Isotopes Exercise

In this lab you will analyze mass spectrometer data for a series of atoms and molecules. Although atoms have only one atomic number, they may have more than one atomic weight. Isotopes are atoms that have the same atomic number but different mass numbers. The atomic weight of an element is the weighted average of the exact masses of the naturally occurring isotopes. Each isotopic mass is multiplied by the fractional abundance of the isotope, and each result must be added to the others.

Example:

Atomic weight of B =

(Mass of ¹⁰B) x (Abundance of ¹⁰B) + (Mass of ¹¹B) x (Abundance of ¹¹B) or

(10.01)x(0.199) + (11.01)x(0.801) = 10.81 Atomic Mass Units (Amu)

 

Answer the following questions.

 

1. Suppose the mass spectrum of a hypothetical monatomic element X contains a signal at mass number 14 and another at mass number 16.
   16. Sketch the mass spectrum assuming the signal at mass number 14 is three times the height of the signal at 16. (You can make your sketch on paper, take a picture on your phone and paste the picture

 

1. How many isotopes are present? Why?
2. What are the fractional abundances of the isotopes?
3. Consider the mass spectrum of neon, Figure 1.

Figure 1. Mass spectrum of neon.

The mass spectrum consists of a signal a major signal at 20 amu, a low abundance signal at 21 amu, and another signal at 22. The heights of these signals are proportional to the number of counts of each mass number and represent the natural abundances of the isotopes. Determine the fractional abundance of each neon isotope from the mass spectrum in Figure 1 and calculate the atomic mass for this element.

 

Mass Number	Measurement with Units	Abundance

 

Atomic Mass (Calculated)    ____________   Atomic Mass (Periodic Table)__________

 

Procedure

 

Using a metric ruler, measure the peak height of each isotope and calculate the abundance of each atomic isotope for mercury molybdenum and bromine.

 

Make sure to include units with all measurements and the correct precision of the measuring device.

 

1. Mercury

 

Figure 2. Mass spectrum of mercury.

Mass Number	Measurement (cm)	Abundance

 

2. Molybdenum

 

Figure 3. Mass spectrum of Molybdenum.

 

Mass Number	Measurement (cm)	Abundance

 

3. Bromine, Br₂
   Consider the fragmentation of diatomic compounds. For example, ¹⁵⁸Br₂ will give a signal at 79 for ⁷⁹Br and one at 79 for ⁷⁹Br. Br has two naturally occurring isotopes, the 79 isotope and the 81 isotope. Determine the origin (formula) of each of the signals for the mass spectrum of molecular bromine. Example, ¹⁵⁸Br₂ is equal to ⁷⁹Br⁷⁹Br.

 

Figure 4. Mass Spectrum of Br₂.

 

Mass Number	Measurement (cm)	Abundance	Formula
	XXXX	XXXX
	XXXX	XXXX
	XXXX	XXXX

 

Calculations

 

1. Calculate the atomic masses of mercury molybdenum and bromine from your data.

 

2. Compare your results to the actual atomic weights of these elements found on the periodic table. Calculate the percent difference for each.

Discussion Question

1. Why does the mass spectrum of Br₂ contain three signals whose heights are approximately in the ratio 1:2:1? What are the origins of these signals?