The History of the AtomThe History of the Atom

A Journey in theA Journey in the

realm of the unseenrealm of the unseen

IntroductionIntroductionWhat does an atom look like? It is so small What does an atom look like? It is so small that it cannot be seen. Yet we know there are that it cannot be seen. Yet we know there are particles like protons, neutrons and electrons particles like protons, neutrons and electrons that make up the atom. that make up the atom.

How did scientists discover these subatomic How did scientists discover these subatomic particles? This presentation will take you particles? This presentation will take you through the scientists who contributed to the through the scientists who contributed to the discovery of the make-up of the atom.discovery of the make-up of the atom.

The early Greeks defined all matter The early Greeks defined all matter As being rooted into the Four ElementsAs being rooted into the Four Elements..

Earth – all things that are dense Earth – all things that are dense solidssolids

Water – all things that are wet.Water – all things that are wet.Fire – Special Fire – Special stuffstuff that have that have both earth aboth earth andnd windwind in them. in them.

Wind – all things that float above.Wind – all things that float above.

DemocritusDemocritus

Proposed that all matter is Proposed that all matter is composed of small bits of composed of small bits of matter too small to be matter too small to be seen. These atoms seen. These atoms CANNOT be further split CANNOT be further split into smaller portions. into smaller portions. There is a void, which is There is a void, which is empty space between the empty space between the small bits.small bits.

He called the bits of He called the bits of matter “ATOMOS” Greek matter “ATOMOS” Greek for indivisible.for indivisible.

Greek philosopher Greek philosopher DemocritusDemocritus (460-371 B.C.) (460-371 B.C.)

AristotleAristotle

Greek philosopherGreek philosopher Opposed Democritus’ Opposed Democritus’

beliefsbeliefs Believed that all matter Believed that all matter

was continuous. was continuous.

Aristotle (384 BC – 322 BC)

Antoine LavoisierAntoine Lavoisier

Father of Modern ChemistryFather of Modern Chemistry

Law of Conservation of MassLaw of Conservation of Mass

In a chemical reaction, the total mass of the In a chemical reaction, the total mass of the reactants will equal the total mass of the reactants will equal the total mass of the

productsproducts

Antoine Laurent Lavoisier, (1743-1794), French chemist

Joseph ProustJoseph Proust Law of Definite Composition (1799)Law of Definite Composition (1799)

A compound always contains the A compound always contains the same elements in definite same elements in definite proportionsproportions

Joseph Proust (1754-1826) Example: pure sugar is composed of: 42.1% carbon, 51.4% oxygen, & 6.5% hydrogen by mass regardless of where you find it.

Law of Definite ProportionsLaw of Definite Proportions

Regardless of how its combined, lead (IV) sulfide will always have the same composition.Regardless of how its combined, lead (IV) sulfide will always have the same composition.

John DaltonJohn Dalton Developed the Law of Developed the Law of

Multiple ProportionsMultiple Proportions First to recognize that First to recognize that

atoms could explain the atoms could explain the laws of: conservation of laws of: conservation of mass, definite composition mass, definite composition and multiple proportions and multiple proportions

Proposed the Atomic Proposed the Atomic Theory in 1803Theory in 1803

Five Part Atomic TheoryFive Part Atomic Theory1.1. All matter is composed of extremely small All matter is composed of extremely small

particles called atoms.particles called atoms.

2.2. Two or more Two or more atomsatoms from the same element from the same element are identical. are identical.

3.3. Atoms cannot be subdivided, created or Atoms cannot be subdivided, created or destroyed.destroyed.

Five Part Atomic TheoryFive Part Atomic Theory

4. The Law of multiple proportions: 4. The Law of multiple proportions:

Atoms from different elements combine in Atoms from different elements combine in simple, whole-number ratios to form simple, whole-number ratios to form chemical compounds. chemical compounds.

5. In chemical reactions, atoms can be 5. In chemical reactions, atoms can be combined, separated, or rearranged.combined, separated, or rearranged.

Dalton’s Model of the AtomDalton’s Model of the Atom

Dalton developed a model of the atom based Dalton developed a model of the atom based on his atomic theory. He felt the atom was an on his atomic theory. He felt the atom was an extremely small, extremely small, indivisibleindivisible particle. particle.

His model of the atom is called the Billiard His model of the atom is called the Billiard Ball Model. Ball Model.

Solid Sphere

JJ ThomsonJJ Thomson Experimented with a CRT Experimented with a CRT

(cathode ray tube)(cathode ray tube) A CRT is an evacuated glass A CRT is an evacuated glass

bulb containing two ends: the bulb containing two ends: the cathode and the anode.cathode and the anode.

