Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Most elements occur naturally as a mixture of isotopes. Shown are the nuclei of three naturally occurring magnesium isotopes. Each isotope of magnesium has 12 protons and a different number of neutrons. What similarities do you see in the symbols for the isotopes of magnesium? What differences? How can you tell that magnesium has 12 protons? How many protons are in C-13, an isotope of carbon? Is it a, b, c or d? The correct answer is a. In the atomic symbol, the mass number is shown in the upper left corner of the symbol, and the atomic number is shown in the lower left corner of the symbol. Carbon, atomic number 6, has 6 protons. Let’s consider in further detail the isotopes of magnesium. Every magnesium atom has an atomic number of 12. Since the atomic number identifies the number of protons, every magnesium atom has 12 protons. Each isotope of magnesium has a different mass number. Remember, the mass number is the number of protons plus neutrons. Thus, each isotope of magnesium has a different number of neutrons. The first isotope of magnesium, M-24, has a mass number of 24. To determine the number of neutrons, we will take 24 minus 12, leaving 12. This means this isotope of magnesium has 12 neutrons. The second isotope, M-25, has a mass number of 25. 25 minus 12 is 13. This means Mg-25 has 13 neutrons. The third isotope, Mg-26, has a mass number of 26 with 12 protons. 26 minus 12 is 14, which is the number of neutrons for Mg-26. C-13 is an isotope of carbon. How many neutrons are in this isotope? Is it a, b, c or d? The correct answer is b. In the atomic symbol, the mass number is shown in the upper left corner of the symbol, and the atomic number is shown in the lower left corner of the symbol. Carbon, atomic number 6, has 6 protons. The number of neutrons is found by subtracting the number of protons, 6, from the mass number. That is, 13 - 6, to give 7 neutrons. What information about an element can we discover from the periodic table? The symbol for an element, like magnesium, is given in the periodic table. Above the symbol is the atomic number, or the number of protons. Below the symbol is the atomic mass in atomic mass units, or amu. This is the weighted average of all the masses of the naturally occurring isotopes of that element, adjusted for their relative abundance. Most elements consist of two or more isotopes, which is one reason that the atomic masses on the periodic table are seldom whole numbers. The total mass of each isotope is dependent on the number of neutrons and protons and their respective masses. Each of these isotopes have different masses because of the differences in the number of neutrons. The atomic mass given on the periodic table is a weighted average of the naturally occurring isotopes. In a naturally occurring sample of Mg, nearly 80% is the Mg-24 isotope. The other two naturally occurring isotopes are found in about 10% abundance each. The atomic mass of magnesium, 24.31 amu, is closest to the mass of Mg-24. This results from a higher percentage of this isotope contributing more to the average atomic mass because it has such a high abundance. The other two isotopes, with their low abundance, do not contribute as much to the weighted average. Let’s look at another element and consider its average atomic mass. Copper is a trace element essential for health. The atomic mass shown on the periodic table is the weighted average of its two naturally occurring isotopes, Cu-63 and Cu-65. Copper’s atomic mass from the periodic table is 63.55 amu. How is this value determined? Atomic mass can be calculated if you know the atomic mass and percent abundance for each isotope of an element. For copper and it’s two isotope’s, the equation would be Cu-63 has 29 protons and 34 neutrons. Cu-65 has 29 protons and 36 neutrons. Their atomic masses are different, given their different mass numbers. When you look at a sample of naturally occurring copper, the two isotopes are found in different abundances. Cu-63 is nearly 70% while Cu-65 is about 30%. Now that we’ve organized the necessary data, let’s walk through the calculation of the weighted average atomic mass for copper. Step 1 is to multiply the mass of each isotope in amu by its percent abundance divided by 100%. For the first isotope, Cu-63, multiply 62.93 amu by 69.15% divided by 100%. This equals 43.52 amu. For Cu-65, it is 64.93 amu times 30.85% divided by 100% to give 20.03 amu. Step 2 is to add the contribution of each isotope to obtain the atomic mass. When the adjusted masses from each isotope are added together, the weighted mass average is 63.55 amu. This is the value given on the periodic table. Two isotopes of chlorine are stable, with Cl-35 at 75.76% (34.97 amu) and Cl-37 at 24.24% (36.97 amu). Calculate the atomic mass for chlorine using the weighted average mass method. Is it a, b, c or d? The correct answer is b, 35.45 amu. To calculate the atomic mass, multiply the mass of Cl-35 by the percentage abundance divided by 100% to give 26.49 amu. Then, multiply the mass of Cl-37 by the percentage abundance divided by 100% to give 8.962 amu. Add these together to get the weighted average mass of 35.45 amu.