Topic: Atomic Concepts Aim: What is an isotope, and how do we determine average atomic mass? Do Now: 1. Which two particles make up most of the mass of a hydrogen-2 atom? (1) electron and neutron (2) electron and proton (3) proton and neutron (4) proton and positron 2. Determine the total number of neutrons in an atom of Si-29. HW: Text p. 152A, #37, 41, 43, 44, 53

Isotopes and Atomic Mass Dalton  all atoms of an element are identical WRONG. Atoms of same element CAN have DIFFERENT #s OF NEUTRONS. ISOTOPES atoms of the same element having different masses due to varying numbers of neutrons

Thallium-201 used to diagnose heart disease Isotopes Can be stable or unstable Smaller elements tend to be more stable than larger elements Unstable isotopes may DECAY and emit RADIATION Thallium-201 used to diagnose heart disease

Examples of Isotopes Hydrogen Carbon H-1, H-2, H-3 Carbon C-12, C-14 The # that follows the symbol is the MASS NUMBER for that isotope.

ISOTOPES

Atomic Mass Each of the isotopes of an element has its own atomic mass (C-12 has a mass of 12, C-14 has a mass of 14) We can calculate the average atomic mass based on the masses and abundances (% of that isotope found in nature) This is why the atomic mass listed on the Periodic Table is a decimal.

Average Atomic Mass Weighted average of all of the naturally occurring isotopes of an element 126C 146C Both of these elements are Carbon with the atomic number 6. The second carbon has 2 more neutrons. (Its mass is 2 units higher) On the periodic table we see an average mass of all of the isotopes represented by the average atomic mass. (12.011) Because the average atomic mass of carbon is so close to 12, we can infer that the C-12 isotope is the most abundant isotope.

Calculating Average Atomic Mass Example: Chlorine has two naturally occurring isotopes – Cl-35 and Cl-37. 76% of the isotopes of Cl are Cl-35, and 24% of its isotopes are Cl-37. Calculate the average atomic mass of chlorine. Step Calculation Multiply each % by the appropriate mass # 76 x 35 = 2660 24 x 37 = 888 Add these answers together 2660 + 888 = 3548 Divide by 100 3548 / 100 = 35.48 Compare your final answer to the mass on the Periodic Table – it should be close!

Examples Data in various chemical handbooks indicate the relative percentages of these naturally occurring isotopes of Magnesium (Mg): 24Mg = 79.3%, 25Mg = 10.1%, 26Mg = 10.6%. Given this data, calculate the average atomic mass for Magnesium. Boron Mass # % Abundance 10 19.9 11 80.1

Sample Problems

Which group of atomic models is listed in historical order from the earliest to the most recent? (1) hard-sphere model, wave-mechanical model, electron-shell model (2) hard-sphere model, electron-shell model, wave-mechanical model (3) electron-shell model, wave-mechanical model, hard-sphere model (4) electron-shell model, hard-sphere model, wave-mechanical model

The total mass of the protons in an atom of gold-198 is approximately (1) 79 atomic mass units (2) 119 atomic mass units (3) 198 atomic mass units (4) 277 atomic mass units

Naturally Occurring Isotopes of Sulfur State, in terms of the number of subatomic particles, one similarity and one difference between the atoms of these isotopes of sulfur. Draw a Lewis electron-dot diagram for an atom of sulfur-33. Show a correct numerical setup for calculating the atomic mass of sulfur. Isotope Atomic Mass Natural Abundance 32S 31.97 94.93 33S 32.97 0.76 34S 33.97 4.29 36S 35.97 0.02