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Early Atomic TheoryChapter 3

Democritus 460 BC- ~

370 BCNothing exists except atoms and

empty space; everything else is

opinion.

Matter is composed of small

indivisible particles, atomos –

meaning “Indivisible”

Plato and Aristotle didn’t buy it…

(play-doe Vs legos)

The AtomAristotle

Matter was continuous. Did not believe in atoms

4 elements – fire, air, earth and water

Blend all 4 of these in different proportions to get all substances

Neither theories supported by experimental evidence

Alchemy 2000 years of Chemical History

Many were scientists & made

important contributions to modern

Chemistry

Some looked to turn metals into

gold

Important contributions:

Discover of Mercury, Sulfur and

Antimony; lab procedures and

preparing acids.

The atom

Atom

The smallest unit of an element that

retains all the properties of that

element

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Foundations of Atomic Theory

Law of conservation of mass (Matter)

Mass is neither created nor

destroyed during ordinary chemical

reactions or physical changes

(Antoine Lavoisier, 1774) 8 X2Y16 X 8 Y+

2.1

Foundations of Atomic Theory

Law of Definite Proportions (Proust)

Chemical compounds contain the same elements in exactly the same proportion regardless of sample size

NaCl Sodium Chloride

39% Sodium and 61% Chlorine

Water will always contain 8g of Oxygen for every 1g of Hydrogen

Dalton’s Atomic Theory (1808)1. Elements are composed of extremely small

particles called atoms.

2. All atoms of a given element are different from

the atoms of all other elements.

3. Compounds are composed of atoms of more

than one element in fixed ratios.

4. A chemical reaction involves only the

separation, combination, or rearrangement of

atoms; it does not result in their creation or

destruction of atoms.2.1

Law of Multiple Proportions

If two or more different compounds are

composed of the same two elements, the

ratio of the masses of elements are always

a ratio of small whole numbers.

CO, CO2, CO32

2.1

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Example of Law of Multiple Proportions

Carbon combines with oxygen to form

CO and CO2 .

Mass of

Carbon(g)

Mass of

Oxygen(g)

Ratio of O

in CO2 to O

in CO

CO 12.01 16.00

CO2 12.01 32.00 2:1

Mysterious world of

the atom

Let’s probe deeper…

Electron

-properties

Model of the atom

Experiments to probe atom:

+ Cathode ray tube

+ Mulliken Oil drop

+ Rutherford scattering

Crookes Tube

1869-1875

“Something” was

making a glowing

shadow

Parts of the Atom

Discovery of Electron J. J. Thomson – English Physicist (1897)

Made a piece of equipment called a

cathode ray tube

Adaptation of Crookes tube

Cathode Ray experiment

Discovery of the electron

Experiments confirmed that electron has

negative charge

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics) 2.2

CATHODE RAY TUBE EXPERIMENT

Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

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n Passing an electric current makes a beam

appear to move from the negative to the

positive end

Thomson’s Experiment

Voltage source

+-Voltage source

Thomson’s Experiment

n By adding an electric field he found that the

moving pieces were negative

+

-

Thomson Summary

Found the electron!

Couldn’t find positive (for a

while)

Said the atom was like plum

pudding

A bunch of positive stuff, with

the electrons able to be removed

Robert Millikan

Robert Millikan

Oil Drop Experiment

Electron mass 9.109 * 10-31 kg

Electron Charge – 1.602 * 10-19

Coulomb

Relative charge (-1)

Measured charge and mass of e- (1923 Nobel Prize in Physics)2.2

Millikan’s Oil Drop Experiment

Rutherford’s experiment

Used radioactivity

Alpha particles - positively charged pieces given off by uranium (2 protons & 2 neutrons)

Shot them at gold foil which can be made a few atoms thick

When the alpha particles hit a florescent screen, it glows.

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Lead

blockUranium

Gold Foil

Florescent

Screen

THOMSON (AND KELVIN)’S PLUM PUDDING

MODEL OF THE ATOM

He Expected

The alpha particles to pass

through without changing direction

very much

Because (he thought) the positive

charges were spread out evenly.

