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Mark S. CracoliceEdward I. Peters

Mark S. Cracolice • The University of Montana

Chapter 5Atomic Theory:

The Nuclear Model of the Atom

5.1 Dalton’s Atomic TheoryDalton’s Atomic Theory (1808)

1. Each element is made up of tiny, individual particles called atoms.

2. Atoms are indivisible; they cannot be created or destroyed.3. All atoms of each element are identical in every respect.4. Atoms of one element are different from atoms of any other

element.5. Atoms of one element may combine with atoms of other

elements, usually in the ratio of small whole numbers, to form chemical compounds.

5.1 Dalton’s Atomic Theory

The atoms are neither created nor destroyed in chemical reactions, but simply arranged

differently to form the products.

5.1 Dalton’s Atomic TheoryLaw of Multiple Proportions

When two elements combine to form more than one compound, the different masses of one element that combine with the same mass of the other element are in a simple ratio of whole numbers.

Subatomic ParticlesGoal 3

Identify the three major subatomic particles by symbol, charge, and approximate atomic mass, expressed in atomic mass units.

5.2 Subatomic ParticlesElectron

Symbol: e or e–

Fundamental charge: 1–Mass (g): 9 × 10–28 gMass (atomic mass unit): 0.000549 uLocation: Outside nucleusDiscovered: 1897 by Thomson

Subatomic ParticlesProton

Symbol: p or p+

Fundamental charge: 1+Mass (g): 1.673 10–24 gMass (u): 1.00728 uLocation: Inside nucleusDiscovered: 1919 by Rutherford

Subatomic ParticlesNeutron

Symbol: n or n0

Fundamental charge: 0Mass (g): 1.675 10–24 gMass (u): 1.00867 uLocation: Inside nucleusDiscovered: 1932 by Chadwick

5.3 The Nuclear Atom

5.3 The Nuclear AtomThe Nuclear Model of the Atom

• An electron with a mass of 1/1837 u could not have deflected an alpha particle with a mass of 4 u.

• Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume. He called this mass the nucleus.

5.3 The Nuclear AtomThe Nuclear Model of the Atom

1. Every atom contains an extremely small, extremely dense nucleus.

2. All of the positive charge and nearly all of the mass of an atom are concentrated in the nucleus.

3. The nucleus is surrounded by a much larger volume of nearly empty space that makes up the rest of the atom.

4. The space outside the nucleus is very thinly populated by electrons, the total charge of which exactly balances the positive charge of the nucleus.

5.3 The Nuclear Atom

TThe diameter of an atom is approximately 10,000 times the diameter of its nucleus.

5.3 The Nuclear AtomPlanetary Model of the Atom (1911)

Electrons travel in circular orbits around the nucleus,much like planets move in orbits around the sun.

5.4 Isotopes

Every atom of any particular elementhas the same number of protons, which is

called the atomic number, Z.

Atoms are electrically neutral, the number of electrons in an atom therefore must be equal

to the number of protons.

5.4 Isotopes

All atoms of any particular element are not identical; the number of neutrons may be

different.

Atoms of the same element that have different numbers of neutrons—different masses—are

called isotopes.

5.4 Isotopes

An isotope is identified by its mass number, A:

Mass number = A =Total number of protons + neutrons

Mass number = # of protons + # of neutrons A = Z + N

5.4 IsotopesName of an Isotope

Elemental name–Mass number

Examples:What is the name of the carbon atom with 6 protons and 6

neutrons?Mass number = 6 + 6 =12, so the name is carbon-12

What is the name of the carbon isotope with 7 neutrons?Mass number is 6 + 7 = 13, so the name is carbon-13

5.4 IsotopesSymbol of an Isotope

Known as a nuclear symbol

Example:What are the nuclear symbols for carbon-12 and carbon-13?

5.5 Atomic MassMasses of atoms are expressed in

atomic mass units, u.

The u is defined as

1 u 1/12 the mass of one carbon-12 atom

1 u mass of 1 nuclear subatomic particle

5.5 Atomic MassPercentage Abundance of Some Natural Isotopes

Symbol Mass (u) Percent 1H 1.0078250321 99.9885 2H 2.0141017780 0.0115

12C 12 (exactly) 98.9313C 13.0033548378 1.07

39K 38.9637069 93.258140K 39.96399867 0.011741K 40.96182597 6.7302

5.5 Atomic MassThis is the mass spectrum of neon.

The relative abundance plotted on the y-axis is the percentage of each isotope found in a natural sample of the pure element.

The mass-to-charge ratio, m/z, is plotted on the x-axis. Since this plot shows 1+ ions only, m/z is the mass of the isotope, expressed in u.

5.5 Atomic MassAtomic mass (of an element):

The average mass of all atoms of anelement as they occur in nature.

Example:Chlorine has two natural isotopes: 75.78% is chlorine-35 at

34.968852721 u and 24.22% is chlorine-37 at 36.96590262 u. What is the atomic mass of chlorine?

Solution:0.7578 × 34.968852721 u = 26.50 u0.2422 × 36.96590262 u = 8.95 3 u

35.45 u

5.6 The Periodic TablePeriodic Table of the Elements

A table that arranges the elements in orderof increasing atomic number.

Elements with similar properties appear in the same column.

5.6 The Periodic Table

The Periodic TableSample box from the periodic table, representing sodium.

5.6 The Periodic Table

PeriodsHorizontal rows in the periodic table.

Numbered 1–7, top to bottom(numbers usually not printed).

Groups (or Chemical Families)Vertical columns in the periodic table.

U.S. numbering: A groups and B groupsIUPAC numbering: 1–18, left to right

5.6 The Periodic TableMain group elements

(sometimes called representative elements)Elements in the U.S. A-groups

Transition elementsElements in the U.S. B-groups

MetalsElements on the left of the stair-step line

NonmetalsElements on the right of the stair-step line

5.7 Elemental Symbols

5.8 Elemental SymbolsMemorize the 35 name–symbol pairs given in Fig. 5.8.

Look at the location of each element in the periodic table while you work on memorization—it will help you remember the

names and symbols.

Whenever a chemical symbol has two letters,the first letter is always capitalized and

the second letter is always written in lowercase.

HomeworkHomework for chapter 5:17, 19, 21, 27,29, 31, 39, 41, 54