Cathode Ray TubeCathode Ray Tube An electrical current passes through the tube An electrical current passes through the tube

from the cathode (negative end) to the anode from the cathode (negative end) to the anode (positive end).(positive end).

Thomson studied the cathode ray that traveled Thomson studied the cathode ray that traveled from the cathode to the anode and noticed that from the cathode to the anode and noticed that the cathode rays were the same the cathode rays were the same regardlessregardless of of the element or metal used to make-up the the element or metal used to make-up the cathode.cathode.

Cathode Ray TubeCathode Ray Tube A magnet was applied to these rays and A magnet was applied to these rays and

always with the same results:always with the same results:

Negative end of magnet repelled cathode ray;Negative end of magnet repelled cathode ray;

Positive end of magnet attracted cathode rays.Positive end of magnet attracted cathode rays.

Thomson’s ConclusionThomson’s Conclusion Cathode ray is made up of an extremely small Cathode ray is made up of an extremely small

particles that are common to all matter.particles that are common to all matter.

The particle have a negative charge.The particle have a negative charge.

Thomson discovered the ELECTRONS.Thomson discovered the ELECTRONS.

Robert MillikanRobert Millikan Oil Drop Experiment Oil Drop Experiment

(1909)(1909) Work contained excellent Work contained excellent

precisionprecision Determined the exact Determined the exact

charge and exact mass of charge and exact mass of an electronan electron

Oil Drop ExperimentOil Drop Experiment

How the Oil Drop Experiment How the Oil Drop Experiment WorkedWorked

A fine mist of oil is sprayed into the chamber.A fine mist of oil is sprayed into the chamber. A few oil drops will fall through the hole in the A few oil drops will fall through the hole in the

positively charged plate at the top.positively charged plate at the top. As the oil drops fall due to gravity, they acquire As the oil drops fall due to gravity, they acquire

extra electrons which are dislodged from gases in extra electrons which are dislodged from gases in the air by X rays.the air by X rays.

As the charged oil drops descend, the electrically As the charged oil drops descend, the electrically charged plates are turned on.charged plates are turned on.

How the Oil Drop Experiment How the Oil Drop Experiment WorkedWorked

The oil drops now have two forces acting on The oil drops now have two forces acting on them. Gravity and electrical charge.them. Gravity and electrical charge.

Using the microscope to observe the oil drops, Using the microscope to observe the oil drops, Millikan could determine the charge needed to Millikan could determine the charge needed to suspend the drops in mid-air.suspend the drops in mid-air.

Millikan calculated the:Millikan calculated the:

exact mass (9.109 x 10exact mass (9.109 x 10-28-28 grams) and grams) and

charge (-1.6 x 10charge (-1.6 x 10-19-19 coulombs) of an electron. coulombs) of an electron.

Results of CRT and Oil Drop Results of CRT and Oil Drop ExperimentExperiment

1.1. Proved that atoms are divisible.Proved that atoms are divisible.

2.2. Atoms are electrically neutral Atoms are electrically neutral therefore they must have a therefore they must have a positive charge equal to the positive charge equal to the negative charge.negative charge.

3.3. Since electrons have such a small mass, Since electrons have such a small mass, atoms must have additional particles to atoms must have additional particles to account for most of their mass.account for most of their mass.

4.4. The Plum pudding model was created & The Plum pudding model was created & confirmed.confirmed.

Ernest Rutherford

Thought that the atom was all empty space.

Used the Gold Foil Experiment to test his hypothesis. (1908 and 1909)

Gold Foil ExperimentGold Foil Experiment

Almost all of the particles pass through with a Almost all of the particles pass through with a slight deflection BUT some particles came back.slight deflection BUT some particles came back.

1 in 8,000 particles ricocheted back to the source1 in 8,000 particles ricocheted back to the source

Gold Foil ExperimentGold Foil Experiment Rutherford said it was “as if you Rutherford said it was “as if you had fired a 15-inch (artillery) shell at had fired a 15-inch (artillery) shell at a piece of tissue paper and it came a piece of tissue paper and it came back and hit you.”back and hit you.”

Why did this happen?Why did this happen? Rutherford reasoned that the fast-moving particles Rutherford reasoned that the fast-moving particles

must be repelled by some powerful force within must be repelled by some powerful force within the atom. Also, whatever caused this repulsion the atom. Also, whatever caused this repulsion must occupy a very small amount of space since must occupy a very small amount of space since only a very few particles ran into it.only a very few particles ran into it.

