1. Unit Objectives
1)understand the evolution of models of the atom: Dalton, Thomson, Rutherford, Bohr, the ‘quantum model
2) understand the meaning of mass number, atomic (proton) number.
3) be able to write and work with full atomic symbols
4) be able to calculate relative atomic, molecular and formula mass

2. Refreshing some Old Terminology
Bohr Rutherford Diagrams
Isotopes and ions
R.A.M. (Relative Atomic Mass)

3. Isotopes Relative abundance
1.8 Mass spectrometer (SB p.20)
Cl has 2 isotopes: Cl-35 and Cl-37
Isotopes
Relative abundance
Cl-35
75%
Cl-37
25%
How can we know?

4. Mass spectrometer consists of 6 parts:
1.8 Mass spectrometer (SB p.20)
Mass spectrometer consists of 6 parts:

5. Mass spectrum of Cl2: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+ 70
1.8 Mass spectrometer (SB p.21)
Mass spectrum of Cl2:
m/e ratio
Corresponding ion 35
35Cl+ 37
37Cl+ 70
35Cl─35Cl+ 72
35Cl ─ 37Cl+ 74
37Cl ─37Cl+

6. Mass spectrum of CH3Cl: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+
1.8 Mass spectrometer (SB p.21)
Mass spectrum of CH3Cl:
m/e ratio
Corresponding ion 35
35Cl+ 37
37Cl+ 50
12CH3─35Cl+ 51
13CH3 ─ 37Cl+ 52
12CH3 ─37Cl+

7. ISOTOPES & AVERAGE ATOMIC OR MOLAR MASS

8. MASS SPECTRA
R.A.M.
Consider neon and its 3 isotopes Ne 21Ne Ne.
We use these values to determine the
average molar mass of an element or
Relative atomic mass, called RAM
m/z
90.92
0.26
8.82
Abundance / %
Calculate the average relative atomic mass of neon using the above information.
Out of every 100 atoms are 20Ne , are 21Ne and are 22Ne
Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) =
100
Relative atomic mass =

9. Calculate the relative atomic mass of the following
– give your answers to 3 significant figures
Bromine : 79 Br 50% , 81 Br 50%
Copper : Cu 69% , 65 Cu 31%
Zirconium : Zr 51.5% , Zr 11.2%, Zr 17.1%,
94 Zr 17.4%, Zr 2.8%
Lead : 204 Pb 1.5% , Pb 23.6%, Pb 22.6%, Pb 52.3%
Neon : Ne 90.9% , Ne 0.2%, Ne 8.9%
63.6
91.3
207.
20.2

10. Can we calculate the % composition if we know the isotopes ?
Naturally occurring potassium consists of potassium-39 and potassium-41.
Calculate the percentage of each isotope present if the average is 39.1.
Assume there are x nuclei of 39K in every 100; so there will be (100-x) of 41K
so x (100-x) =
100
therefore x x = 3910
thus x =
and x = 95
ANSWER There will be 95% 39K and
5% 41K

11. Right, ok now you try, see the handout