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