QUESTION: Which of the following has the largest mass number? A. 56Mn, B. 56Fe, C. 56Cr, D. they all have the same mass number The mass number is the number written as a superscript to the left of the symbol. All of these atoms have the same mass number: 56. SCRIPT: Which of the following has the largest mass number? A. 56Mn, B. 56Fe, C. 56Cr, D. they all have the same mass number PAUSE CLICK The mass number is the number written as a superscript to the left of the symbol. All of these atoms have the same mass number: 56. If you picked iron, you probably looked at the periodic table and made a wrong assumption. CONTINUED ON NEXT SLIDE

Here are chromium, manganese, and iron. The whole numbers shown here…24, 25, and 26… are the atomic numbers. CALLOUT The other numbers shown here are the average atomic masses, not mass numbers. A typical periodic table doesn’t give mass numbers So what’s the difference between mass number and average atomic mass? CLICK CONTINUED ON NEXT SLIDE

Naturally-occurring isotopes of iron Fe-54: 53.94 u, 5.8% 28 neutrons Fe-56: 55.93 u, 91.8% 30 neutrons Fe-57: 56.94 u, 2.1% 31 neutrons Fe-58: 57.93 u, 0.3% 32 neutrons The atomic number of iron is 26. All iron atoms have 26 protons. But they can have different numbers of neutrons. The ones we find in nature either have 28, 30, 31, or 32 neutrons We call these four possibilities the four naturally-occurring isotopes HIGHLIGHT isotopes if iron. Atoms with the same number of protons but different number of neutrons are called isotopes of the same element. The mass number of the atoms with 28 neutrons is 54. HIGHLIGHT 26 protons plus 28 neutrons, gives us a mass number of 54. These atoms are called iron 54. Each of these atom actually has a mass of 53.94 atomic mass units, Which is 54 atomic mass units if we round off to the nearest whole number. Similarly, iron atoms with 30 neutrons have a mass number of 56, … These atoms each have a mass of 55.93 amu, or 56 amu if we around off to the nearest whole number. So how are these masses related to the number we see in the periodic table? HIGHLIGHT 55.85 The number we see in the periodic table is the average mass of atoms in a naturally-occurring sample. We can look up the relative abundance of the isotopes of iron. For iron-54, it’s 5.8%. That’s about 6%. HIGHLIGHT 5.8% That means, if you were to randomly pick a hundred iron atoms from a naturally-occurring sample, about six out of 100 atoms would be iron 54. Similarly, the likelihood of getting an iron-56 atom is 91.8%. HIGHLIGHT 91.8% If you were to randomly pick a hundred iron atoms from a naturally-occurring sample, about 96 out of 100 atoms would be iron 56. You can see that since iron 56 is the predominant isotope, the average mass .. 55.85… is closer to the mass of an iron 56 atom. A good source of information about isotopes is the website called webelements.com PAUSE CLICK END RECORDING Go to webelements.com

Video ID: 1-11-2 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08