Waals forces.

• Molecules are versatile and can exist in various physical states depending on the conditions.
• They are essential to both organic and inorganic chemistry, playing crucial roles in chemical reactions.

Unlike ionic compounds with lattice structures, molecules exist distinctly, which affirms the correct usage of terms like "molecule of H_2O."

Macromolecules

Macromolecules are giant molecules that often consist of thousands of atoms. Unlike conventional molecules, which contain a definite number of atoms, the structure of macromolecules can be quite variable. They are primarily formed through covalent bonding but can create incredibly large networks.

Macromolecules include biological polymers like proteins, nucleic acids, and synthetic polymers like plastics. Diamond is a classic example of a macromolecular structure, where each carbon atom is bonded to four other carbon atoms in a continuous network. This creates a robust, stable lattice that is immensely strong and inflexible.

• Macromolecules have unique properties, such as high mechanical strength, resilience, and thermal stability.
• They are fundamental to both biology and materials science.

Due to their extensive and continuous structure, terms like "molecule of diamond" are not technically correct. Instead, they are better described as crystalline or giant macromolecular structures.