Models of the Models of the AtomAtom

Ancient GreeceAncient Greece300 BC in Greece300 BC in Greece

School of thought that matter is School of thought that matter is made up of tiny indivisible, invisible, made up of tiny indivisible, invisible, indestructible, fundamental units of indestructible, fundamental units of matter called atoms (means “that matter called atoms (means “that which cannot be divided”)which cannot be divided”)Democritus of Abdera most well Democritus of Abdera most well known atomist. known atomist.

Did no experimentsDid no experimentsNo proofNo proofPhilosopherPhilosopher

1770s1770sAntoine Lavosier made observations Antoine Lavosier made observations and did experiments resulting in and did experiments resulting in the Law of Conservation of Massthe Law of Conservation of MassLaw of Conservation of MassLaw of Conservation of Mass

Matter cannot be created Matter cannot be created or destroyedor destroyed

Mass of materials before the Mass of materials before the chemical reaction is the same chemical reaction is the same mass after the reactionmass after the reaction

1799- Joseph 1799- Joseph ProustProust

Law of Definite Proportions:Law of Definite Proportions: that the that the proportion by mass of the elements in a proportion by mass of the elements in a compound is always the same (CO vs. compound is always the same (CO vs. COCO22))

Part of our definition of a chemical - any Part of our definition of a chemical - any substance with a definite compositionsubstance with a definite composition

1808- John Dalton1808- John Dalton

English School TeacherEnglish School TeacherTheory based on the Greek idea of Theory based on the Greek idea of the atomthe atom

Atoms were tiny indestructible Atoms were tiny indestructible particlesparticles

Dalton’s Atomic Dalton’s Atomic TheoryTheory1.1. All matter is made of invisible and All matter is made of invisible and

indestructible particles (atoms)indestructible particles (atoms)2.2. Atoms of same element are identicalAtoms of same element are identical3.3. Atoms of different elements differ in physical Atoms of different elements differ in physical

and chemical propertiesand chemical properties4.4. Atoms of different elements combine in simple Atoms of different elements combine in simple

whole number ratios to form compoundswhole number ratios to form compounds5.5. Chemical Reactions occur when atoms Chemical Reactions occur when atoms

separate, join or rearrange. Atoms of one separate, join or rearrange. Atoms of one element NEVER change into atoms of another element NEVER change into atoms of another element.element.

Problems with Dalton’s Problems with Dalton’s ModelModel

Atoms are destructible (protons, Atoms are destructible (protons, electrons, neutrons)electrons, neutrons)Atoms of the same element are not Atoms of the same element are not completely completely identical, isotopes existidentical, isotopes existAtoms of the same element combine Atoms of the same element combine with each other (Owith each other (O22))

Atoms can “turn into” other elements Atoms can “turn into” other elements through nuclear decay through nuclear decay

1897 1897 Sir J.J.ThomsonSir J.J.Thomson

British physicistBritish physicistTheory: Theory: cathode ray cathode ray is a stream of is a stream of negatively charged negatively charged particlesparticlesExperiment: Experiment:

cathode ray cathode ray experimentexperiment

ExperimentExperimentCathode ray: a tube filled with gas that Cathode ray: a tube filled with gas that would glow when an electric current was would glow when an electric current was passed through tube (like a neon sign)passed through tube (like a neon sign)

Thomson held the “+” pole of magnet next Thomson held the “+” pole of magnet next to tube, and the light bent towards magnetto tube, and the light bent towards magnet

Therefore he Therefore he concluded that there must be concluded that there must be some negative charge in atom!! some negative charge in atom!! (discovered electron)(discovered electron)

Also hypothesized that there must be some Also hypothesized that there must be some positive charge in atom (positive charge in atom (as atoms are as atoms are neutral)neutral)

Plum Pudding Plum Pudding ModelModel

Electrons Electrons are are embedded embedded in atomin atom

Positive Positive charge charge floats floats around rest around rest of atomof atom

Thomson Thomson Misconceptions?Misconceptions?Placement of electronPlacement of electron

Location of positive chargeLocation of positive charge

1909 -Ernst 1909 -Ernst RutherfordRutherford

Physicist and Chemist from New Physicist and Chemist from New ZealandZealandProposedProposed::

that the theory of Thomson was that the theory of Thomson was actually correctactually correctAny charge that occurs in atom must Any charge that occurs in atom must occur in whole number ratiosoccur in whole number ratios

Ex: +1 or -1, not + 1.5Ex: +1 or -1, not + 1.5

Rutherford's Gold Foil Rutherford's Gold Foil ExperimentExperiment

Fired small radioactive particles (basically a Fired small radioactive particles (basically a helium nucleus) at a piece of gold foilhelium nucleus) at a piece of gold foil

Expected that most of these particles would Expected that most of these particles would pass right through the gold foil, deflecting pass right through the gold foil, deflecting only a little bitonly a little bit

* what actually happened was that while that * what actually happened was that while that majority did pass right through, those that majority did pass right through, those that deflected deflected at many different anglesdeflected deflected at many different angles

Think of throwing a grain of salt at a chain link Think of throwing a grain of salt at a chain link fence! fence!

