Question

The atomic mass of fluorine is \(19.00 \mathrm{amu}\), and all fluorine atoms in a naturally occurring sample of fluorine have this mass. The atomic mass of chlorine is \(35.45 \mathrm{amu}\), but no chlorine atoms in a naturally occurring sample of chlorine have this mass. Provide an explanation for the difference.

Short answer

Fluorine has only one stable isotope, which is why all fluorine atoms have the same atomic mass of 19.00 amu. In contrast, chlorine has two stable isotopes with different masses, and no single atom has the average atomic mass of 35.45 amu.

Step-by-step solution

Understanding Atomic Mass

Atomic mass, often called atomic weight, is the average mass of atoms of an element, measured in atomic mass units (amu). It takes into account the various isotopes of an element and their abundance in a naturally occurring sample.

Examining Fluorine

The atomic mass of fluorine is stated as 19.00 amu, which indicates that all fluorine atoms have a mass very close to this value. This suggests that fluorine has only one stable isotope, or very overwhelmingly one isotope that contributes to this atomic mass.

Analyzing Chlorine

Chlorine's atomic mass is 35.45 amu, which is not equal to the mass of any single isotope of chlorine. This indicates that chlorine has more than one stable isotope contributing to this average atomic mass.

Understanding Isotopes

Isotopes of an element have the same number of protons but differ in the number of neutrons. Natural chlorine has two stable isotopes, chlorine-35 (with an approximate atomic mass of 35 amu) and chlorine-37 (with an approximate atomic mass of 37 amu). Their relative abundances in nature lead to the average atomic mass of 35.45 amu.

Key concepts

Isotopes

Isotopes are variations of elements that have the same number of protons but different numbers of neutrons in their nuclei. Although these isotopes share chemical properties due to their identical number of protons, their physical properties may vary slightly because of the difference in mass.

Take chlorine as an example. Chlorine has two stable isotopes: chlorine-35 and chlorine-37. The '35' and '37’ denote the mass number, which is the sum of protons and neutrons for that isotope. Chlorine-35 has 18 neutrons (35-17=18, since chlorine has 17 protons), and chlorine-37 has 20 neutrons. Isotopes are particularly important when calculating atomic masses because their existence means that the atomic mass of an element is actually an average of the masses of all its naturally occurring isotopes, weighted by their abundance.

Atomic Mass Units (amu)

The atomic mass unit (amu) is a standardized unit of mass that quantifies mass on an atomic or molecular scale. One atomic mass unit is defined as one-twelfth the mass of a carbon-12 atom.In our context, when we speak of the atomic mass of an element like fluorine with a value of 19.00 amu, we're essentially referring to how much a typical atom of fluorine weighs compared to one-twelfth of a carbon-12 atom. This unit allows scientists to compare the mass of different atoms on a scale that makes sense, given how minuscule these masses are. The simplicity of stating that fluorine has an atomic mass of 19.00 amu belies the complex calculus that can go into determining the atomic mass of elements with multiple isotopes. When an element consists mostly of a single isotope, its atomic mass will closely approximate the mass number of that dominant isotope.

Natural Abundance of Isotopes

The term 'natural abundance' refers to the relative percentage of different isotopes of an element as they occur in nature. For instance, chlorine exists primarily as chlorine-35 and chlorine-37. However, they don't occur in equal proportions. Natural abundance affects the calculation of the average atomic mass of an element because each isotope contributes proportionately to the total mass.

For example, if there is a higher fraction of chlorine-35 in a naturally occurring sample, the average atomic mass will skew closer to 35 amu. Differential natural abundance can explain discrepancies in measured atomic mass: while all samples of fluorine may have an atomic mass of 19.00 amu due to a single or overwhelmingly predominant isotope, the atomic mass of chlorine is 35.45 amu since no single isotope solely exists or is predominant enough to solely determine the atomic mass. Weighing these proportional contributions allows chemists to ascertain the atomic mass that you'll find on the periodic table.