2. The Atom

2.1 Evidence for the existence

of the electron • The universe consists of energy & matter.

Matter occupies space and has mass.

• Law of conservation of mass states matter is neither created nor destroyed in the course of a chemical reaction

• All matter composed of particles (atoms, molecules or ions)

• Evidence is shown by expt involving the diffusion of ammonia gas and HCl gas in a glass tube

Diffusion

Diffusion: is the movement of

gas from areas of high gas

concentration to areas of low

gas concentration

Definition!

Diffusion of ammonia and HCl

gases

2.1 Evidence for the existence

of the electron

• Atoms very minute particles

• e.g. H atom in its normal state has

radius of about 4 x 10-11m

• The atom is made of even smaller

particles

• The atom is considered to be the

basic unit from which all substances

are formed

2.2 History of the

atom

Greeks • Some ancient Greeks

noticed that

substances such as

iron, gold, lead, silver

were 'pure'

• They could not be

broken down into

simpler substances

by heat.

400 BC

• The Greek philosophers

were the first to propose

that matter is made up of

tiny particles.

• They didn’t do any

experiments to back this

up.

Greeks

The Greek word for ‘indivisible’ is

‘atomos’ from where we get the

word atom to describe these

indivisible particles

• Over the next two millennia, major advances in

chemistry were achieved by alchemists. Their

major goal was to convert certain elements into

others by a process called transmutation.

A Brief History of Chemistry

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

The Greeks History of the Atom

• Not the history of atom, but the idea of the atom

• The Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air.

• Democritus and Leucippus Greek philosophers

~ ~

Greek Model

• Greek philosopher

• Idea of ‘democracy’

• Idea of ‘atomos’

– Atomos = ‘indivisible’

– ‘Atom’ is derived

• No experiments to support

idea Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus

“To understand the very large,

we must understand the very small.”

DEMOCRITUS (400 BC) – First Atomic Hypothesis

Atomos: Greek for “uncuttable”. Chop up a piece of matter until you reach the atomos.

Properties of atoms:

• indestructible.

• changeable, however, into different forms.

• an infinite number of kinds so there are an infinite number of elements.

• hard substances have rough, prickly atoms that stick together.

• liquids have round, smooth atoms that slide over one another.

• smell is caused by atoms interacting with the nose – rough atoms hurt.

• sleep is caused by atoms escaping the brain.

• death – too many escaped or didn’t return.

• the heart is the center of anger.

• the brain is the center of thought.

• the liver is the seat of desire.

“Nothing exists but atoms and space, all else is opinion”.

Democritus

Anaxagoras (Greek, born 500 B.C.) –Suggested every substance had its own kind of “seeds” that clustered together to make the substance, much as our atoms cluster to make molecules.

Some Early Ideas on Matter

O’Connor Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 26,

Empedocles (Greek, born in Sicily, 490 B.C.)

–Suggested there were only four basic seeds – earth, air, fire, and water. The elementary substances (atoms to us) combined in various ways to make everything.

Democritus (Thracian, born 470 B.C.)

–Actually proposed the word atom (indivisible) because he believed that all matter consisted of such tiny units with voids between, an idea quite similar to our own beliefs. It was rejected by Aristotle and thus lost for 2000 years.

Aristotle (Greek, born 384 B.C.)

–Added the idea of “qualities” – heat, cold, dryness, moisture – as basic elements which combined as shown in the diagram (previous page).

Hot + dry made fire; hot + wet made air, and so on.

Who was Right?

• Greek society was slave based

• Beneath famous to work with hands

• did not experiment

• Greeks settled disagreements by

argument

• Aristotle was more famous

• He won!

• His ideas carried through middle ages.

• Alchemists change lead to gold

John Dalton

• 1808

• Proposed

an Atomic

Theory

• Developed a theory – that matter

is composed of tiny, indivisible

particles called atoms that can

neither be created nor

destroyed i.e. no change in mass

‘law of conservation of mass’

John Dalton (Atomic theory)

.. the existence of atoms

Conservation of Atoms

John Dalton

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

2 H2 + O2 2 H2O

4 atoms hydrogen

2 atoms oxygen

4 atoms hydrogen

2 atoms oxygen

H

H

O

O

O

O

H

H

H

H

H

H

H2

H2

O2

H2O

H2O

+

Legos are Similar to Atoms

Lego's can be taken apart and built into many different things.

