1. Isotopes and Atomic Mass
L.O. To understand isotopes and atomic mass

2. What do you notice about these elements?

3. Isotopes of carbon
Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14.
6 protons
6 neutrons
6 electrons
6 protons
7 neutrons
6 electrons
6 protons
8 neutrons
6 electrons

4. Why does the mass number change in an isotope?
Particle
Mass
Charge
proton 1 +1
neutron
electron
almost 0 -1
The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

5. Properties of isotopes
The isotopes of an element are virtually identical in their chemical reactions.
This is because they have the same number of protons and the same number of electrons.
The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.
Natural samples of elements are often a mixture of isotopes.

6. Isotopes of hydrogen
Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist.
hydrogen
deuterium
tritium
1 proton
0 neutrons
1 electron
1 proton
1 neutron
1 electron
1 proton
2 neutrons
1 electron

7. Isotopes of chlorine
About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37.
17 protons
18 neutrons
17 electrons
17 protons
20 neutrons
17 electrons

8. What are the particle numbers in each isotope?
Isotopes of oxygen
Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18.
What are the particle numbers in each isotope?
oxygen-16
oxygen-18
protons
neutrons
electrons 8 8 8 10 8 8

9. Atomic Mass of elements
The atoms of each element have a different mass.
The relative atomic mass is a way of saying how heavy atoms are in comparison to others.
Carbon is given a relative atomic mass (RAM) of 12.
The RAM of other atoms compares them with carbon.
Eg. Hydrogen has a mass of only one twelfth that of carbon and so has a RAM of 1.
Below are the RAMs of some other elements.
Element
Symbol
Times as heavy as carbon
R.A.M
Helium He
one third
Beryllium Be
three quarters
Molybdenum Mo
Eight
Krypton Kr
Seven
Oxygen O
One and one third
Silver Ag
Nine
Calcium Ca
Three and one third 4 9 96 84 16
108 40

10. Complete the Isotopes and atoms sheet

11. Isotopes and RAM
Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this.
To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass and add them together.
For example, chlorine exists as two isotopes: chlorine-35 (75%) and chlorine-37 (25%).
= (0.75 x 35) + (0.25 x 37) =
= 35.5
RAM of chlorine = (75% x 35) + (25% x 37)

12. Calculating RAM
Bromine contains 50.5% bromine-79 and 49.5% bromine-81.
What is the RAM of naturally-occurring bromine?
= (0.505 x 79) + (0.495 x 81) =
= 79.99
= 80 (the RAM is usually rounded to the nearest whole number)
RAM of bromine = (50.5% x 79) + (49.5% x 81)

13. RAM Questions
Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56.
What is the average atomic mass of silicon if % of its atoms have a mass of amu, 4.07 % have a mass of amu, and 3.09 % have a mass of amu?
 Calculate the average atomic mass for neon if its abundance in nature is 90.5% neon-20 ( amu), 0.3% neon-21 ( amu), and 9.2% neon-22 ( amu).
 Calculate the average atomic mass of silver if 13 out of 25 atoms are silver-107 and 12 out of 25 atoms are silver-109.
 Calculate the average atomic mass of chromium.