The Scientific MethodThe Scientific Method

Scientific MethodScientific MethodA series of logical steps to follow, A series of logical steps to follow, including experimentation, in order including experimentation, in order to solve problems or answer to solve problems or answer questions. questions.

The Process is NOT linear but The Process is NOT linear but circular.circular.

Observe

Question

Hypothesis

Evaluate Data

Results partially

support hypothesis

Reflect/generatenew questions

Repeat

experiment

Repeat expt.

Revise

hypothesis

.

Publish results

Results don’t

support hypothesis

Design/Conduct

Experiment

OR

Results support

hypothesis

HypothesisHypothesisPreceded by a questionPreceded by a question

Takes the general form:Takes the general form:

If If ““questionquestion”” , , thenthen ““predictionprediction”” or or ““outcomoutcomee””, , becausebecause ““tentative explanationtentative explanation””..

Example: Example: QUESTIONQUESTION: why doesn’t my : why doesn’t my flashlight light?flashlight light?

HYPOTHESISHYPOTHESIS: : IfIf a flashlight doesn’t light a flashlight doesn’t light thenthen the batteries are exhausted, the batteries are exhausted, becausebecause the power for the flashlight comes from the power for the flashlight comes from the batteries. the batteries.

A scientific hypothesis MUST be testable!!A scientific hypothesis MUST be testable!!

• In Science, a Hypothesis is an attempt at an explanation for the events that have been observed.  A hypothesis has to be testable.

• If a lot of evidence (data) is collected through experiments to support the hypothesis, then scientists accept the hypothesis as a good explanation.

• An accepted hypothesis may, given even more testing and agreement, become an accepted theory. Usually takes many years. 

THEORYS – Explain

• Scientific Laws DESCRIBE observations, they DO NOT EXPLAIN them.

• Experiments are not usually associated with Laws.

Remember this very important idea:  Isaac Newton knew what gravity did, but he could not explain why gravity did it.  Even today the topic of “what is gravity” is an active topic for scientific discussion using Einstein’s general theory of relativity.

LAWS – Describe

You Can Never Turn A Theory Into A Law!!

• Scientific Method is Circular• Science is Social (scientists talk

to scientists)• A Law Describes• A Theory explains

• Can’t change a law into a

theory or vice versa.

Summary

The Modern Atomic The Modern Atomic Model of the AtomModel of the Atom

Dalton, Thomson and Dalton, Thomson and RutherfordRutherford

What do We Know What do We Know About the Atom?About the Atom?

and…and…

How do we know what we know?How do we know what we know?

OutlineOutlineEarly ideas about the atomEarly ideas about the atom

Discovery of the electronDiscovery of the electron

Discovery of the nucleus and protonsDiscovery of the nucleus and protons

Atomic Number, Mass NumberAtomic Number, Mass Number

Atomic MassAtomic Mass

Greek Model (Democrites)Greek Model (Democrites)

Everything is composed of earth, fire, wind Everything is composed of earth, fire, wind and waterand water

Atoms are the smallest building block of Atoms are the smallest building block of mattermatter

Atoms are indivisibleAtoms are indivisible

All atoms are made of the same thing.All atoms are made of the same thing.

Atoms of different materials are different Atoms of different materials are different shapes. shapes. (tree atoms look like little trees, rock atoms (tree atoms look like little trees, rock atoms

look like little rocks) look like little rocks)

DaltonDalton’’s View of the Atom s View of the Atom (1803)(1803)

Elements are made of tiny indivisible particles called atoms.Elements are made of tiny indivisible particles called atoms.

All atoms of one element are unique from atoms of other All atoms of one element are unique from atoms of other elementselements

All atoms of a given element are identical and have the All atoms of a given element are identical and have the same properties.same properties.