H

H

O

O

O

O

H

H

H

H

H

H H2

H2

O2

H2O

H2O

+

Atoms can be rearranged into different substances.

45 g H2O ? g H2O

Conservation of Mass

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

High

voltage

Before reaction

electrodes

glass

chamber

5.0 g H2

80 g O2

300 g (mass

of chamber)

+ 385 g total

H2 O2

High

voltage

After reaction

0 g H2

40 g O2

300 g (mass

of chamber)

+ 385 g total

O2

H2O

John Dalton proposed his atomic

theory.

1. All matter is made up of small

particles called atoms.

2. Atoms are indivisible – they cannot

be broken down into smaller

particles. False!

3. Atoms cannot be created or

destroyed. False!

FACT

We now know that there are

particles within atoms –

subatomic particles e.g. protons,

neutrons and electrons.

Radioactive atoms can change

into other atoms.

Dalton’s Symbols

John Dalton

1808

Daltons’ Models of Atoms

Carbon dioxide, CO2

Water, H2O

Methane, CH4

Structure of Atoms

• Scientist began to wonder what an atom was like.

• Was it solid throughout with no internal structure or was it made up of smaller, subatomic particles?

• It was not until the late 1800’s that evidence became available that atoms were composed of smaller parts.

Discovery of the Electron

• In the 1800’s electricity was new,

exciting and the subject of a lot of

study.

William Crookes

• 1875

• Crookes passed an

electric current through

a complete vacuum just

to see what would

happen.

Crookes experiment

• Electricity seemed to leap across the

vacuum from the negative plate to the

positive plate.

• The radiation given off from the negative

cathode was called Cathode rays.

Crookes Tube

William Crookes

Mask holder

Cathode

(-)

Anode

(+)

Crookes tube (Cathode ray tube)

Mask holder

Glow

William Crookes

A discharge tube

• A long glass tube fitted with a metal electrode at each end

• When the gas inside is at a very low pressure, passing electricity through the tube results in invisible rays travelling in straight lines from the cathode (-ve) to the anode (+ve)

• Small objects placed in the path of the rays cast a sharp shadow in the fluorescence at the end of the tube

Crooke’s Vacuum tube

Cathode (-)

Anode (+) High

Voltage

Vacuum tube means most of the air has been pumped out of it

Maltese Cross

The Effect of an Obstruction on

Cathode Rays

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117

High

voltage

cathode

source of

high voltage

yellow-green

fluorescence

shadow

Anode

Crooke’s Vacuum tube

An electric current passing through air causes the air to glow

Cathode (-)

Anode (+)

Rays coming from the –ve electrode (cathode) cast a shadow

at the far end of the tube

Shadow of cross

on glass

Cathode rays Maltese cross

• The cathode rays caused shadows and could spin light paddle wheels.

Crooke’s experiment

• This proved that the cathode rays

carried energy, and that they

might be made of particles.

• This also indicates that the rays

(particles) moved from the

cathode to the anode.

William Crookes

(cathode rays) (Hons)

• Showed some form of radiation passes from the -ve electrode (cathode) to the +ve electrode (anode) when a current passes through a vacuum tube

-ve cathode +ve anode

CNAP

• He called these cathode rays and was convinced they consisted of particles

J J Thomson

• Was very interested in the Crooke’s discoveries.

• 1897 He used a cathode ray tube that had a positive and negative plate between which the rays had to travel.

Background Information

Cathode Rays

• Form when high voltage is applied across electrodes in a partially evacuated tube.

• Originate at the cathode (negative electrode) and move to the anode (positive electrode)

• Carry energy and can do work

• Travel in straight lines .

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Cathode Ray Experiment

1897 Experimentation

• Using a cathode ray tube, Thomson was

able to deflect cathode rays with an

electrical field.

• The rays bent towards the positive pole,

indicating that they are negatively

charged.

