1. Dalton was proved incorrect and his theory was modified
Isotope notes
Dalton was proved incorrect
and
his theory was modified

2. Protons define the element
Atoms that have the same number of protons are always atoms of a specific element.
Example: Carbon

3. Neutrons can vary
BUT
atoms can have different numbers of neutrons and still be an atom of a specific element.

4. Isotopes
This is because elements can have isotopes (basically atoms of the same element with a different number of neutrons in their nuclei).

5. Dalton was wrong
When Dalton stated his atomic theory in the early 1800’s, he assumed that all of the atoms of a given element were identical.

6. James Chadwick
Over 100 years after Dalton, James Chadwick discovered that the nuclei of most atoms contains neutrons as well as protons.

7. Dalton’s Theory Changes
Dalton’s theory now states:
All atoms of the same element contain the same number of protons and electrons, but atoms of a given element may have different numbers of neutrons.

8. The Isotopes of Hydrogen
Also written H-1
Also known as
protium
Hydrogen has an atomic number of 1, so it has 1 proton
The hyphen notation above tells us that the mass number of H-1 is 1
Number of neutrons = mass number – atomic number
So H-1 must have 0 neutrons

9. The Isotopes of Hydrogen
Also written H-2
Also known as
Deuterium
Hydrogen has an atomic number of 1, so it has 1 proton
The hyphen notation above tells us that the mass number of H-2 is 2
Number of neutrons = mass number – atomic number
So H-2 must have 1 neutron

10. The Isotopes of Hydrogen
Also written H-3
Also known as
Tritium
Hydrogen has an atomic number of 1, so it has 1 proton
The hyphen notation above tells us that the mass number of H-3 is 3
Number of neutrons = mass number – atomic number
So H-3 must have 2 neutron

11. Calculate the number of neutrons
For chlorine found on the periodic table (the most common form of chlorine that is found in nature) Chlorine-35
For Chlorine-37
Chlorine-35 = 18 neutrons, Chlorine-37 = 20 neutrons
For Cobalt found on the periodic table Cobalt-59
For Cobalt-60
Cobalt-59 = 32 neutrons, Cobalt-60 = 33 neutrons

12. Calculating Average Atomic Mass
Average atomic mass is the atomic mass that appears on the periodic table. For example – Copper has an average atomic mass of amu.

13. Calculating Average Atomic Mass
Yet, in nature, most elements are found as mixtures of two or more isotopes. For example, copper consists of
69.17% copper-63 which has a relative atomic mass of amu
AND
30.83% copper-65 which has a relative atomic mass of amu

14. Calculating Average Atomic Mass
To find the average atomic mass, multiply the decimal equivalent of the percent (for example 69.17% = ) of each isotope by the respective relative atomic mass and add the results.
( X amu) + ( X amu) = amu
Isotope
Copper – 63
Copper – 65
Relative abundance in nature
69.17%
30.83%
Relative atomic mass
62.94 amu
64.93 amu

15. Practice Calculating Average Atomic Mass
Boron – 10 is found 19.9% of the time in nature and has a relative atomic mass of amu
Boron – 11 is found 80.1% of the time in nature and has a relative atomic mass of amu
Calculate the average atomic mass of Boron

16. Practice Calculating Average Atomic Mass
Boron (0.199 X ) + (0.801 X ) = amu
Isotope
Boron – 10
Boron – 11
Relative abundance in nature
19.9%
80.1%
Relative atomic mass
amu
amu

17. Practice Calculating Average Atomic Mass
Magnesium – 24 is found 78.99% of the time in nature and has a relative atomic mass of amu
Magnesium – 25 is found 10.00% of the time in nature and has a relative atomic mass of amu
Magnesium – 26 is found 11.01% of the time in nature and has a relative atomic mass of amu
Calculate the average atomic mass of Magnesium

18. Practice Calculating Average Atomic Mass
Magnesium ( X ) + ( X ) + ( X ) = amu
Isotope
Magnesium – 24
Magnesium – 25
Magnesium – 26
Relative abundance in nature
78.99%
10.00%
11.01%
Relative atomic mass
amu
amu
amu