Chapter 4
Structure of the Atom
4.3 How Atoms Differ
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.
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.
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.
Atomic Number (AN)
AN = # of protons
For any neutral element
# of protons = # of electrons
Different ANs  different elements
• Lithium, Li
• Carbon, C
AN = 3
AN = 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)
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
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
Potassium, K (Figure 4.17)
K AN = 19 (19 protons, 19 electrons)
K-39
19e-
19e-
19p
19p++
20n
20n00
K-40
19p+
21n0
K-41
19e-
19p+
22n0
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
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
22
Isotope = neon-22
Ne symbol
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)
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
Masses of Subatomic Particles
Protons and neutrons do not have
mass of exactly 1 amu
Proton (p+1) and neutron (n0) masses
slightly different
Particle Mass (amu)
Electron mass ~
Electron
0.000549
+1
0
1/1840 (p or n )
Proton
1.007276
Neutron
1.008665
Mass Spectrum & Mass Spectrometer
Q. How do you “weigh” these atoms to get
their masses?
A. 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
Mass Spectrometer
Detector
Electric
field
accelerates
ions
Least
massive
ions
+ ions
Slits
Magnetic Field
Heating coil to vaporize sample
Most
massive
ions
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)
Mass Spectrometer
Ionization
Acceleration
Electromagnet
Vaporized
Sample
To vacuum
pump
Deflection
Detection
Mass Spectrum – Mercury Isotopes
Mass Spectrum - Mercury
Natural abundance
Relative 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%
Mass Number
Hg-204, 6.85%
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)
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
Chlorine Example – Fig 4.18, page 119
35 Cl
17
Atomic mass = 34.969 amu
% abundance = 75.770%
Contribution to weighted avg = 26.496 amu
37 Cl
17
Atomic mass = 36.966 amu
% abundance = 24.230%
Contribution to weighted avg = 8.9569 amu
AVGwt = 26.496 + 8.9569 = 35.453 amu
This is value listed in period table for Cl
Example Problem 4.3, page 121
Unknown element X
6X 6.015 amu
7.59% abundance
7X 7.016 amu 92.41% abundance
Calculate contributions to weighted avg
6.015 amu  0.0759 = 0.457 amu
7.016 amu  0.9241 = 6.483 amu
Sum to find mass; Atomic mass =
0.457 + 6.483 = 6.940 amu
Matches atomic mass of lithium (Li)
Practice
At mass of from isotope abundance
Practice Problems, page 121
18 - 19
Section Assessment, page 121
23 - 24
Chapter Assessment, page 129
71 (data source?), 76 – 78
Appendix Suppl. Problems, page 978
7, 8