The NucleusThe Nucleus

So how small So how small isis the nucleus? the nucleus? How large is an atom’s volume compared to its How large is an atom’s volume compared to its

nucleus?nucleus? Think of a football field and place a dime in the Think of a football field and place a dime in the

center of the 50 yard line.center of the 50 yard line.

Rutherford’s AtomRutherford’s Atom The dime represents the nucleus of the atom The dime represents the nucleus of the atom

while the outer edge of the football field would while the outer edge of the football field would represent the outer edge of the atom.represent the outer edge of the atom.

Rutherford concluded that the atomRutherford concluded that the atomis is mostlymostly empty space. empty space.

Rutherford’s model of the Rutherford’s model of the atom is the nuclear model.atom is the nuclear model.

A PuzzleA Puzzle If an atom has a positive center and the If an atom has a positive center and the

negative electrons are on the outside of the negative electrons are on the outside of the atom, why don’t the electrons fall into the atom, why don’t the electrons fall into the center?center?

………………………………..Centrifugal force {..Centrifugal force {an an inertia force due to e- traveling in circles.}inertia force due to e- traveling in circles.}

Niels BohrNiels Bohr Developed the Planetary Developed the Planetary

Model in 1913Model in 1913 Electrons move around Electrons move around

the nucleus like planets the nucleus like planets move around the sun.move around the sun.

BohrBohr Bohr suggested that electrons travel in a specified Bohr suggested that electrons travel in a specified

path around the nucleus which he called energy path around the nucleus which he called energy levels. These energy levels are designated distances levels. These energy levels are designated distances from the nucleus in which electrons may be found. from the nucleus in which electrons may be found.

The maximum number of The maximum number of electrons found in an energy electrons found in an energy level can be determined by level can be determined by the formula 2nthe formula 2n22, where, where n = energy level.n = energy level.

Werner HeisenbergWerner Heisenberg 1927 – Heisenberg 1927 – Heisenberg

Uncertainty PrincipleUncertainty Principle

It is not possible to It is not possible to know both the velocity know both the velocity and the position of an and the position of an electron at the same electron at the same time.time.

Erwin ShrErwin ShrÖÖdinger dinger

           

Austrian Physicist who Austrian Physicist who developed an e- formula.developed an e- formula.

His theory was able to His theory was able to determine the most likely determine the most likely AREA an e- is to be found. AREA an e- is to be found.

These areas are called These areas are called OrbitalsOrbitals

James ChadwickJames Chadwick Discovered the neutron in Discovered the neutron in

19321932 The neutron is a particle The neutron is a particle

in the nucleus that has in the nucleus that has about the same mass as a about the same mass as a proton, but has no charge. proton, but has no charge.

Modern Day ModelModern Day Model Two Main PartsTwo Main Parts

The NucleusThe Nucleus Positively Charged Positively Charged

PROTONSPROTONS

Neutral NEUTRONS Neutral NEUTRONS

Held together by the Held together by the STRONG NUCLEAR STRONG NUCLEAR FORCEFORCE

The Electron The Electron CloudCloud

ee- - moving about the moving about the nucleus in nucleus in

3-D ORBITALS (s,p,d,f). 3-D ORBITALS (s,p,d,f). The e- ORBITALS are The e- ORBITALS are

positioned in ENERGY positioned in ENERGY LEVELSLEVELS

Properties of Subatomic Properties of Subatomic Particles Particles

ParticleParticle ChargeCharge Mass Mass # #

Relative mass Relative mass (a.m.u.)(a.m.u.)

Actual mass Actual mass (grams)(grams)

Electron e- -1 0 0.0005486 9.109 x10 -28

Proton p+ +1 1 1.007276 1.673x 10 -24

Neutron n 0 1 1.008665 1.675x 10 -24

Fuzzy Blob of Uncertainty Fuzzy Blob of Uncertainty The Modern ModelThe Modern Model

Modern Day Model of the Modern Day Model of the ATOMATOM

The modern day model is a collection of all The modern day model is a collection of all the contributions of the previous scientists, the contributions of the previous scientists, from Dalton to Chadwick.from Dalton to Chadwick.

Today we would have to include quarks Today we would have to include quarks which make-up the protons and neutrons in which make-up the protons and neutrons in the nucleus. the nucleus.

MURRAY GELL-MANN named the 6 MURRAY GELL-MANN named the 6 Quarks after a line in the play “Finnegan’s Quarks after a line in the play “Finnegan’s Wake.” A good bonus questions might be to Wake.” A good bonus questions might be to name these six name these six flavorsflavors of quarks. of quarks.

THE ENDTHE END Are you ready for Are you ready for

the history of the the history of the Atom quiz?Atom quiz?