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 AN = 3 • Carbon, C 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 mass atomic

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 K-39 K-40 K-41 19e 19e 19p + + 20n n 0 0 19p + 21 n 0 19p + 22 n 0

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 Isotope = neon-22 22 10 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 exactly a mass of 12 atomic mass units (amu) => One amu = 1/12 mass of 12 6 C 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 (n 0 ) masses slightly different Particle Mass (amu) Electron mass ~ 1/1840 (p +1 or n 0 ) Electron 0.000549

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

Acceleration Ionization Electromagnet To vacuum pump Vaporized Sample Deflection Detection

Mass Spectrum – Mercury Isotopes

Mass Spectrum - Mercury

Mass Number 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%

Atomic Mass - Elements

Atomic mass of element is weighted average of the isotopes of that element AM(element) = AVG wt = 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 17 Cl Atomic mass = 34.969 amu % abundance = 75.770% Contribution to weighted avg = 26.496 amu 37 17 Cl Atomic mass = 36.966 amu % abundance = 24.230% Contribution to weighted avg = 8.9569 amu AVG wt = 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 6 X 6.015 amu 7.59% abundance 7 X 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