Which of the following elements does NOT exist as a diatomic molecule in its standard elemental form?

• A. Oxygen (O_2)
• B. Chlorine (Cl_2)
• C. Helium (He)
• D. Nitrogen (N_2)

Answer: (C)

Verified step by step guidance

1. Understand that diatomic molecules are molecules composed of two atoms of the same element bonded together. Common diatomic molecules in their elemental form include O_2, N_2, and Cl_2.
2. Recall that elements in the noble gas group, such as Helium (He), exist as single atoms (monatomic) rather than bonded pairs because they have a full valence shell and are chemically stable on their own.
3. Identify that Oxygen (O_2), Chlorine (Cl_2), and Nitrogen (N_2) are all well-known diatomic molecules under standard conditions, meaning they naturally exist as pairs of atoms.
4. Recognize that Helium (He) does not form diatomic molecules; it exists as individual atoms due to its inert nature and complete electron shell.
5. Conclude that among the given options, Helium (He) is the element that does NOT exist as a diatomic molecule in its standard elemental form.