1. Mass Number and Atomic Mass
Protons
Neutrons
Electrons
Mass Number and Atomic Mass
James Chadwick

2. James Chadwick, the student of Rutherford, performed experiments to verify the presence of neutrons within the nucleus of atoms. This was an important discovery that broadened our understanding of the atom and the mass of elements.
James Chadwick
Ernest Rutherford

3. Hydrogen-1
The discovery of the neutron led to the understanding that not every atom of an element is identical. Some atoms have a different number of neutrons. Atoms of the same element with different number of neutrons are known as ________.
Hydrogen-2
isotopes
Hydrogen-3

4. 1 1 Understanding the mass of atoms:
When discussing subatomic particles, scientists often talk about their relative masses. This just means how their masses compare.
Protons
Neutrons
Electrons
Relative
Mass 1 1 1
1840

5. Understanding the mass of atoms:
Because electrons are 1/1840th the size of protons and neutrons, they are too small to concern ourselves with when we are discussing mass.
Electrons 1
1840

6. Atoms get their mass numbers from the total number of protons and neutrons inside their nucleus. This is NOT the same thing as atomic mass. We will learn how they are related today.
Hydrogen-2
Lithium-7
Helium-4

7. # of neutrons = mass number – atomic number
Mass number does NOT tell you the number of neutrons. This is also NOT on the periodic table because it is not consistent for every atom of an element. However, determining the number of neutrons for an isotope is a simple calculation.
# of neutrons = mass number – atomic number
Helium-5
5 – 2 = 3 neutrons

8. 5 protons and 6 neutrons = _______________
Knowing the number of protons and neutrons allows you to identify an isotope.
Boron-11
5 protons and 6 neutrons = _______________
5 protons tells you that the element is
Boron
Mass Number = protons + neutrons
5 + 6 = 11
These are connected by a dash (-) to show that they belong together.

9. Determine how many neutrons are in each of the isotopes listed on your notes.7
147 2 6 10 20
Write the names of the isotopes, including their mass numbers, on your notes.
Mercury-201
Calcium-40
Zinc-65
Helium-5
Argon-40

10. Because atoms are so small, we use a very large number to talk about them. This number is known as Avogadro's number.
Avogadro's number =
Avogadro's number = 6.02 × 1023
6.02 × 1023 particles is known
as a of particles
mole

11. Remember, the number of neutrons is not listed on the periodic table because it is not consistent for every atom. However the ratio of isotopes is consistent for a sample.
Within a mole of atoms of an element, the percentage (%) of each isotope in the sample is very consistent.
In a mole of chlorine, about 75% of the atoms are chlorine-35, and about 25% of the atoms are chlorine-37

12. In a mole of chlorine, about 75
In a mole of chlorine, about 75.0% of the atoms are chloring-35, and about 25.0% of the atoms are chlorine-37
Because the ratio of isotopes is consistent, the mass of 1 mole of an element is also consistent. The mass of 1 mole of an element is the atomic mass, and because it is consistent we can record it on the periodic table.
The atomic mass can be calculated from the ratio of each isotope that is found in the element.

13. (35 × 75.0%) + (37 × 25.0%) = 35.5 grams (35 × 0.750) + (37 × 0.250) =
In a mole of chlorine, about 75.0% of the atoms are chloring-35, and about 25.0% of the atoms are chlorine-37
To calculate atomic mass from the composition of isotopes, you simply multiply the percentage by the mass number for each isotope then add them all together.
(35 × 75.0%) + (37 × 25.0%) =
35.5 grams
(35 × 0.750) + (37 × ) =
It may be easier to type the percentages into your calculator as decimals.
35.5 grams

14. What is the approximate atomic mass of Boron based on this data?
In a mole of boron, about 80% of the atoms are boron-11, and about 20% of the atoms are boron-10
Photo by X. Vazquez
11B = 80.0% 10B = 20.0%
What is the approximate atomic mass of Boron based on this data?
(11 × 80.0%) + (10 × 20.0%) =
10.8 grams

15. In a sample of an unknown element, X, the ratio of isotopes is examined. It is determined that 89.80% of the sample is composed of 112X, and the other 10.20% is composed of 114X. Based on this data, what would be the atomic mass of element X?
(112 × 89.80%) + (114 × 10.20%) =
112.2 grams

16. Let this simple little tune get stuck in your head, and you will always remember how to solve these problems.
Mass times percentage plus mass times percentage
Plus mass times percentage plus mass times percent….
Sing along as a class!
Click Speaker to Listen