1.4 Isotopes, Radioisotopes, and Atomic Mass B3.1 explain the relationship between the atomic number and the mass number of an element, and the difference between isotopes and radioisotopes of an element B3.2 explain the relationship between isotopic abundance of an element’s isotopes and the relative atomic mass of the element

Atomic Mass If a proton has a mass of 1, and a neutron has a mass of 1, how come the elements on the periodic table have masses that are decimals? What unit does the periodic table use to measure mass?

Mass of an Atom In the head of a pin there are approximately 8.0 x atoms of iron. Because atoms are so small, standard units of measurement are not practical. Scientists use a smaller unit to describe the mass of atoms, the Atomic Mass Unit (u or amu).

Relative Atomic Mass Mass 12 C atom = × g 1 p= amu 1 n = amu 1 e - = amu 1 amu= 1 / 12 the mass of a 12 C atom × g / 12 = 1.66 x g = 1 amu Nucleus Electrons Nucleus Neutron Proton Carbon-12 Neutrons 6 Protons 6 Electrons6

Isotopes The number of protons inside the nucleus at the centre of an atom decides what element it is. Dif­ferent atoms with the same number of protons and a dif­ferent number of neutrons are known as isotopes. For example, there are three nat­ur­ally occur­ring isotopes of carbon: carbon-12, carbon-13 and carbon-14. Most (98.9%) of the natural carbon is carbon-12 and the remaining 1.1% is made up of stable carbon-13 and radio­active carbon-14.

Isotopes Nucleus Electrons Nucleus Neutron Proton Carbon-12 Neutrons 6 Protons 6 Electrons6 Nucleus Electrons Carbon-14 Neutrons 8 Protons 6 Electrons Nucleus Neutron Proton

3 p + 3 n 0 2e – 1e – 3 p + 4 n 0 2e – 1e – 6 Li 7 Li Nucleus Electrons Nucleus Neutron Proton Lithium-6 Neutrons 3 Protons 3 Electrons3 Nucleus Electrons Nucleus Neutron Proton Lithium-7 Neutrons 4 Protons 3 Electrons

Naming Isotopes Put the mass number after the name of the element carbon- 12 carbon -14 uranium-235 California WEB

Isotopic Abundance

Isotopic Abundance Why is the average mass not the same as the mass on the periodic table? The abundance of each isotope has to be taken into consideration. A regular average calculation treats each isotope the same

Isotopic Abundance A weighted average takes into account the abundance of each isotope

Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an AVERAGE value. Average Atomic Mass = weighted average of the masses of the isotopes of an element

Average Atomic Mass Weighted average of all isotopes The mass indicated on the Periodic Table Usually rounded to 2 decimal places Courtesy Christy Johannesson Avg. Atomic Mass = (mass)(%) + (mass)(%) … 100

Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. Courtesy Christy Johannesson Avg. Atomic Mass = (16)(99.76) + (17)(0.04) + (18)(0.20) 100 = 16 amu

Average Atomic Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Courtesy Christy Johannesson Avg. Atomic Mass = (35)(8) + (37)(2) 10 = 35 amu

. mass spectrometry is used to experimentally determine isotopic masses and abundances interpreting mass spectra average atomic weights - computed from isotopic masses and abundances - significant figures of tabulated atomic weights gives some idea of natural variation in isotopic abundances Weighing atoms gas sample enters here filament current ionizes the gas ions accelerate towards charged slit magnetic field deflects lightest ions most ions separated by mass expose film The first mass spectrograph was built in 1919 by F. W. Aston, who received the 1922 Nobel Prize for this accomplishmentF. W. Aston1922 Nobel Prize Copyright © by Fred SeneseFred Senese

Negative grid Magnet Detector To vacuum pump Mass numbers Evacuated glass tube Heated filament Electron beam Neon gas inlet (+) (-) Image Copyrighted by Houghton Mifflin Company Mass Spectrophotometer

Abundance Mass Mass spectrum of chlorine. Elemental chlorine (Cl 2 ) contains only two isotopes: amu (75.53%) and (24.47%) AAM = (34.97 amu)(0.7553) + (36.97 amu)(0.2447) AAM = ( amu) + ( amu) AAM = amu Cl-35 Cl-37 Cl

Mass Spectrophotometer electron beam magnetic field gas stream of ions of different masses lightest ions heaviest ions Dorin, Demmin, Gabel, Chemistry The Study of Matter 3rd Edition, page 138

Mass Spectrum for Mercury Mass number Relative number of atoms Mass spectrum of mercury vapor The percent natural abundances for mercury isotopes are: Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % (The photographic record has been converted to a scale of relative number of atoms)

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest has a mass of amu. Cu IsotopePercentAbundanceMass Cu Cu

ProtonsNeutronsElectronsMass number Cu-65AB29C ArgonDEF40 Ba 2+ 56GHI A. B. C. D. E. F. G. H. I. Given the average atomic mass of an element is amu and it has three isotopes (“A”, “B”, and “C”): isotope “A” has a mass of amu and is 87.14% abundant isotope “B” has a mass of amu and is 12.36% abundant Find the mass of isotope “C”. Show work for credit. Extra Credit: What is a cation?

Given the average atomic mass of an element is amu and it has three isotopes (“A”, “B”, and “C”): isotope “A” has a mass of amu and is 87.14% abundant isotope “B” has a mass of amu and is 12.36% abundant Find the mass of isotope “C”. Show work for credit.

The percent natural abundances for mercury isotopes are: Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % Hg % ( )(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x = x x = amu Hg (% "A")(mass "A") + (% "B")(mass "B") + (% "C")(mass "C") + (% "D")(mass "D") + (% "E")(mass "E") + (% F)(mass F) + (% G)(mass G) = AAM ABCDEFGABCDEFG

Using a periodic table and what you know about atomic number, mass, isotopes, and electrons, fill in the chart: ElementSymbolAtomic Number Atomic Mass # of protons # of neutron # of electron charge 888 Potassium 39+1 Br Atomic Number = Number of Protons Number of Protons + Number of Neutrons = Atomic Mass Atom (no charge) : Protons = Electrons Ion (cation) : Protons > ElectronsIon (anion) : Electrons > Protons

Atomic Mass Magnesium has three isotopes % magnesium 24 with a mass of amu, 10.00% magnesium 25 with a mass of amu, and the rest magnesium 26 with a mass of amu. What is the atomic mass of magnesium? If not told otherwise, the mass of the isotope is the mass number in amu. California WEB IsotopePercentAbundanceMass Mg Mg Mg amu