chapter 4 structure of the atom
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Chapter 4Structure of the Atom
4.3 How Atoms Differ
Section 4.3 How Atoms Differ
• 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.
The number of protons and the mass number define the type of atom.
Objectives
Section 4.3 How Atoms Differ
• 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.
The number of protons and the mass number define the type of atom.
Objectives (cont)
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 AN = 3• Carbon, C AN = 6
PracticeAtomic 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)
19e- 19e- 19e-
19p+
20n019p+
20n019p+
21n019p+
22n0
K-39 K-40 K-41
Alternate Notation for Isotopes
Mass Number
Atomic Number
Rather than using silver-107, silver-109 for isotopes, often preferable to use
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 = 10Neutral atom, # electrons = protons=10Mass number = # protons + # neutrons# neutrons = 22 – 10 = 12Isotope = neon-22 Ne
22
10symbol
Practice
Atomic & mass number and isotope symbolsPractice Problems, page 118
16, 17Chapter Assessment, page 128-9
60 – 62, 64, 67-69, 72-74Appendix Suppl. Problems, page 978
4, 5, 6(a-f)
Mass of Individual Atoms
So far, only discussed mass numberAtomic mass has definition in terms of a chosen atomic standardCarbon-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 ParticlesProtons 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 0.000549
Proton 1.007276
Neutron 1.008665
Mass Spectrum & Mass SpectrometerQ. How do you “weigh” these atoms to get
their masses?A. Mass spectroscopyCharge (ionize) atom or moleculeAccelerate in electric fieldLaws of physics predict path of ion in a known magnetic fieldSpecific path and place where it strikes a detector depends on ion’s massSee page 125
Mass Spectrometer
Magnetic Field
Detector
Heating coil to vaporize sample
+ ions
Electric field
accelerates ions
Slits
Least massive
ions
Most massive
ions
Mass SpectrometerLaunch video from misc
Mass Spectroscopy (Royal Soc Chem) (7m 58s)
Essentials: from start to 1:54 & from 3:25 to 4:48Mass Spectroscopy (Royal Soc Chem)
Mass Spectrometer
IonizationAcceleration
Deflection
Detection
Vaporized Sample
Electromagnet
To vacuum pump
Mass Spectrum – Mercury Isotopes
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
Rel
ativ
e A
bund
ance
Atomic Mass - Elements
Atomic mass of element is weighted average of the isotopes of that elementAM(element) =
AVGwt = Mass(1) x Abundance(1) +
Mass(2) x Abundance(2) +
…
where Mass(i) = atomic mass of isotope(i)
Atomic Mass - ElementsAtomic 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
3517Cl Atomic mass = 34.969 amu
% abundance = 75.770%
Contribution to weighted avg = 26.496 amu
3717Cl 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
Chlorine Example – Fig 4.18, page 119
Example Problem 4.3, page 121
Unknown element X6X 6.015 amu 7.59% abundance7X 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 amuSum to find mass; Atomic mass = 0.457 + 6.483 = 6.940 amuMatches atomic mass of lithium (Li)
PracticeAt mass of from isotope abundancePractice Problems, page 121
18 - 19Section Assessment, page 121
23 - 24Chapter Assessment, page 129
71 (data source?), 76 – 78Appendix Suppl. Problems, page 978
7, 8
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