CATHODE RAYS IN ELECTRIC

FIELDS

Thompson’s cathode ray tube

-

PROVIDES

ELECTRICAL FIELD

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 117

The Effect of an Electric Field on

Cathode Rays

High

voltage

cathode

source of

high voltage

positive

plate

negative

plate

anode

_

+

Thomson’s Experiment

+ -

vacuum tube

metal disks

voltage

source

Thomson’s Experiment

+ -

voltage

source OFF

ON

Passing an electric current makes a beam appear

to move from the negative to the positive end

Thomson’s Experiment

+ -

voltage

source OFF

ON

+

-

By adding an electric field…

he found that the moving pieces were negative.

• He found the cathode rays were

attracted towards the positive

plate.

• The rays had to be negative (as

opposites attract).

Thomson

• 1897

• Thomson announced that cathode

rays consisted of negative charged

particles.

• Thomson had discovered the

ELECTRON!

• (He called them corpuscles)

Thomson 1897

Discovered the electron

George Stoney

1891 UCG

• Proposed that the

smallest particle that

has a negative

charge should be

called an electron.

• So Thomson's

corpuscles were

called electrons.

An Irish scientist

Electrons

Cathode rays are streams of

negatively charged particles

called electrons

• But where had these tiny particles come

from?

• Since they were so small, Thomson

suggested that they could only have come

from inside atoms.

• So Dalton's idea of the indestructible atom

had to be revised.

1808 AD

• John Dalton proposed his atomic theory.

1. All matter is made up of small particles

called atoms.

2. Atoms are indivisible – they cannot be

broken down into smaller particles.

False!

3. Atoms cannot be created or destroyed.

• This discovery implied that Dalton was

wrong and the atom is not the smallest

particle of matter.

• It looked like the atom could be broken

down into smaller pieces – electrons.

JJ Thomson

• He did another experiment.

• He placed an electro magnet around the

vacuum tube.

• He changed the strength of the magnetic

field to match the force of the electric field.

• Using the magnetic field he was able to

bring the beam of electrons back down to

its original position.

Cathode Ray Experiment

JJ Thomson

Electromagnet

JJ Thomson (Hons)

• From the results of his expt:

• He was able to calculate the ratio of the

charge of an electron to its mass.

• This is the of the electron. e

m

JJ Thomsom (hons)

• But he couldn’t determine the

individual value of either number

• He found that these –vely charged

particles were about 2000 times

lighter than H atoms and were to be

found in all matter

JJ Thomsom

• Also proposed the ‘plum pudding’

model of the atom

• He visualised an atom as a sphere

of +ve charge into which –ve

electrons were embedded at

random

Thomson Model of the Atom

• William Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere.

• The electrons were like currants in a plum pudding.

• This is called the ‘plum pudding’ model of the atom.

- electrons -

-

-

-

-

-

-

Plum-Pudding Model

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

Robert Millikan (Hons)

• Oil Drop Experiment

• Determine the mass

& charge of the

electron

Actual apparatus used

Robert Millikan

• 1911, Millikan used the ‘Oil Drop’ expt to measure accurately the charge on the electron (see book)

• This then allowed its mass to be calculated

• The mass is much smaller than that of any atom

• Confirming that the atom is made up of smaller particles

Millikan’s oil drop expt

Millikan’s oil drop expt

• He sprayed tiny drops of oil between two

charged plates.

• He used x-rays to cause the molecules in

the air to lose electrons.

• As the oil droplets passed through, they

picked up electrons from the air. i.e. the

oil droplets were now –vely charged

Molecules in air Oil droplets -

Electrons -

Millikan’s oil drop expt

Oil Drop Experiment

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oil droplets

oil droplet

under observation

Charged plate

Small hole

Charged plate

-

+

Telescope

oil atomizer

Robert Millikan

X-rays

Millikan’s oil drop expt

• By attaching a battery to the

plates, he created an electric field

between the plates that would act

on the charged oil drops .

• i.e. the –ve oil droplets were

attracted to the +ve charged plate

Millikan’s oil drop expt

• He adjusted the voltage

• till the electric field force would

just balance

• the force of gravity on an oil drop,

• and the oil drop would hang

suspended in mid-air.

When the oil

droplet was

stationary, the

weight of the oil

droplet due to

gravity must have

been equal to the

electrical force

pulling the droplet

upwards

Robert Millikan

• Using this information , He was able to

calculate:

1. The size of the charge on

the electron

2. The mass of the electron.

JJ Thomson

• Since atom is neutral

• There must be a

positive charge to

neutralise the negative

electrons.

