Isotopes and Atomic Weighs 24 Mg 25 Mg 26 Mg Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Chapter 2 Atoms and Molecules

2 Isotopes are atoms of the same element that have different mass numbers. have the same number of protons, but different numbers of neutrons. Isotopes

3 A nuclear symbol represents a particular atom of an element. gives the mass number in the upper left corner and the atomic number in the lower left corner. Example: An atom of sodium with atomic number 11 and a mass number 23 has the following atomic symbol: mass number 23 Na atomic number 11 Nuclear Symbol

4 From the nuclear symbol, we can determine the number of protons (p + ), neutrons, (n), and electrons (e - ) in a particular atom O P Zn p + 15 p + 30 p + 8 n16 n 35 n 8 e - 15 e - 30 e - Information from Nuclear Symbols

5 Naturally occurring carbon consists of three isotopes, 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of the following. 12 C 13 C 14 C protons ______ ______ ______ neutrons ______ ______ ______ electrons______ ______ ______ Learning Check

6 12 C 13 C 14 C protons 6 p + 6 p + 6 p + neutrons 6 n7 n 8 n electrons 6 e - 6 e - 6 e - Solution

7 Write the nuclear symbols for atoms with the following subatomic particles. A. 8 p +, 8 n, 8 e - ___________ B. 17p +, 20n, 17e - ___________ C. 47p +, 60 n, 47 e - ___________ Learning Check

8 A. 8 p +, 8 n, 8 e - 16 O 8 B. 17p +, 20 n, 17e - 37 Cl 17 C. 47p +, 60 n, 47 e -107 Ag 47 Solution

9 Learning Check 1. Which of the following pairs are isotopes of the same element? 2. In which of the following pairs do both atoms have 8 neutrons? A. 15 X 15 X 8 7 B. 12 X 14 X 6 6 C. 15 X 16 X 7 8

10 Solution B. 12 X 14 X 6 6 Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8. C. 15 X 16 X 7 8 An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.

11 Isotopes of Magnesium In naturally occurring magnesium, there are three isotopes. 24 Mg 25 Mg 26 Mg Isotopes of Mg

12 Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32 S S S S S, 33 S, 34 S, 36 S Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

13 Relative Masses of Atoms Use atomic weights of the elements to calculate molecular weights (MW) of compounds Relative masses provide a simple way of comparing the masses of atoms. Ex. Mass of neon atoms is and Mass of calcium atoms is

14 Ca atom mass Ne atom mass = = Mass of Calcium is 2x than Ne He atom mass H atom mass = = Mass of Helium is 4x than Hydrogen The exact relationship between two masses calculated

15 Definitions Atomic mass unit (amu) is a unit used to express the relative masses of atoms. Atomic Weight is the mass of an average atom of an element expressed in atomic mass units Molecular Weight is the relative mass of a molecule expressed in atomic mass units and calculated by adding together the atomic weights of the atoms in the molecule.

16 Calculating the atomic weight of compounds MW = C n H m O k MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O) H 2 O the MW is MW = 2(at. Wt. H) + 1(at. Wt. O) MW = 2(1.008 u) + 1( u) MW = u or u for water

17 Use atomic weighs from the periodic table inside the front cover to determine the molecular weight of urea, CH 4 N 2 O, the chemical form in which much nitrogenous body waste is excreted in the urine. Learning Check a u b u c u

18 Solution MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O) Urea chemical formula is CH 4 N 2 O, MW = 1(12.01 u) + 4(1.008) + 2(14.01) + 1(16.00) MW = u or rounded off u

19 Atomic Mass The atomic mass of an element is listed below the symbol of each element on the periodic table. gives the mass of an “average” atom of each element compared to 12 C. is not the same as the mass number. Na 22.99

20 Isotopes of Some Elements and Their Atomic Mass Most elements have two or more isotopes that contribute to the atomic mass of that element. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

21 Atomic Mass for Cl The atomic mass of chlorine is due to all the Cl isotopes. not a whole number. the average of two isotopes: 35 Cl and 37 Cl. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

22 Using the periodic table, specify the atomic mass of each element. A.calcium__________ B. aluminum__________ C. lead__________ D. barium__________ E. iron__________ Learning Check

23 Using the periodic table, specify the atomic mass of each element: A. calcium40.08 amu B. aluminum26.98 amu C. lead207.2 amu D. barium137.3 amu E. iron55.85 amu Solution