Models of the AtomModels of the Atom

The Greeks, Alchemists, The Greeks, Alchemists, Dalton, Thomson and Dalton, Thomson and

RutherfordRutherford

Greek PhilosophersGreek Philosophers

• More than 2000 years ago, Democritus More than 2000 years ago, Democritus and Aristotle contributed to the first and Aristotle contributed to the first thought of atomic particlesthought of atomic particles

• Aristotle believed in four elements: Earth, Aristotle believed in four elements: Earth, Air, Wind and FireAir, Wind and Fire

• Democritus believed that matter was Democritus believed that matter was made of small particles called atomsmade of small particles called atoms

• Aristotle and Democritus used Aristotle and Democritus used observations and inferences to explain the observations and inferences to explain the existence of everythingexistence of everything

AlchemistsAlchemists

• European Scientists, 800-900 years European Scientists, 800-900 years agoago

• Their developed into what is now Their developed into what is now known as Chemistryknown as Chemistry

• Their work was inspired by the quest Their work was inspired by the quest to turn ordinary objects into to turn ordinary objects into

John Dalton (1803)John Dalton (1803)Dalton was the first to find experimentalDalton was the first to find experimentalsupport for the following: support for the following: • Atoms were the smallest possible Atoms were the smallest possible particle of matter.particle of matter.

• Each element had its own atoms that Each element had its own atoms that differed from others in massdiffered from others in mass

• Atoms were the fundamental building blocks of Atoms were the fundamental building blocks of matter and could not be splitmatter and could not be split

• In chemical reactions, atoms would rearrange In chemical reactions, atoms would rearrange themselves and combine with other atoms in new themselves and combine with other atoms in new waysways

The Billiard Ball Model of The Billiard Ball Model of AtomAtom• Dalton’s model built on the ideas of Dalton’s model built on the ideas of

Democritus that the atom was Democritus that the atom was indivisibleindivisible

J.J. Thomson’s J.J. Thomson’s Cathode RayCathode Ray

• A gaseous element is in the A gaseous element is in the end of a tubeend of a tube

• When the power source is When the power source is connected, a shadow is connected, a shadow is observed at the end of the observed at the end of the tubetube

• Conclusion: an invisible Conclusion: an invisible beam of “something” must beam of “something” must be “fired” from the cathode be “fired” from the cathode and is being blocked by the and is being blocked by the “mica” (a thin sheet of “mica” (a thin sheet of mineral)mineral)

Atoms Are Not the SmallestAtoms Are Not the Smallest• Observation:Observation: In Thomson’s In Thomson’s

experiments, the “invisible beam” experiments, the “invisible beam” always behaved the same regardless of always behaved the same regardless of what type of element was used for the what type of element was used for the gas or cathode.gas or cathode.

• Conclusion:Conclusion: The ray or particle is The ray or particle is common to all atoms of the elements so common to all atoms of the elements so the atom is not the smallest the atom is not the smallest fundamental building block of matterfundamental building block of matter

What is a cathode ray?What is a cathode ray?

• Thomson “fired” cathode rays again in his modified cathode ray Thomson “fired” cathode rays again in his modified cathode ray tube (CRT)tube (CRT)

• He noticed that the “ray” bent towards the positive magnet (plate)He noticed that the “ray” bent towards the positive magnet (plate)

• Conclusion: the “ray” must be negatively charged because Conclusion: the “ray” must be negatively charged because opposite charges attractopposite charges attract

• Above is an example of a cathode Above is an example of a cathode ray (negative charge) which is “bent” ray (negative charge) which is “bent” by a positively charged plateby a positively charged plate

Thomson’s “Plum Pudding”Thomson’s “Plum Pudding”ModelModel

• The The particles making up the raysparticles making up the rays acted acted as if they were negatively charged.as if they were negatively charged.

• Since elements are neutral, there must be Since elements are neutral, there must be positive particles balancing out the positive particles balancing out the negative particles. negative particles.

• Thomson theorized that an atom consistedThomson theorized that an atom consisted

of a “pudding” of evenly distributed of a “pudding” of evenly distributed positive and negative particles.positive and negative particles.

Plum Pudding ModelPlum Pudding Model

• Thomson called the Thomson called the negative particles negative particles corpusclescorpuscles. .

• Later, English Later, English physicist G.J. physicist G.J. Stoney coined the Stoney coined the name name electronelectron

ThomsonThomson – used a modified cathode – used a modified cathode ray tube to prove what he had ray tube to prove what he had hypothesized; the existence of a hypothesized; the existence of a positively charged subatomic particle positively charged subatomic particle called a called a protonproton. It has equal but . It has equal but oppositeopposite charge to the electron charge to the electron (+1)(+1) and a mass and a mass 18401840 times heavier times heavier than an electron.than an electron.

Robert MillikanRobert Millikan (1909) used his oil (1909) used his oil drop experiment drop experiment to prove that the to prove that the charge on of an charge on of an electron is electron is -1.-1.

Ernest Rutherford Ernest Rutherford (1911) (1911)

• Bombarded a thin metal foil Bombarded a thin metal foil

with positively charged with positively charged

alpha particles. alpha particles.

