CHEM 6A Chapter Notes - Chapter 3: Electron Affinity, Atomic Orbital, Isoelectronicity

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of Br2 consists of three peaks. Mass (amu) Relative Size157.836 0.2569159.834 0.4999161.832 0.2432(a) What is the origin of each peak (of what isotopes does each consist)? (Enter all particles in the form AX.)mass 157.836 amu 159.834 amu 161.832 amu molecule (b) What is the mass of each isotope? (Calculate using the given masses.)lighter isotope amuheavier isotope amu(c) Determine the average molecular mass of a Br2 molecule. amu(d) Determine the average atomic mass of a bromine atom. amu(e) Calculate the abundances of the two isotopes.

Mass spectrometry 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 amu; abundance 0.0115%). (a) How many peaks will the mass spectrum have? (b) Give the relative atomic masses of each of these peaks. (c) Which peak will be the largest, and which the smallest?