The atomic mass of fluorine is 18.998 amu, and its mass spectrum shows a large peak at this mass. The atomic mass of chlorine is 35.45 amu, yet the mass spectrum of chlorine does not show a peak at this mass. Explain the difference.

Key Concepts

Atomic Mass and Isotopes

Atomic mass is the weighted average mass of an element's isotopes, measured in atomic mass units (amu). Fluorine has a single stable isotope, which corresponds closely to its atomic mass of 18.998 amu, resulting in a prominent peak in its mass spectrum. In contrast, chlorine has multiple isotopes, primarily chlorine-35 and chlorine-37, which leads to a distribution of peaks rather than a single peak at its average atomic mass.

Mass Spectrum Interpretation

A mass spectrum displays the relative abundance of ions as a function of their mass-to-charge ratio (m/z). Peaks in the spectrum correspond to specific isotopes or molecular fragments. For chlorine, the presence of multiple isotopes results in several peaks, reflecting the different masses of its isotopes, rather than a single peak at the average atomic mass.

Relative Abundance of Isotopes

The relative abundance of isotopes affects the appearance of a mass spectrum. Chlorine's atomic mass of 35.45 amu indicates a mixture of isotopes, with chlorine-35 being more abundant than chlorine-37. This abundance ratio leads to distinct peaks in the mass spectrum, rather than a single peak at the average atomic mass, as seen with fluorine.