4.5 Isotopes and Atomic Mass The atomic symbol for an isotope of magnesium with 12 neutrons. Learning Goal Determine the number of protons, electrons, and neutrons in one or more of the isotopes of an element; calculate the atomic mass of an element using the percent abundance and mass of its naturally occurring isotopes.

Isotopes Isotopes are atoms of the same element. have different mass numbers. have the same number of protons but different numbers of neutrons. can be distinguished by their atomic symbols. Core Chemistry Skill Writing Atomic Symbols for Isotopes

Atomic Symbols: Subatomic Particles Given the atomic symbols, determine the number of protons, neutrons, and electrons. ANALYZE Atomic Mass Number of THE PROBLEM Number Number Protons number in number in equal to lower left upper left atomic corner corner number

Atomic Symbols: Subatomic Particles Determine the number of protons, neutrons, and electrons. Isotope Atomic Number Mass Number of Protons Neutrons 8 16 8 (16–8) 15 31 16 (31–16) 30 65 35 (65–30)

Isotopes of Magnesium Magnesium, with three naturally occurring isotopes, has an atomic mass of 24.31 amu.

Isotopes of Magnesium

Study Check Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the three isotopes.

Solution Naturally occurring carbon consists of three isotopes: 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the three isotopes.

Study Check Write the atomic symbols for atoms with the following subatomic particles: A. 8 protons 8 neutrons 8 electrons B. 17 protons 20 neutrons 17 electrons C. 47 protons 60 neutrons 47 electrons

Solution Write the atomic symbols for atoms with the following subatomic particles: A. 8 protons 8 neutrons 8 electrons B. 17 protons 20 neutrons 17 electrons C. 47 protons 60 neutrons 47 electrons

Study Check 1. Which of the pairs below are isotopes of the same element? In which of the pairs below do both atoms have eight neutrons? A. B. C.

Solution 1. Which of the pairs are isotopes of the same element? B. 2. In which of the pairs do both atoms have eight neutrons? C.

Calculating Atomic Mass Atomic mass is the weighted average of all naturally occurring isotopes of that element. number on the periodic table below the chemical symbol. Chlorine, with two naturally occurring isotopes, has an atomic mass of 35.45 amu.

Calculating Atomic Mass To calculate atomic mass, use the experimental percent abundance of each isotope of the element. multiply the percent abundance by the atomic mass of that isotope. sum the total mass of all isotopes.

Calculating Atomic Mass To calculate atomic mass of chlorine, use experimental data for both isotopes. Atomic mass of Mg = 18.88 amu + 2.531 amu + 2.902 amu = 24.31 amu (weighted average mass)

Atomic Mass of Some Elements

Study Check Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the more prevalent one. (According to the periodic table, the atomic mass of lithium is 6.941 amu.)

Solution Lithium consists of two naturally occurring isotopes, 6Li and 7Li. Use the periodic table to predict which isotope is the more prevalent one. (According to the periodic table, the atomic mass of lithium is 6.941 amu.) Since the total atomic mass of 7Li is closer to 7, the mass number of 7Li, the more prevalent isotope is 7Li.

Study Check Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69Ga (atomic mass 68.926) atoms 39.90% of 71Ga (atomic mass 70.925) atoms What is the atomic mass of gallium?

Solution STEP 1 Multiply the mass of each isotope by its percent abundance divided by 100.

Solution STEP 2 Add the contribution of each isotope to obtain the atomic mass. 31 Ga 69.72