1. Atomic Mass and Isotope
Objectives:
SWBAT calculate the average atomic mass of an element when given the relative abundance of the isotopes.
Bellringer:
What does the atomic
mass tell you about the
atom?

2. RELATIVE MASSES Chemists more than 200 years ago used a
relative scale to compare weights of atoms to
each other.
Dalton assigned a H atom a mass of 1.
According to his scale a helium atom has a
relative mass of 4 because it is 4 times as heavy.

3. RELATIVE MASSES
Using Dalton’s scale a carbon atom has a relative mass of 12
because a carbon atom is twelve times
heavier than a hydrogen atom

4. Comparing Masses
In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC-
International Union of Applied Physics and Chemistry.

5. IUPAC RELATIVE MASSES IUPAC decided that the most common isotope
of C which is 12C would be used as a reference
standard and assigned an atom of 12C a mass of
12 exactly.
Using this scale the helium isotope is assigned
a relative mass of 4
Comparing a helium atom to a carbon atom
The He atom is 3 times lighter

6. RELATIVE ATOMIC MASSES
A Krpton atom that is given a relative mass of 36
A Kr atom is three times heavier than a 12C
atom

7. Atomic Symbols atomic number 11 Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11

8. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37

9. RELATIVE ATOMIC MASSES
All isotopes of elements are given a relative
isotopic mass compared to the 12C isotope.
There are 3 isotopes of Mg
24Mg
25Mg
26Mg
These 3 atoms are different because
they have different numbers of neutrons

10. Abundances of Isotopes
In a sample of pure Mg you will find the isotopes of
Mg always occur in the following quantity
78.7% 24Mg
10.13% 25Mg
11.17% 26Mg
Like Magnesium most elements exist as a mixture of isotopes.
Eg 1H, 2H and 3H
Deuterium (heavy hydrogen) is used in heavy water moderated fission reactors, usually as liquid D2O (heavy water), to slow neutrons without high neutron absorption of ordinary hydrogen.
During World War II, Nazi Germany was known to be conducting experiments using heavy water as moderator for a nuclear reactor design. Such experiments were a source of concern because they might allow them to produce plutonium for an atomic bomb. Ultimately it led to the Allied operation called the "Norwegian heavy water sabotage", the purpose of which was to destroy the Vemork deuterium production/enrichment facility in Norway. At the time this was considered important to the potential progress of the war.

11. Guided Practice 1
Calculate the average relative atomic mass of Magnesium given:
Isotope Relative Mass Abundance
24Mg %
25Mg %
26Mg %
Assume we have 100 atoms of Mg.
mass contributed by the 24Mg isotope is x 78.7
mass contributed by the 25Mg isotope is x 10.13
mass contributed by the 26Mg isotope is x 11.17
Total mass of 100 Mg atoms
= (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17)
Step one: multiply each relative mass by the relative abundance %. (this will give us the total mass of 100 mg atoms)

12. Finding Ar Total mass of 100 Mg atoms
= (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17)
Ar(Mg) = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17)
100
Ar(Mg) = 24.3
This is not the true mass of a Mg atom but its
relative mass compared to a 12C atom.
Step 2: Divide by 100. Ar stands for relative atomic mass

13. Finding Ar The general rule is:
Ar = Σ(relative isotopic mass x abundance)
100

14. Guided Practice 2 Find the relative atomic mass of Chorine.
Isotope Relative Mass Abundance
35Cl %
37Cl %
Ar(Cl) = ( x 75.8) + ( x 24.2)
100
Ar(Cl) = 35.45

15. Find Ar(O)
Isotopes Relative Isotopic Mass Abundance
16O %
17O %
18O %
Ar(O) = ( x 99.76) + ( x 0.04) + ( x 0.2)
100
Ar(O) = 16

16. Atomic Mass of Magnesium
Isotopes Mass of Isotope Abundance
24Mg = 24.0 amu %
25Mg = amu %
26Mg = amu %
Ar(Mg) =(24 x78.7) + (25 x10.13) + (26 x 11.17)
100
Ar(Mg) =

17. HW: Practice Quiz tomorrow on: Atom History Atomic Structure
Drawing Bohr Models
Finding Relative Abundance (?)