Alone they were not enough to

stop the alpha particles

GOLD FOIL EXPERIMENT

What he expected:

Because, he thought the mass was

evenly distributed in the atom

What he got

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How he explained it…

+

Atom is mostly empty

Small dense,

positive piece

at center

Alpha particles

are deflected by

it if they get close enough

Gold Foil experiment

+

Discovery of Atomic

Nucleus

Rutherford’s Gold foil experiment

Concluded atom had a positively

charged nucleus

Volume of nucleus small compared

to volume of atom

Most of the mass of the atom

contained in nucleus (dense)

Composition of Atomic

NucleusProton – positive charge

Number of protons in nucleus determines atoms identity – Atomic Number

Mass 1.673 * 10-27 kg

Relative Charge (+1)

Neutron – “neutral” no charge

Mass 1.675 * 10-27 kg

Summary of Subatomic particles

Electron

Proton

Neutron

Name Symbol Charge

Relative

mass

Actual

mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Structure of the atom

Atoms composed of smaller particles

Two regions

Nucleus – Protons and Neutrons

Electron Cloud

Number and arrangement of these particles determine chemical properties

Neutral – charges add up to zero

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Size of an atom

Atoms are small.

Measured in picometers, 10-12 meters

ex: Hydrogen atom, 32 pm radius

Nucleus tiny compared to atom

If the atom were the size of a stadium, the

nucleus would be the size of a marble.

Radius of nucleus ~ 10-15 m

Density near 1014 g/cm

Counting the Pieces

Atomic Number = number of protons

# of protons determines kind of atom

the same as the number of electrons in the

neutral atom

Mass Number = the number of protons +

neutrons

All the things with significant mass

Isotopes

Atoms of the same element (same # of

protons) can have different numbers of

neutrons

different mass numbers

called isotopes

Ions

Elements can also gain or lose electrons.

Each electron has a -1 charge so;

Adding an electron = -1

Removing an electron = +1

An element with a charge (gained or lost

electrons) is an ion..

Cations & Anions

Cations are ions with positive charges

Lost one or more electrons

Ca+ion

Think cats are nice (positive) but always

looking around for their electron

Anions are ions with negative charges

Gained one or more electrons

Nuclear Symbols

Contain the symbol of the element, the mass

number and the atomic number

XMass

number

Atomic

number

Charge

(ion only)

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Nuclear Symbols

Find the

number of protons

number of neutrons

number of electrons

Atomic number

Mass Number

Cation, Anion or Neither?

F19

9

Nuclear Symbols

n Find the

–number of protons

–number of neutrons

–number of electrons

–Atomic number

–Mass Number

–Cation, Anion or Neither?

Br80

35

-1

Symbols

n If an element has an atomic number of

34 and a mass number of 78 what is

the ?

–number of protons

–number of neutrons

–number of electrons

–Complete symbol

Symbolsn if an element has 91 protons and

140 neutrons and a +2 charge

what is the ?

–Atomic number

–Mass number

–Number of electrons

–Complete symbol

Symbols

n if an element has a +1 charge, 78

electrons and 117 neutrons what is the

?

–Atomic number

–Mass number

–number of protons

–Complete symbol

Naming Isotopes

Hyphen Notation

Put the mass number after the

name of the element

carbon- 12

carbon -14

uranium-235

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Atomic Mass

How heavy is an atom of oxygen?

There are different kinds of oxygen atoms.

More concerned with average atomic mass.

Based on abundance of each isotope in nature.

Don’t use grams because the numbers would be

too small

Measuring Atomic Mass

Unit is the Atomic Mass Unit (amu)

Defined as one twelfth the mass of a carbon-

12 atom.

Each isotope has its own atomic mass we

need the average from percent abundance.

Calculating averages

A student has an 81 test average, a 89 quiz

average, a 90 lab average, and 100

homework average.

Average grade is…?

81+ 89 + 90 + 100 = 360

360 =

4

90 average!

Calculating weighted

averages

Tests = 50%; Quizzes 20%; Labs 15 %; Homework 15%

Weighted average = (.5 x 81) + (.2 x 89) + (.15 x 90) + (.15 x 100) =

Weighted average = 40.5 + 17.8 + 13.5 + 15

Weighted average = 86.8 (rounds to 87)

Atomic Mass

Boron has 2 isotopes. 20% is Boron-10,

and 80% is Boron-11. What is the

average atomic mass?

(.20 X 10)+(.80 X 11) = ?

10.8 amu

Check out the periodic table!

Atomic Mass

Calculate the atomic mass of lead if 1.37% is

lead-204, 26.26% is lead-206, 20.82% is lead-

207, and 51.55% is lead 208.

207.21 amu

If not told otherwise, the mass of the isotope

is the mass number in amu

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Atomic Mass

Is not a whole number because

it is an average.

The average is based on the

how often each isotope appears

in nature

Explains the decimal numbers

on the periodic table.