Which of the following substances is held together by metallic bonds between its atoms?

Introduction / Context:
This question focuses on recognising metallic bonding among different types of substances. Metals have a characteristic type of bonding where valence electrons are delocalised and shared among many metal atoms, forming a lattice with a sea of electrons. Identifying which formula corresponds to a metallic substance helps students differentiate between molecular compounds, ionic compounds, and metallic elements.

Given Data / Assumptions:
- The options include glucose (C6H12O6), mercury (Hg), water (H2O), and sodium chloride (NaCl).
- We assume standard bonding descriptions: metallic bonding in elemental metals, covalent bonding in molecular compounds, and ionic bonding in salts.

Concept / Approach:
Metallic bonds occur in elemental metals and alloys, where metal atoms release valence electrons into a shared electron cloud. This electron sea model explains properties such as electrical conductivity, malleability, and luster. Mercury is an elemental metal and, even though it is liquid at room temperature, its atoms are bound by metallic bonding. Glucose and water are molecular compounds held together internally by covalent bonds and between molecules by intermolecular forces. Sodium chloride is an ionic compound, where ions are held together by electrostatic attraction, not metallic bonding.

Step-by-Step Solution:
Step 1: Recognise that Hg is the chemical symbol for mercury, a metal that is liquid at room temperature but still metallic in nature.
Step 2: In mercury, positively charged Hg2 plus like centres are surrounded by a cloud of delocalised electrons, forming metallic bonds throughout the sample.
Step 3: C6H12O6 is the formula for glucose, an organic molecule whose atoms are connected by covalent bonds. Between glucose molecules, only weak intermolecular forces operate.
Step 4: H2O is water, a molecular compound formed by covalent bonds between hydrogen and oxygen. Intermolecular hydrogen bonding holds water molecules together, not metallic bonding.
Step 5: NaCl is an ionic solid where Na plus and Cl minus ions are arranged in a lattice and held together by ionic bonds, not metallic bonds.
Step 6: Therefore, the only substance in the list that contains metallic bonds is mercury, Hg.

Verification / Alternative check:
You can verify this by thinking about physical properties. Mercury is a good conductor of electricity and has metallic luster, which are typical properties of metals due to metallic bonding. Glucose and water are nonconductors in solid or pure liquid forms and do not show metallic properties. Sodium chloride conducts electricity only when molten or in solution, due to mobile ions, but in solid form it is a typical ionic crystal. These observations support the conclusion that metallic bonding occurs only in mercury among the given options.

Why Other Options Are Wrong:
- C6H12O6 (glucose): This is a covalently bonded organic molecule with no metallic bonding present.

Why Other Options Are Wrong (continued):
- H2O (water): Water molecules are held together by covalent bonds internally and hydrogen bonding between molecules, not by metallic bonding.
- NaCl (sodium chloride): This compound has an ionic lattice, with electrostatic attraction between Na plus and Cl minus ions, which is different from the electron sea model of metallic bonding.

Common Pitfalls:
Students sometimes think that any compound containing a metal symbol might have metallic bonds, but this is not correct. Once a metal forms ions and combines with nonmetals, the compound is usually ionic. Metallic bonds exist in elemental metals and alloys where metal atoms share delocalised electrons. Another confusion comes from the fact that mercury is a liquid at room temperature, leading some learners to forget that it is still a metal. Remember that being liquid does not change its bonding type; it remains metallic.

Final Answer:
Hg (mercury) is the substance whose atoms are held together by metallic bonds.