JJ Thomson

• He developed the

idea that

• atoms are made of

negative electrons

embedded in a gel of

positive charge

• (a "plum pudding"

model).

Other pieces

• Proton - positively charged pieces

– 1840 times heavier than the electron

• Neutron - no charge but the same mass as

a proton.

• How were these pieces discovered?

• Where are the pieces?

1909

• Scientists began to question

the simple view of atom.

Ernest Rutherford

Discovered the nucleus & proton

• Worked in JJ

Thomson’s lab

• overturned Thomson's

atom model in 1911

• with his gold foil

experiment

• in which he

demonstrated that the

atom has a nucleus.

(The modern view of the atom was developed by Ernest Rutherford)

Ernest Rutherford

• Two of Ernest Rutherford's

students, Hans Geiger and Ernest

Marsden, were doing an

experiment at Manchester

University with radiation.

Ernest Rutherford

• They were using the dense,

positively charged particles

(called alpha particles)

• as 'bullets' to fire at a very thin

piece of gold foil.

Rutherford ‘Scattering’

particle

source

Lead collimator Gold foil

a

q

• They expected the

particles to pass straight

through the gold atoms.

GOLD ATOM

Experiment method

• Bombarded a thin leaf of gold foil

with alpha particles (+vely

charged)

• Used a zinc sulfide screen to

detect scattered alpha particles

Rutherford’s Goil Foil Experiment

Experiment Results

Gold atom

Ruherford’s experiment

• Most alpha particles were

observed to pass straight

through the gold foil.

• They had passed through

basically empty space!

Rutherford’s experiment

• A few, were scattered at small

and large angles (90°), and

some even bounced back

toward the source.

Interpreting the

Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

.

.

.

.

.

.

.

.

.

.

.

.

.

.

gold foil

deflected particle

undeflected

particles

.

. beam of

alpha

particles .

Density and the Atom

• Since most of the particles went through, the atom was mostly empty.

• Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

• Small volume and big mass = big density

• This small dense positive area is the nucleus

California WEB

Discovery of Nucleus

• Only a positively charged and

• relatively heavy particle,

• such as the proposed

nucleus, could account for

such strong repulsion.

New model of Atom

• Tiny, dense, positively charged

core called a nucleus,

• around which the light, negative

electrons, circulate at some

distance, much like planets

revolving around the Sun.

• Top: Expected results of Rutherford's gold foil experiment: alpha particles passing through the plum pudding model of the atom undisturbed.

• Bottom: Observed results: Some of the particles were deflected, and some by very large angles. Rutherford concluded that the positive charge of the atom must be concentrated into a very small location: the atomic nucleus.

Rutherford’s

Gold-Leaf

Experiment

Conclusions:

Atom is mostly empty space

Nucleus has (+) charge

Electrons float around nucleus

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Discovery of Proton

• Rutherford

continued to

bombard various

elements with

Alpha particles.

Ernest Rutherford (Hons)

• With light atoms e.g. Oxygen and

Nitrogen

• The alpha particles broke up the

nucleus to release positive

charged particles.

Rutherford

had discovered the

proton.

James Chadwick

Discovered the Neutron

• Bombarded a

sample of Beryllium

with alpha

particles.

Discovery of Neutron (Hons)

James Chadwick

• Some type of radiation with no

charge came from the

Beryllium.

James Chadwick

• The particles had the same

mass as protons but no

charge.

• He called them neutrons

Models of the Atom

Dalton’s model

(1803)

Thomson’s plum-pudding

model (1897)

Rutherford’s model

(1909)

Greek model

(400 B.C.)

+ - -

-

-

- e

e

e

+

+ +

+

+ +

+ +

e

e e

e

e

e e

"In science, a wrong theory can be valuable and better than no theory at all."

- Sir William L. Bragg

2.3 Properties of sub-atomic

particles

Particle Relative

charge

Relative

mass

Location

Proton +1 1 Nucleus

Neutron 0 1 Nucleus

Electron -1 1/1838 ‘Shells’

Recheck

• Evidence for the existence of small particles

• History of the atom

• Discovery of the electron

• Thomsom’s plum pudding model of the atom

• Millikan’s oil drop expt

• Discovery of the nucleus

• Discovery of the proton

• Discovery of te neutron

• Properties of sub-atomic particles