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Atomic

StructureAn introduction to Atomic Theory

Instructional

Goals• Define Dalton‟s atomic theory

• Differentiate between historical atomic models

• Describe the tentative nature of science.

Overview• Leucippus, Democritus and Aristotle

• John Dalton

• J.J. Thompson

• Ernst Rutherford

• Neils Bohr

• Modern Quantum Mechanical Model of

the atom

Leucippus 480-420 BC

• He was a natural philosopher.

• He thought matter to be finite.

• His student was Democritus.

• Only the quotes others have of Leucippus and Democritus been found.

He believed matter is

finite.

Finite means having

boundaries or limits,

like “No refills.”

Democritus 460-371 BC

Democritus’ Hypothesized:

If matter is cut again

and again, eventually a

single indivisible part

will remain. This part

called atomos is Greek

for indivisible. His

idea of matter gave us

the word ATOM.

According to Aristotle

• Matter was continuous

or infinitely smaller .

• Waterfalls produce

smaller and smaller

particles we call mist.

• Four basic elements”

Earth, Fire, Water and

Air.

Aristotle 384-322 BC

• His works were

rediscovered in

Western Europe

around 1200 AD.

• Aristotle‟s theory

about the atom was

widely accepted and

promoted by the

Catholic Church for

generations.

• Was an English school teacher who

independently studied the nature of matter.

• Recorded observations of his experiments.

• Proposed the first atomic theory based on

empirical evidence.

http://maple.lemoyne.edu/~giunta/dalton.html

http://www.tannerm.com/index.htm

John Dalton 1766-1844

Dalton‟s Theory• All elements are composed of submicroscopic

indivisible particles called atoms.

• Atoms of the same element are identical.

• Atoms of different elements can mix together, or chemically combine with one another to form compounds.

• Chemical reactions occur when atoms are separated, combined, or rearranged, but atoms of one element do not change into

another element.

Dalton‟s Model of the atom

History Summary

Performed

experiments to

support his theory.

Scientific theory- Matter

finite: is composed of

indivisible particles

called “atoms.”

Scientist:

Dalton

His idea or matter

was not supported by

later experiments.

Matter is infinite:

composed of basic

elements: earth, air,

water, and fire.

Aristotle

His ideas agreed

with later theories,

but lacked

experimental

support.

Matter is finite:

composed of tiny

invisible, indestructible

particles.

Democritus

Highlights PhilosophyPhilosophers

The discovery of the Electron

• J.J. Thomson (1856-1940) used results from

cathode ray tube (commonly abbreviated CRT)

experiments to discover the electron.

http://dbhs.wvusd.k12.ca.us/Disc-of-Electron-Images.html

http://dbhs.wvusd.k12.ca.us/Disc-of-Electron-Intro.html

J.J. Thompson’s Experiments

Thomson devised a cathode ray tube with a paddle

wheel built inside. When the high voltage

electricity was turned on the paddle wheel began

to rotate and move away from the cathode and

towards the anode.

J.J. Thompson’s Experiments

Thomson devised another cathode ray tube with positive

and negatively charged plates. Thomson observed that

the particles were being repelled by the negatively

charged plate and attracted to the positive plate.

http://lausd.k12.ca.us/~kmcmahon/Atoms%20%26%20Period%20Table/page_id_4745.html

Magnetic vs Electric field

deflection ratio

J. J. Thomson's experiment resulted

in a value for e/m (the ratio of the

charge of the electron to the mass of

the electron).

Millikan’s Oil Drop Experiment

http://webphysics.davidson.edu/applets/pqp_preview/cont

ents/pqp_errata/cd_errata_fixes/section4_5.html

Thompson’s Ions MineIn the dusty lab‟ratory,

„Mid the coils and wax and twine,

There the atoms in their glory.

Ionize and recombine.

Chorus

O my darlings! O my darlings!

O my darling ions mine!

You are lost and gone forever

When just once you recombine.

In a tube quite electrodeless,

They discharge around a line,

And the glow they leave behind them

Is quite corking for a time.

Chorus

…In the weird magnetic circuit

See how lovingly they twine,

As each ion describes a spiral

Round its own magnetic line.

J.J. Thompson‟s ModelAtom is like

raisin bread.

The atom‟s

“dough” is

positive so

the electrons

stick like

raisins.

This is also referred to as the plum pudding model.

Rutherford‟s Hypothesis

Rutherford thought if J.J.‟s model was correct, alpha particles

would not be deflected through a thin piece of gold foil.

http://micro.magnet.fsu.edu/electromag/java/rutherford/

ZnS

screen

What he expected

Why? Because, he thought the mass

was evenly distributed in the atom.

He thought if the mass

was evenly distributed

in the atom…

alpha particles would pass straight through.

Rutherford‟s ExperimentMost of the alpha

particles went straight

through.

But some were

deflected; the atomic

model had to change.

+

How he explained it

• A small dense,

positive piece is at the center of the atom.

