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.

Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons. This variation in the number of neutrons leads to different atomic masses. For example, carbon has two stable isotopes, carbon-12 and carbon-13. The number refers to the isotopic mass, which is roughly the sum of protons and neutrons in the atom's nucleus.

When we consider atomic mass, it’s the weighted average of these isotopes, considering how abundant each one is in nature. This average is what we typically find on the periodic table for each element. Since isotopes vary widely in their abundance, some extremely rare isotopes have a negligible impact on the average atomic mass, while the more common ones dominate the calculation. For elements with only one stable isotope such as fluorine-19, the atomic mass effectively equals the mass of that single isotope.

Mass spectrum analysis is a method used in chemistry and physics to determine the types and abundances of isotopes in a sample of an element. This is done using an instrument called a mass spectrometer, which ionizes chemical species and sorts the ions based on their mass-to-charge ratio. The peaks on a mass spectrum represent isotopes, with their height corresponding to relative abundance.

In the provided exercise, fluorine's mass spectrum showing a large peak at 18.998 amu denotes the presence of a single stable isotope, fluorine-19. Such a clear spectrum is rare, as many elements display a pattern of peaks representing multiple stable isotopes. Understanding a mass spectrum plot thus gives us key information about the isotopic composition of an element.

The concept of relative isotopic abundance relates to how common an isotope is compared to others of the same element in a given sample or in nature. It is expressed as a percentage that indicates the isotope's contribution to the total sample. For instance, if an element like chlorine has two stable isotopes, chlorine-35 and chlorine-37, present at a 3:1 ratio, chlorine-35 will have a relative isotopic abundance of 75%, and chlorine-37 will have an abundance of 25%.

Relative isotopic abundance is critical in determining the average atomic mass of an element, as by multiplying the mass of each isotope by its relative abundance and summing these products, the average mass that represents the element in nature can be calculated.

Chlorine is a common example used to explain isotopic abundance and atomic mass because it has two primary isotopes with significant natural abundances: chlorine-35 and chlorine-37. The former is more abundant at approximately 75%, and the latter makes up about 25% of all chlorine atoms. The fact that chlorine's mass spectrum doesn't show a peak at its average atomic mass of 35.45 amu is because this value is a weighted average based on the two isotopes' individual masses and their respective abundances.

The natural occurrence in a particular ratio leads to a combined average atomic mass that is between the masses of the individual isotopes. This underlines why chlorine's mass spectrum displays two distinct peaks instead of one matching the average atomic mass as in the case of fluorine, which only has one stable isotope.