1. Chapter 4 Structure of the Atom
4.3 How Atoms Differ

2. Section 4.3 How Atoms Differ
The number of protons and the mass number define the type of atom.
Objectives
Explain the role of atomic number in determining the identity of an atom.
Define an isotope; be able to identify and give an example of one
Write an isotope in any one of the 3 standard notations for them.
Explain how the atomic mass unit (amu) is defined
Describe in semi-quantitative terms the relative masses of the neutron, the proton and the electron.

3. Section 4.3 How Atoms Differ
The number of protons and the mass number define the type of atom.
Objectives (cont)
Explain why atomic masses are not whole numbers.
Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.
Calculate the atomic mass of an element given the isotope masses and abundances (and variations of this problem).
Explain what a mass spectrum is, identify the instrument used to obtain it and describe the basic ideas behind its operation.

4. Section 4.3 How Atoms Differ
Key Concepts
The atomic number of an atom is given by its number of protons. The mass number of an atom is the sum of its neutrons and protons.
atomic number = number of protons = number of electrons
mass number = atomic number + number of neutrons
Atoms of the same element with different numbers of neutrons are called isotopes.
The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom
The atomic mass of an element is a weighted average of the masses of all of its naturally occurring isotopes.

5. Atomic Number (AN) AN = # of protons For any neutral element
# of protons = # of electrons
Different ANs  different elements
Lithium, Li AN = 3
Carbon, C AN = 6

6. Practice Atomic Number Practice Problems, page 116 12 – 15
Chapter Assessment, page 128-9
58, 59, 66, 75
Appendix Suppl. problems, pp 977-8
1(a-f), 2&3(a-d)

7. Isotopes / Mass Number
Isotopes have same number of protons but a differing number of neutrons
Same # protons  Same element
Mass number = sum of number of protons and neutrons in the nucleus
Mass number = atomic number + number of neutrons
Mass number not the same as atomic mass

8. Isotopes / Mass Number
To distinguish a particular isotope from another, use element name followed by dash followed by mass number
Potassium-39
Hydrogen-3 (aka tritium)
Can also use chemical symbol
K-39
H-3

9. K AN = 19 (19 protons, 19 electrons)
Potassium, K (Figure 4.17)
K AN = 19 (19 protons, 19 electrons)
19e-
19p+
20n0
21n0
22n0
K K K-41

10. Alternate Notation for Isotopes
Rather than using silver-107, silver-109 for isotopes, often preferable to use
Mass Number
Atomic Number
Note that numbers are to left of symbol

11. Ne Example Problem 4.2, page 118
One of neon’s isotopes found to have atomic number 10 & mass number 22
Find # protons, electrons, neutrons
Name isotope and give its symbol
AN = # protons = 10
Neutral atom, # electrons = protons=10
Mass number = # protons + # neutrons
# neutrons = 22 – 10 = 12
Isotope = neon-22 Ne 22 10
symbol

12. Practice Atomic & mass number and isotope symbols
Practice Problems, page 118
16, 17
Chapter Assessment, page 128-9
60 – 62, 64, 67-69, 72-74
Appendix Suppl. Problems, page 978
4, 5, 6(a-f)

13. Mass of Individual Atoms
So far, only discussed mass number
Atomic mass has definition in terms of a chosen atomic standard
Carbon-12 atom assigned a mass of exactly 12 atomic mass units (amu)
=> One amu = 1/12 mass of 126C
All masses of atoms or atomic particles expressed in terms of amu

14. Masses of Subatomic Particles
Protons and neutrons do not have mass of exactly 1 amu
Proton (p+1) and neutron (n0) masses slightly different
Electron mass ~ 1/1840 (p+1 or n0)
Particle
Mass (amu)
Electron
Proton
Neutron

15. Mass Spectrum & Mass Spectrometer
Q. How do you “weigh” these atoms to get their masses?
Mass spectroscopy
Charge (ionize) atom or molecule
Accelerate in electric field
Laws of physics predict path of ion in a known magnetic field
Specific path and place where it strikes a detector depends on ion’s mass
See page 125

16. Mass Spectrometer Detector Electric field accelerates ions
Least massive ions
+ ions
Most massive ions
Slits
Magnetic Field
Heating coil to vaporize sample

17. Mass Spectrometer Launch video from misc
Mass Spectroscopy (Royal Soc Chem) (7m 58s)
Essentials: from start to 1:54 & from 3:25 to 4:48
Mass Spectroscopy (Royal Soc Chem)

18. Mass Spectrometer Acceleration Ionization Electromagnet
To vacuum pump
Vaporized Sample
Deflection
Detection

19. Mass Spectrum – Mercury Isotopes

20. Mass Spectrum - Mercury
Natural abundance
Hg-196, 0.146%
Hg-198, 10.02%
Hg-199, 16.84%
Hg-200, 23.13%
Hg-201, 13.22%
Hg-202, 29.80%
Hg-204, 6.85%
Mass Number
Relative Abundance

21. Atomic Mass - Elements
Atomic mass of element is weighted average of the isotopes of that element
AM(element) =
AVGwt = Mass(1) x Abundance(1) +
Mass(2) x Abundance(2) + …
where Mass(i) = atomic mass of isotope(i)

22. Atomic Mass - Elements
Atomic masses don’t have integer values because:
a) Protons and neutrons have masses close to but not exactly 1 amu, so mass of a given isotope not integer
b) Even if isotope masses had integer values, process of doing weighted average over isotopes generally gives result which is not an integer

23. Chlorine Example – Fig 4.18, page 119
3517Cl Atomic mass = amu
% abundance = %
Contribution to weighted avg = amu
3717Cl Atomic mass = amu
% abundance = %
Contribution to weighted avg = amu
AVGwt = = amu
This is value listed in period table for Cl

24. Example Problem 4.3, page 121 Unknown element X
6X amu % abundance
7X amu % abundance
Calculate contributions to weighted avg
6.015 amu  = amu
7.016 amu  = amu
Sum to find mass; Atomic mass =
= amu
Matches atomic mass of lithium (Li)

25. Practice At mass of from isotope abundance Practice Problems, page 121
Section Assessment, page 121
Chapter Assessment, page 129
71 (data source?), 76 – 78
Appendix Suppl. Problems, page 978
7, 8