In mass spectrometry, which of the following best describes a molecular ion?

• A. An ion formed by the addition of a proton to a molecule
• B. An ion formed by the fragmentation of a molecule into smaller pieces
• C. An ion formed by the removal of one electron from a molecule, resulting in a species with the same molecular mass as the original molecule
• D. An ion formed by the loss of a neutron from a molecule

Answer: (C)

Verified step by step guidance

1. Understand that in mass spectrometry, a molecular ion is the ion that represents the entire molecule with minimal fragmentation.
2. Recall that the molecular ion is typically formed when a molecule loses one electron, resulting in a positively charged ion with the same molecular mass as the original molecule.
3. Recognize that this ion is often denoted as M\u00b7+ (M with a dot and a plus sign), indicating it is a radical cation.
4. Differentiate this from other ions such as protonated molecules (which have an added proton and thus a different mass) or fragment ions (which are smaller pieces of the original molecule).
5. Conclude that the best description of a molecular ion is an ion formed by the removal of one electron from a molecule, maintaining the molecular mass.