Models of the Models of the AtomAtom

September 24September 24thth & 25 & 25thth, 2009, 2009

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, made up of tiny indivisible, invisible, indestructible, invisible, indestructible, fundamental units of matter called fundamental units of matter called atoms (means “that which cannot atoms (means “that which cannot be divided”)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 Antoine Lavosier made observations and did experiments observations and did experiments resulting in the Law of resulting in the Law of Conservation of MassConservation of Mass

Law of Conservation of MassLaw of Conservation of MassMatter cannot be created Matter cannot be created

or destroyedor destroyedMass 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

Showed that the proportion by mass of Showed that the proportion by mass of the elements in a compound is always the elements in a compound is always the same (CO vs. COthe same (CO vs. CO22))

Now called the Law of Definite Now called the Law of Definite ProportionsProportions

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 Teacher

Wanted to learn in what ratios Wanted to learn in what ratios different elements combinedifferent elements combine

Based on the Greek idea of the Based on the Greek idea of the atomatom

Atoms were tiny indestructible Atoms were tiny indestructible particlesparticles

Which clay model is this?Which clay model is this?

Dalton’s Atomic Dalton’s Atomic TheoryTheory

1.1. All matter is made of indivisible, invisible, All matter is made of indivisible, invisible, and indestructible particles called atomsand indestructible particles called atoms

2.2. Atoms of the same element are identicalAtoms of the same element are identical

3.3. Atoms of different elements differ in their Atoms of different elements differ in their physical and chemical propertiesphysical and chemical properties

4.4. Atoms of different elements combine in Atoms of different elements combine in simple whole number ratios to form simple whole number ratios to form compoundscompounds

5.5. Chemical Reactions occur when atoms are Chemical Reactions occur when atoms are separated, joined or rearranged. Atoms of separated, joined or rearranged. Atoms of one element are NEVER changed into atoms one element are NEVER changed into atoms of another element.of another element.

Problems with Dalton’s Problems with Dalton’s ModelModel

Atoms are not indestructible, they are Atoms are not indestructible, they are made of smaller parts like protons, made of smaller parts like protons, neutrons, electrons and quarksneutrons, electrons and quarks

Atoms of the same element are not Atoms of the same element are not completely completely identical, isotope exist with identical, isotope exist with different massesdifferent masses

Atoms of the same element can combine Atoms of the same element can combine with each otherwith each other

Atoms can “turn into” other elements Atoms can “turn into” other elements through nuclear decay (though there are through nuclear decay (though there are byproducts)byproducts)

1897 1897 Sir Sir

J.J.ThomsonJ.J.ThomsonProposed that the Proposed that the cathode ray formed in cathode ray formed in the tube is a stream of the tube is a stream of negatively charged negatively charged particlesparticles

Atoms have parts that Atoms have parts that are negatively chargedare negatively charged

Since atoms are Since atoms are neutral, they must also neutral, they must also have positively have positively charged areascharged areas

Gave rise to the plum Gave rise to the plum pudding model of the pudding model of the atomatom

1909 -Ernst 1909 -Ernst RutherfordRutherford

Experimented with the positively charged Experimented with the positively charged particleparticle

He figuredHe figuredAtoms have no chargeAtoms have no chargeElectrical charges are properties of Electrical charges are properties of mattermatterElectrical charges exist in single whole Electrical charges exist in single whole number units + or - (no fractions)number units + or - (no fractions)Electrical Charges cancel each other, Electrical Charges cancel each other, therefore a + and a - together = neutral therefore a + and a - together = neutral atomatom

Rutherford’s Rutherford’s modelmodel

Key Idea -NUCLEAR ATOMKey Idea -NUCLEAR ATOM

All of the mass of the positive particles All of the mass of the positive particles (protons) is at the center region of the (protons) is at the center region of the atomatom

Center region called the NUCLEUSCenter region called the NUCLEUS

Electrons surround the nucleus in a Electrons surround the nucleus in a “cloud”“cloud”

Plum PuddingPlum Pudding vsvsNuclearNuclear

Which clay models are Which clay models are these?these?

Bohr’s Atom- Bohr’s Atom- 19131913

There are certain energy levels that There are certain energy levels that mathematically allow the electron to stay mathematically allow the electron to stay in the cloud (not crash into the nucleus)in the cloud (not crash into the nucleus)

Key idea - There are certain circular Key idea - There are certain circular ORBITS in which an electron can travel ORBITS in which an electron can travel around the nucleus without losing energyaround the nucleus without losing energy

The farther away from the nucleus, the The farther away from the nucleus, the higher the energy levelhigher the energy level

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 discovery

All have important key ideasAll have important key ideas

Atoms are made up of subparticlesAtoms are made up of subparticles

Atoms are divisible, but not by ordinary Atoms are divisible, but not by ordinary chemical meanschemical means

Atoms of elements can vary (isotopes)Atoms of elements can vary (isotopes)

Electrons reside in ORBITALS, not orbitsElectrons reside in ORBITALS, not orbits

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