Massspectrometry is more often applied to molecules than to atoms. We will see in Chapter 3 that the molecular weight of a molecule is the sum of the atomic weights of the atoms in the molecule. The mass spectrum of H₂ is taken under conditions that prevent decomposition into H atoms. The two naturally occurring isotopes of hydrogen are ¹H (atomic mass = 1.00783 amu; abundance 99.9885%) and ²H (atomic mass = 2.01410; abundance 0.0115%). (c) Which peak will be the largest, and which the smallest?

Verified step by step guidance

1. Step 1: Understand the problem. We are asked to determine which peak in the mass spectrum of H2 will be the largest and which will be the smallest. The mass spectrum is a graph plotting the relative abundance of different isotopes against their mass-to-charge ratio. In this case, we are dealing with two isotopes of hydrogen: 1H and 2H.
2. Step 2: Consider the abundance of each isotope. The abundance of 1H is 99.9885% and the abundance of 2H is 0.0115%. This means that in a sample of hydrogen, the vast majority of atoms will be 1H.
3. Step 3: Consider the possible combinations of isotopes in a H2 molecule. There are three possibilities: two 1H atoms, one 1H and one 2H atom, or two 2H atoms.
4. Step 4: Determine the relative abundance of each combination. The combination of two 1H atoms will be the most common, because 1H is the most abundant isotope. The combination of one 1H and one 2H atom will be less common, and the combination of two 2H atoms will be the least common, because 2H is the least abundant isotope.
5. Step 5: Conclude that the peak corresponding to the combination of two 1H atoms will be the largest, because this combination is the most common. The peak corresponding to the combination of two 2H atoms will be the smallest, because this combination is the least common.

Key Concepts

Molecular Weight

Molecular weight is the sum of the atomic weights of all the atoms in a molecule. It is a crucial concept in mass spectrometry, as it helps determine the mass-to-charge ratio of ions produced from the molecule. Understanding how to calculate molecular weight allows for the interpretation of mass spectra and the identification of molecular species.

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. In the case of hydrogen, the two isotopes are protium (¹H) and deuterium (²H). The relative abundance of these isotopes affects the peaks observed in a mass spectrum, as each isotope contributes differently to the overall mass of the molecule.

Mass Spectrum

A mass spectrum is a graphical representation of the mass-to-charge ratios of ions produced in a mass spectrometer. Each peak in the spectrum corresponds to a specific ion, with the height of the peak indicating the relative abundance of that ion. In the context of the question, the largest peak will correspond to the most abundant isotope, while the smallest peak will represent the less abundant isotope.