1. The Atom
Lesson 2 - Isotopes

2. Different Forms of the Same Element
In any specific element, the # of protons is always constant.
Unlike the number of protons, the number of electrons and neutrons can vary within atoms of an element without changing the identity of the element.
Ex. Carbon (C) ALWAYS has 6 protons, but it can have anywhere from 6-8 neutrons and 2-10 electrons

3. Isotopes C 6 p+, 6 no C 6 p+, 7 no C 6 p+, 8 no
Isotopes are atoms of the same element (same number of protons) but with different number of neutrons
Carbon has three isotopes 12
C 6 p+, 6 no 6
Notice how the atomic # (# of protons) does NOT change but the mass number does. 13
C 6 p+, 7 no 6 14
C 6 p+, 8 no 6

4. The atomic mass on the periodic table is an average of all the known isotopes of each element.
It is not the mass of any individual atom.
To determine the mass of a specific isotope, you need to add the number of protons to the number of neutrons. This is mass number.

5. Practice A lithium atom has 3 protons, 3 electrons, and 3 neutrons. 6
A nitrogen atom has 8 neutrons
An unknown element has 92 protons and 143 neutrons.
Element = A = 6 15 U
235

6. Representing Isotopes
Option # 1
Top number is mass number (A), bottom number is atomic number (Z) U K C
Option #2
U – 235
K - 40
C – 14
Only the mass number (A) is listed, the atomic number (Z) can be found on the Periodic Table
235 92 40 19 14 6

7. Finding average atomic mass
To find the average atomic mass of an element you need two pieces of information:
The mass numbers of the different isotopes (this is NOT the average mass found on the periodic table)
Neon has three: Ne - 20 , Ne - 21, Ne - 22
The relative abundance of each isotope
Of all the neon measured, 90.60% is Ne-20, 0.200% is Ne-21, and 9.200% is Ne-22

8. Finding average atomic mass
Even though it is the least massive, Ne- 20 accounts for the vast majority of Neon.
Isotope
Mass
Abundance
Mass Contribution
Ne-20
20 amu
90.60%
18.12
Ne-21
21 amu
0.20%
0.042
Ne-22
22 amu
9.200%
2.024
Avg. mass =
20.19 amu

9. Examples
Gallium-69 has a relative abundance of 60.11% and Gallium-71 has a relative abundance of 39.89%. What is the average atomic mass of Gallium?
Isotope
Mass
Abundance
Mass Contribution
Ga-69
69 amu
60.11%
41.48
Ga-71
71 amu
39.89%
28.32
Avg. mass =
69.80 amu

10. Examples
Thallium has two stable isotopes, Thallium-203 and Thallium-205. Thallium-203 has a relative abundance of 29.52%.Thallium-205 has a relative abundance of 70.48%. What is the average atomic mass of Thallium?
Isotope
Mass
Abundance
Mass Contribution
Tl-203
203 amu
29.52%
59.93
Tl-205
205 amu
70.48%
144.5
Avg. mass =
204.4 amu