4.2 ATOMIC NUMBER Elements are different because they contain different numbers of protons. Atomic number = # protons = # electrons in a neutral atom Atomic number Chemical symbol Chemical name Average atomic mass 82 O 8 Zn 30 82 8 30 Atomic Mass Units amu: atomic mass units 1 atomic mass unit = 1.66053892 × 10-24 grams Unit used to measure the mass of VERY SMALL particles (atoms) Standard is the carbon-12 atom 1 carbon-12 atom = 12 amu 1 amu = 1/12 carbon-12 atom mass 1 proton = 1 amu 1 neutron = 1 amu Mass Number Mass Number – Mass of a specific atom Mass number = # protons + # neutrons REVIEW: Atomic Number: The total number of protons Atomic number = # protons = # electrons Mass Number: The total number of protons and neutrons Mass number = # protons + # neutrons Isotopes • Each element may have different numbers of neutrons => ISOTOPES • The mass number is different for different isotopes. Isotopes – Naming (Two ways) • Element name – mass number • Ex. carbon-12 or neon-22 • Using the element symbol: Isotopes - Naming • How would you write Uranium-235 using the symbol? (atomic number = 92) Atomic Mass The weighted average mass of the isotopes of that element. *different than mass number Calculating Average Atomic Mass All isotopes of an element do not exist in equal abundance in the world Percent abundance in world: 90.48% 0.27% 9.25% Example #1 Calculate the average atomic mass of Neon Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + …. Isotope Mass (amu) (M) Percent abundance (P) Neon-20 20.0 90.48% Neon-21 21.0 0.27% 0.057 Neon-22 22.0 9.25% 2.03 Average atomic mass (amu) M x P÷100 18.1 20.2 Atomic Mass Which isotope is most abundant? Neon-20 If Neon has 3 isotopes: Neon-20, Neon-21, Neon-22, Weighted average atomic mass is 20.2 amu. Example #1 Calculate the average atomic mass of Neon Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + …. Another way to write it out: Atomic Mass = (90.48/100)(20.0) + (0.27/100)(21.0) + (9.25/100)(22.0) = 20.2 amu Example #1 Calculate the average atomic mass of Lithium Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + …. Isotope Mass (amu) (M) Percent abundance (P) Lithium-6 6.01 7.5% Lithium-7 7.01 92.5% Average atomic mass (amu) M x P÷100 0.45 6.49 6.94 OR Atomic Mass = (7.5/100)(6.01) + (92.5/100)(7.01) = 6.94 amu Example #3 Calculate the average atomic mass of Cesium Atomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + …. Isotope Mass (amu) (M) Percent abundance (P) M x P÷100 Cesium-132 132.0 75.0% 99.0 Cesium-133 133.0 20.0% 26.4 Cesium-134 134.0 5.0% 6.7 Average atomic mass (amu) OR 132.1 Atomic Mass = (75.0/100)(132.0) + (20.0/100)(133.0) + (5.0/100)(134.0) = 132.1 amu Questions 1) What particles make up an atom? Protons, Neutrons and Electrons 2) What are the charges on these particles? Protons (+), Neutrons (none), Electrons (-) 3) What particles make up the nucleus? Protons and Neutrons More Questions: Atomic Number tells us: The identity of the element; the number of protons and electrons Atomic Mass tells us: The average mass in amu of all naturally occuring isotopes Mass The Development of Atomic Models The timeline shoes the development of atomic models from 1803 to 1911. 5.1 Defining the Atom > The Development of Atomic Models The timeline shows the development of atomic models from 1913 to 1932. Slide 20 of 18 © Copyright Pearson Prentice Hall End Show