In basic covalent bonding, which of the following diatomic molecules is held together by a double covalent bond between its two atoms?

Introduction / Context:
Diatomic molecules consist of two atoms chemically bonded together. Depending on the number of shared electron pairs between the atoms, these molecules may contain single, double, or triple covalent bonds. Knowing the type of bond in common diatomic molecules such as Cl2, O2, and N2 helps in understanding bond strength, bond length, and reactivity. This question asks you to identify which molecule specifically has a double covalent bond.

Given Data / Assumptions:

• The molecules listed are Cl2, O2, N2, and He2.
• Covalent bonds involve sharing of electron pairs between atoms.
• Single bonds share one pair, double bonds share two pairs, and triple bonds share three pairs of electrons.
• We assume ground state, stable molecular forms as commonly taught in school chemistry.

Concept / Approach:
Chlorine (Cl2) is a diatomic molecule in which each chlorine atom shares one electron with the other, forming a single covalent bond. Oxygen (O2) has a double bond: two pairs of electrons are shared between the oxygen atoms, resulting in a typical O=O bond. Nitrogen (N2) is bound by a triple bond, sharing three electron pairs, making it very stable and relatively inert. Helium is a noble gas that does not readily form stable diatomic molecules like He2 under normal conditions. Hence, among the given options, O2 is the molecule that has a double covalent bond between its atoms.

Step-by-Step Solution:
Step 1: Consider chlorine, Cl2. Each chlorine atom has 7 valence electrons and shares 1 electron with the other, forming a single bond Cl–Cl. Step 2: Consider oxygen, O2. Each oxygen atom has 6 valence electrons and needs 2 more to complete an octet; they share 2 pairs, forming a double bond O=O. Step 3: Consider nitrogen, N2. Each nitrogen atom has 5 valence electrons and forms a triple bond N≡N by sharing 3 pairs of electrons. Step 4: Consider helium, He2. Helium already has a stable duet of electrons; it does not need to form bonds, and He2 is not a stable covalent molecule under normal conditions. Step 5: Select the molecule with a double bond: O2.

Verification / Alternative check:
Lewis dot structures and valence bond diagrams consistently show O2 as having a double bond between the two oxygen atoms, with each atom achieving an octet. Bond order tables list O2 with bond order 2, N2 with bond order 3, and Cl2 with bond order 1. Spectroscopic and structural data, such as bond lengths and energies, further support this: the O=O double bond is shorter and stronger than a single bond but weaker than the N≡N triple bond. Helium, being a noble gas with a filled valence shell, does not form such bonds. This evidence clearly confirms that O2 is the double bonded molecule among the choices.

Why Other Options Are Wrong:
Option A (Cl2): Chlorine forms a single covalent bond between its atoms, not a double bond. Option C (N2): Nitrogen forms a triple covalent bond, one of the strongest covalent bonds known. Option D (He2): Helium does not form a stable diatomic molecule under normal conditions; it remains monatomic due to its full valence shell.

Common Pitfalls:
A frequent mistake is to assume that all diatomic molecules have single bonds, which is not true for O2 and N2. Another confusion comes from misremembering whether oxygen or nitrogen has the triple bond. A useful memory aid is that N2, with 5 valence electrons, needs to share 3 pairs to reach an octet, leading to a triple bond, while O2, with 6 valence electrons, needs to share 2 pairs, leading to a double bond. Keeping this valence electron counting method in mind helps avoid such errors.

Final Answer:
The diatomic molecule that is joined by a double covalent bond is O2.