1. 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

2. 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

3. 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

4. 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.

5. ISOTOPES

6. 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.

7. Average Atomic Mass
Weighted average of all of the naturally occurring isotopes of an element
126C C
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.

8. Calculating Average Atomic Mass
Example: Chlorine has two naturally occurring isotopes – Cl-35 and Cl % 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
= 3548
Divide by 100
3548 / 100 = 35.48
Compare your final answer to the mass on the Periodic Table – it should be close!

9. 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

10. Sample Problems

11. 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

12. 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

13. 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