What Rutherford ObservedWhat Rutherford Observed

Called the Gold Foil Experiment

Analysis of Rutherford’s Analysis of Rutherford’s ExperimentExperiment• Observation:Observation: Most particles flew right Most particles flew right

through the foil as if there were nothing therethrough the foil as if there were nothing there

• Conclusion:Conclusion: The foil was mostly empty space The foil was mostly empty space

• Observation:Observation: A small number of particles A small number of particles were bounced back to their sourcewere bounced back to their source

• Conclusion:Conclusion: There must be a small, dense There must be a small, dense nucleus with a positive chargenucleus with a positive charge

The Rutherford Model of theThe Rutherford Model of theAtomAtom

• A dense, positively-charged nucleus A dense, positively-charged nucleus containing most of the total mass of containing most of the total mass of the atom the atom

• Negatively-charged electrons Negatively-charged electrons orbiting the nucleus in the empty orbiting the nucleus in the empty space of the atomspace of the atom

Rutherford’s Model: Not all it Rutherford’s Model: Not all it is cracked up to be!is cracked up to be!

• Rutherford’s model is referred to as the Rutherford’s model is referred to as the “Planetary Model” of the atom –PROBLEM!!!!!!!“Planetary Model” of the atom –PROBLEM!!!!!!!

• According to experiments dealing with electric According to experiments dealing with electric charge, the negatively charged electron should charge, the negatively charged electron should have been attracted to the positively charged have been attracted to the positively charged protons in the nucleus. protons in the nucleus.

• However, if this were true, the electrons would be However, if this were true, the electrons would be pulled into the nucleus causing the atom to pulled into the nucleus causing the atom to collapse.collapse.

• Scientists knew that the atom was stable and this Scientists knew that the atom was stable and this suggested that a change in atomic structure was suggested that a change in atomic structure was still neededstill needed

• Rutherford theorized that the nucleus Rutherford theorized that the nucleus although positively charged must although positively charged must contain other particles in itcontain other particles in it

• He theorized this based on the fact that He theorized this based on the fact that the mass of the total protons in the the mass of the total protons in the nucleus was less than the total mass of nucleus was less than the total mass of the nucleus the nucleus

•Something else must be in the nucleus? But what? The mystery continues…

Solution to the Problem?

• The same mass as a protonThe same mass as a proton

• Was not affected by electric Was not affected by electric or magnetic fieldsor magnetic fields

• ConclusionConclusion: the particles did : the particles did not have a charge (neutral)not have a charge (neutral)

• Chadwick had discovered the Chadwick had discovered the NEUTRONNEUTRON

• Chadwick received the Nobel Chadwick received the Nobel Prize for his discovery (the Prize for his discovery (the poor Curies)poor Curies)

• In 1932 Fredric and Irene Joliot-In 1932 Fredric and Irene Joliot-Curie discovered a Curie discovered a beam of beam of particles particles when they when they bombarded beryllium with bombarded beryllium with alpha radiationalpha radiation

• They did not realize the They did not realize the importance of their discoveryimportance of their discovery

• James ChadwickJames Chadwick: a student of : a student of BohrBohr

• He used the Curie’s work to He used the Curie’s work to demonstrate that the beam of demonstrate that the beam of particles had:particles had:

Mystery solved?

Neutrons have no charge so how does Neutrons have no charge so how does their existence in the nucleus prevent their existence in the nucleus prevent protons from pulling electrons into the protons from pulling electrons into the nucleus and having the atom collapse?nucleus and having the atom collapse?

Why do protons not repel each other?Both of these are valid questions that need answering!

How do the existence of neutrons solve the problems with the

structure of the atom?

Neutrons and protons are Neutrons and protons are made up of subatomic particles made up of subatomic particles

called called quarksquarks• Quarks are shown Quarks are shown

to the right held to the right held together by a together by a strong force called strong force called gluonsgluons

• The gluons are The gluons are represented by the represented by the coiled springscoiled springs

A proton comprises 2 up and 1 A proton comprises 2 up and 1 down quarks, while the down quarks, while the neutron comprises 1 up and 2 neutron comprises 1 up and 2 down quarksdown quarks • Gluons hold the types Gluons hold the types

of quarks togetherof quarks together

• Gluons also play a Gluons also play a role in creating a role in creating a force between force between neutrons and neutrons and protons.protons.

• This force is called This force is called the the stong nuclear stong nuclear forceforce

The hydrogen atom does not The hydrogen atom does not collapse even though it has no collapse even though it has no neutrons. Why?neutrons. Why?

• It was this question that many scientists It was this question that many scientists struggled withstruggled with

• It led to a new theory of the atom based on It led to a new theory of the atom based on a new way of thinking called a new way of thinking called

Quantum MechanicsQuantum Mechanics

Subatomic ParticlesSubatomic Particles

ProtonProton ElectronElectron NeutronNeutron

SymbolSymbol p+p+ e-e- nn

ChargeCharge 1+1+ 1-1- 00

MassMass 11 1/18371/1837 11