• Alpha particles are deflected by it if they get closeenough

•The atom is

mostly empty

space

Rutherford‟s

Model of the AtomClick here for

simulation

Summary

Have your neighbor

• explain the reason we

no longer accept the

plum pudding/raisin

bread model of the

atom

•Describe the Rutherford

model of the atom.

•What is the nucleus?

•What is in the nucleus?

•How big is the nucleus?

•Where are the electrons?

Because less than 1% of

the alpha particles were

deflected, so 99% must

have passed through

empty space.

Protons and neutrons

1/10,000 of an atom

Nucleus is a dense center of mass

surrounded by empty space

Neils Bohr 1885-1962

• Atoms have energy levels that contain electrons.

• Electrons must have a specific or quanta of energy to change levels.

• His model is the first quantum model of the atom.

http://www.colorado.edu/physics/2000/quantumzone/index.html

Neils Bohr’s Theory• The dense nucleus is composed of positive

protons

• Electrons travel in elliptical orbits of definite energies

• A force of attraction draws electrons to the nucleus

• Centrifugal force pulls them outward

• Electron energy increases with distance from the nucleus

Bohr Model

Problems with Bohr‟s Model

• The electrons must radiate energy when they move

• Loss of energy should propel them closer to the nucleus

• All atoms therefore must collapse

• The model does not predict reality- atoms don‟t collapse

• Hydrogen is the only atom the model seems to explain

The modern theory of the atom

Electrons are negatively charged

particles found around the nucleus.

Atoms are the building blocks of matter.

The nucleus

concentrates

protons and

neutrons. This

region in the

atom has the

most mass,

and the least

volume.

Protons have a

positive

charge.

The Quantum Model• Is a mathematical model of the atom as

opposed to a simple visual model

• Predicts electron positions in terms of probability clouds

• Describes electron positions and energies, as approximations

• Answers questions raised by previous theory’s

Current Animated Atomic Model

Atomic Model SummaryDalton‟s Thompson‟s Rutherford‟s

Bohr‟s Quantum

Mechanical

Model

For Review• How did Democritus idea of matter differ from

Aristotle’s?

• What is John Dalton’s Atomic Theory?

• What evidence demonstrated electrons had charge?

• Describe the evidence for a small dense nucleus.

• Describe the modern quantum model of the atom.

• Explain, using examples, the tentative nature of science.

Relative Mass

Atoms are compared according to how much mass they have. For example, it takes 12 hydrogen atoms to equal the mass of 1 carbon atom.

VocabularyRelative Atomic Mass

Mass Number

Atomic Mass UnitAverage Atomic Mass

Isotope

Electron

Neutron

Proton Video lessons

http://web.jjay.cuny.edu/~acarpi/

NSC/3-atoms.htm

So a relative mass unit was invented, the

AMU, or atomic mass unit.

Carbon has 12 times the mass of hydrogen,

or we say hydrogen weighs 1 amu and

carbon weighs 12 amu.

12 X 1 amu hydrogen = 1 X 12 amu carbon

So if carbon has a mass of 12 amu that also

means it has a counted number of 12

subatomic particles in its nucleus: 6

neutrons and 6 protons. So 12 amu is also

called the MASS NUMBER.

To find the number of protons, look up the

atomic number. To find the number of neutrons,

subtract the atomic number from the mass

number.

Vocabulary Terms

Mass number = protons plus neutrons = Z

(Z or Mass number = #p+ added to #no)

Atomic number = number of protons = A

Nuclear symbol Z 14

A 6

carbon-14

http://www.colorado.edu/physics/2000/isotopes/

Atoms of the same element can have different

numbers of neutrons; the different possible

versions of each element are called isotopes. For

example, the most common isotope of hydrogen

has no neutrons at all; there's also a hydrogen

isotope called deuterium, with one neutron, and

another, tritium, with two neutrons.

Isotopes

Hydrogen Deuterium Tritium

Atomic Theory and the

Periodic Table

• Atoms are the smallest particles of an element that display the properties of that element

• There are 92 naturally occurring elements

• Recently scientists found theoretically predicted elements 114 and 118 and____?

• Modern Atomic Theory is helping to complete the Periodic Table of Elements

How does this story illustrate

the definition of science?

Science is a tentative, truth seeking, self correcting relevant way of learning.

An Introduction to the

Elements

http://www.privatehand.com/flash/elements.html

References

• http://inst.augie.edu/~jkbjerga/hist.html

• http://www.tannerm.com/index.htm

• http://www.watertown.k12.wi.us/hs/teachers/buescher/atomtime.html

• http://dbhs.wvusd.k12.ca.us/Greeks.html

• http://www.tannerm.com/atoms/chem_elements/list.htm

• http://www.tannerm.com/atoms/periodic_table/Periodic.htm

• http://www.liv.ac.uk/Chemistry/Links/links.html

• http://windows.engin.umich.edu/cgi-bin/tour_def?link=/sun/Solar_interior/Sun_layers/Core/atom_model.html&sw=false&cd=false&fr=f&edu=mid