There are two different isotopes of bromine atoms. Under normal conditions, elem
ID: 747941 • Letter: T
Question
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 Size 157.836 0.2569 159.834 0.4999 161.832 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 amu heavier 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.Explanation / Answer
I don't fully understand your question but think of it this way. Bromine has two isotopes 79Br and 81 Br are they are roughly 50:50 in abundance. Let's just call them 79 and 81. There are three possible outcomes when you make molecular bromine. You can get two 79 atoms in which case the mass displayed would be 158 amu You can get one 79 atom and one 81 atom (order is not important) in which case the mass displayed would be 160 amu You can get two 81 atoms in which case the mass displayed would be 162 amu The probability of getting any combination is 0.25 (0.5 x 0.5) but remember the middle option can have 79+81 or 81+79 so the probability for this one is 2 x 0.25 = 0.5. So the Br2 spectrum has three peaks where the first and third are 1/2 the height of the middle one. The average atomic mass is the sum of the mass of each isotope x it's percentage abundance. So you are correct the average atomic mass for bromine is 80 amu. You can get this by using the following formula RAM = (79 x 0.5) + (81 x 0.5) = 80. The values for "avg. Br2 molecule" is just 2 x relative atomic mass - ie 160 amu I'm not sure what you mean about the value for "one Br2 molecule".