Rutherford’s Rutherford’s ExperimentExperiment

Rutherford’s Rutherford’s ConclusionConclusion

Key Idea -NUCLEAR ATOMKey Idea -NUCLEAR ATOMAll of the mass of the positive All of the mass of the positive particles (protons) is at the center particles (protons) is at the center region of the atomregion of the atomCenter region called the NUCLEUSCenter region called the NUCLEUSElectrons surround the nucleus in a Electrons surround the nucleus in a “cloud”“cloud”Atom is mostly empty spaceAtom is mostly empty space

Rutherford’s Rutherford’s MisconceptionsMisconceptions

Still could not place electrons correctlyStill could not place electrons correctly

Was not aware of the neutronWas not aware of the neutron

Niels BohrNiels BohrDanish PhysicistDanish Physicist

Proposed his model in 1915 (this is the one Proposed his model in 1915 (this is the one most of you know)most of you know)

He coined the term, “Planetary Model” of atomHe coined the term, “Planetary Model” of atomElectrons orbit the nucleus of atom like planets Electrons orbit the nucleus of atom like planets around the sunaround the sunDifferent orbits exist, each having a specified Different orbits exist, each having a specified level of energylevel of energySuggested that outermost energy levels can hold Suggested that outermost energy levels can hold more electrons than inner energy levelsmore electrons than inner energy levels

Bohr’s Bohr’s ExperimentExperiment

Studied Studied line emission spectrum of line emission spectrum of hydrogenhydrogen

Noticed that when electrons were excited Noticed that when electrons were excited (basically electrified) they would “jump”. (basically electrified) they would “jump”.

Further, he noticed that these jumps were Further, he noticed that these jumps were similar in energysimilar in energy

This helped to solidify his theory of This helped to solidify his theory of electrons orbiting the nucleus in specific electrons orbiting the nucleus in specific energy levelsenergy levels

Bohr’s AtomBohr’s Atom

Conclusions:Conclusions:There are certain circular ORBITS in There are certain circular ORBITS in which an electron can travel around which an electron can travel around the nucleusthe nucleusThe farther away from the The farther away from the nucleus, the higher the energy nucleus, the higher the energy levellevel

Problems with Problems with BohrBohr

Electrons do not exist in orbits!Electrons do not exist in orbits!

Quantum Quantum Model/SchrodingeModel/Schrodinge

r - 1925r - 1925Theory:Theory:

states that electron can act as a particle states that electron can act as a particle and a waveand a waveelectrons can exist in any of an unlimited electrons can exist in any of an unlimited number of energy levelsnumber of energy levels

Experiment:Experiment: Schrodinger wave equation Schrodinger wave equation

Quantum Quantum Model/SchrodingeModel/Schrodinge

rrConclusions:Conclusions:

Electrons do not orbit nucleus, they exist in Electrons do not orbit nucleus, they exist in orbitalsorbitalsOrbitals are probable locations of the electronOrbitals are probable locations of the electronThere is an unlimited number of energy levels in There is an unlimited number of energy levels in which electrons can existwhich electrons can existEnergy increases as you get farther from nucleusEnergy increases as you get farther from nucleusMisconceptions?Misconceptions?

Quantum Quantum Model/SchrodingeModel/Schrodinge

rr

What do What do we know we know

now??now??All models are not exactly correct, but they All models are not exactly correct, but they lead to further understanding and discoverylead to further understanding and discoveryAll have important key ideasAll have important key ideasAtoms are made up of subparticlesAtoms are made up of subparticlesAtoms are divisible, but not by ordinary Atoms are divisible, but not by ordinary chemical meanschemical meansAtoms of elements can vary (isotopes)Atoms of elements can vary (isotopes)Electrons reside in ORBITALS, not orbitsElectrons reside in ORBITALS, not orbits

Parts of Parts of AtomAtom

Particle Location Relative Mass & Size

Charge

Protons Nucleus Heavy mass1.673 E - 24 g

+

Neutron Nucleus Heavy mass1.675 E - 24 g

No charge

Electron Orbitals outside of nucleus

1.1 E -29 gmass is 1/1836 that of proton

_

What happens when you What happens when you change…change…Protons: Since the number of protons is the Protons: Since the number of protons is the

same as the atomic number, protons are what same as the atomic number, protons are what define an element.define an element.

If you change the protons you change the If you change the protons you change the elementelement

Neutrons: The neutrons add to the mass of the Neutrons: The neutrons add to the mass of the element.element.

If you change the neutrons you change the If you change the neutrons you change the mass of the element (make an isotope)mass of the element (make an isotope)

Electrons: The electrons balance the charge of Electrons: The electrons balance the charge of the protons.the protons.

If you change the electrons you change the If you change the electrons you change the charge of the element (make an ion)charge of the element (make an ion)

Quotes about Quotes about atomsatoms

From From A Short History of Nearly A Short History of Nearly Everything Everything by Bill Bryson by Bill Bryson

““protons give an atom protons give an atom its identity, electrons its its identity, electrons its personality” p. 140personality” p. 140““if an atom were if an atom were expanded to the size of expanded to the size of a cathedral, the nucleus a cathedral, the nucleus would be only about the would be only about the size of a fly- but a fly size of a fly- but a fly many thousands of many thousands of times heavier than the times heavier than the cathedral” p. 141cathedral” p. 141