Overhead Isotope Detector
Publication No. 12083
With few exceptions, the atomic mass listed for each element in the periodic table is an average mass and does not represent the mass of an atom of that element. This is due to the existence of isotopes. Most elements exist in nature in the form of two or more different isotopes. What are isotopes and how can they be detected?
In 1900, the number of known elements was approximately 80, and they were arranged in rows and columns in the periodic table in order of increasing atomic mass. The prevailing theory at the time stated the periodic properties of elements were a function of their atomic masses. Unfortunately, this led to contradictions in the placements for some elements, notably iodine after the heavier tellurium, and argon before the lighter potassium. In 1913, the English physicist Henry G. J. Moseley, using X-ray diffraction data, showed that atomic number, not mass, determined the properties of elements. But the larger question still remained—Why aren’t the atomic masses of elements always consistent with their corresponding atomic numbers?
Also in 1913, the English physicist J. J. Thomson began experimenting with positively charged beams of ionized neon gas, bending the ion stream in a magnetic field and letting the deflected ions strike a photographic plate. Two distinct patches appeared on the plate, leading Thomson to conclude that neon consisted of atoms with two unique masses, or isotopes. The existence of isotopes, that is, atoms of the same element with the same atomic number, but different masses, explained the apparent anomalies inherent in the atomic mass–based periodic table. It was left to the discovery of the neutron to explain the existence of isotopes.
The purpose of this demonstration is to simulate the separation of charged isotopes in a magnetic field using steel spheres and a magnet. Steel spheres of different masses will be separated based on the degree to which they are deflected when passing over a magnet. The demonstration is performed on an overhead projector stage. Based on the separation, students will determine the mass ratios of the steel sphere “isotopes” and the average atomic mass of the spheres.
(for each demonstration)
Circle tabs, 5*
Graph paper transparency*
Launching ramp with set pin*
Neodymium magnets, ½" x ⅛", 2*
Picture frame and glass*
Steel spheres, from ½" to ¾" diameter, 3*
Washable marking pen
*Materials included in kit.
No hazards are associated with this demonstration. If performed in a laboratory setting, please follow all laboratory safety guidelines.
Copy and distribute the hand-out materials to each student, including the graph, instruction sheet, example calculation sheet, protractor and ruler.
Student Worksheet PDF
Answers to Questions
The overhead isotope detector simulates the basic principle in the design of a mass spectrometer. A schematic of the system is shown in Figure 4.
The system involves a small picture frame with a ramp mounted on it and a magnet secured below the frame. The height of the ramp provides the acceleration due to gravity and because all the steel spheres are launched from the same point, all the spheres have the same velocity as they enter the magnetic field.
The magnetic property of the steel plays the same role as the charge on an ion, causing the path of the sphere to bend in the magnetic field. For steel spheres moving at identical initial velocities in a constant magnetic field, the equation relating mass (m) and radius (r) of curvature is:
To calculate the radius of circular travel for each deflected steel sphere, start by using a protractor to construct a line perpendicular to the diagonal line at d (see Figure 5). Label the end point H.
Next, draw a line from d to A, the point of impact of the deflected sphere with the side of the frame. Find the midpoint of this line segment and label this point G. Drop a perpendicular line from the line segment dA at G. Label the intersection of this perpendicular line and the line segment dH with the letter F. Label the angle formed by AdH as ΘA.
The triangle ▲GdF is a right triangle and of dF, the hypotenuse of the right triangle, is also equal to the radius of the curvature for the path of the steel sphere. In this configuration,
The line segment dF is equal to the the deflected path and dG is equal to ½ dA.
Substituting and rearranging Equation 2 yields:
In this demonstration, the radius of curvature r for each sphere will be calculated from the angle and cord measurements.
Special thanks to Irwin Talesnik and John Eix for providing the idea and the instructions for this activity to Flinn Scientific.