2IsotopesAtoms of the same element (same number of protons) that differ in the number of neutrons.Ex: Neon-20, Neon-21, Neon-22Atomic Mass- average mass of all the isotopesBut it’s not a normal average!!!
3Mass SpectrometerThe most direct and accurate means for determining atomic and molecular weights.Use the gas form of the element or compound.
4Basic IdeaSuppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose- pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.
5Basic IdeaBut suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection.The same idea can be applied to atomic sized particles.The lighter the atom, the more it’s deflected and so on.
6An outline of what happens in a mass spectrometer Stage 1: IonisationThe atom is ionised by knocking one or more electrons off to give a positive ion.Mass spectrometers always work with positive ions.
7An outline of what happens in a mass spectrometer Stage 2: AccelerationThe ions are accelerated so that they all have the same kinetic energy.
8An outline of what happens in a mass spectrometer Stage 3: DeflectionThe ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.
9An outline of what happens in a mass spectrometer Stage 4: DetectionThe beam of ions passing through the machine is detected electrically.
15The Number of IsotopesThe 5 peaks in the mass spectrum shows that there are 5 isotopes of zirconium - with relative isotopic masses of 90, 91, 92, 94 and 96 on the 12C scale.
16The Abundance of the Isotopes This time, the relative abundances are given as percentages.zirconium-90 (51.5%), zirconium- 91 (11.2%), zirconium-92 (17.1 %)zirconium-94 (17.4%), zirconium- 96 (2.8%)
17How to Calculate!To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.The total mass of these 100 typical atoms would be(0.515 x 90) + (0.112 x 91) + (0.171 x 92) + ( x 94) + (0.028 x 96) = 91.3
18PracticeNaturally occurring magnesium has the following isotopic abundances:What is the average atomic mass of Magnesium?Sketch the mass spectrum of Mg.IsotopeAbundanceAtomic Mass (amu)Magnsium-2478.99%Magnesium-2510.00%Magnesium-2611.01%
19Did you know that mass spectrometry is used to... Detect and identify the use of steroids in athletesMonitor the breath of patients by anesthesiologists during surgeryDetermine the composition of molecular species found in spaceDetermine whether honey is adulterated with corn syrupLocate oil deposits by measuring petroleum precursors in rockMonitor fermentation processes for the biotechnology industryDetect dioxins in contaminated fishDetermine gene damage from environmental causesEstablish the elemental composition of semiconductor materials