Atoms of one element can combine with atoms of other Atoms of one element can combine with atoms of other elements in whole number ratios to form compounds. (Law elements in whole number ratios to form compounds. (Law of constant composition)of constant composition)

Atoms are not created or destroyed in a chemical reaction Atoms are not created or destroyed in a chemical reaction they are just rearranged into new compounds. (Law of they are just rearranged into new compounds. (Law of conservation of matter)conservation of matter)

CrookeCrooke’’s Tubes Tube

Cathode (-)

MA

GN

ETDC Power

Supply

Anode (+)

Cathode ray

ThomsonThomson’’s Cathode Ray Tube s Cathode Ray Tube (1897)(1897)

+

-

Cathode (-)

Anode (+)

Second DC Power

Supply

CRT Tube Video (Click to start)CRT Tube Video (Click to start)

ThomsonThomson’’s experiment resultss experiment resultsBeam was made of a particle 2000 times Beam was made of a particle 2000 times smaller than the smallest atom (hydrogen)smaller than the smallest atom (hydrogen)

Beam was negatively charged – attracted Beam was negatively charged – attracted to the positive electrode of a second to the positive electrode of a second power supply. (electron)power supply. (electron)

CouldnCouldn’’t find the corresponding positive t find the corresponding positive particle.particle.

Assumed atom had a Assumed atom had a ““fogfog”” of positive of positive charge in which the electrons were charge in which the electrons were dispersed.dispersed.

““Plum PuddingPlum Pudding”” Model Model

Some Information about RadiationSome Information about Radiation

Alpha particles are large positively Alpha particles are large positively charged particles; they are helium nucleicharged particles; they are helium nuclei

Beta particles are smaller negatively Beta particles are smaller negatively charged particles; they are electronscharged particles; they are electrons

Gamma rays are electrically neutral, high Gamma rays are electrically neutral, high energy electromagnetic radiation.energy electromagnetic radiation.

RutherfordRutherford’’s Experiments Experiment

Box

Radioactive α

Animation

RutherfordRutherford’’s Gold Foil Experiments Gold Foil Experiment

Trying to Prove Plum Pudding ModelTrying to Prove Plum Pudding Model

Shot pos. (+) charged alpha particles at Shot pos. (+) charged alpha particles at thin gold foil.thin gold foil.

Expected all to go straight through.Expected all to go straight through.

Most did go straight through Most did go straight through but a few but a few (1 in 20,000) bounced off. (1 in 20,000) bounced off.

Gold Foil

Rutherford’s Astonishing Rutherford’s Astonishing ConclusionConclusion

Atoms are mostly empty space !Atoms are mostly empty space !

There is a very dense, positively charged There is a very dense, positively charged nucleus that contains most of the mass.nucleus that contains most of the mass.

SummarySummaryThomson and RutherfordThomson and Rutherford

ThomsonThomson – the atom is made up of smaller – the atom is made up of smaller particlesparticles

ThomsonThomson – one of the smaller particles is – one of the smaller particles is negatively chargednegatively charged

RutherfordRutherford - The atom has a dense - The atom has a dense positive center.positive center.

RutherfordRutherford - The atom is mostly empty - The atom is mostly empty space.space.

Rutherford ModelRutherford Model• Center – positive nucleus with neg. Center – positive nucleus with neg.

electrons dispersed around nucleuselectrons dispersed around nucleus

• Atom is mostly empty space.Atom is mostly empty space.Large Positive Nucleus

Electrons (randomly dispersed)

Empty space

Along Comes Neils BohrAlong Comes Neils Bohr

Bohr ModelBohr Model

Electrons are in discreet energy levels Electrons are in discreet energy levels orbiting around the nucleus.orbiting around the nucleus.

……And Then de Broglie - The Wave ModelAnd Then de Broglie - The Wave Model

If electrons are particles and they orbit If electrons are particles and they orbit the nucleus, why donthe nucleus, why don’’t they eventually t they eventually spiral into the nucleus? spiral into the nucleus?

Electrons are waves not particles! Electrons are waves not particles!

Electrons are everywhere aroundElectrons are everywhere around

The nucleus all at once.The nucleus all at once. The Double Slit ExperimentThe Double Slit Experiment

Wave Model (de Broglie)Wave Model (de Broglie)

Electron are traveling at near the speed of Electron are traveling at near the speed of light.light.

Electrons are everywhere all at once in Electrons are everywhere all at once in their energy level or orbital around the their energy level or orbital around the nucleusnucleus

Electrons are in standing waves of Electrons are in standing waves of potential around the nucleus potential around the nucleus at discreet at discreet distances from the nucleus (Bohr)distances from the nucleus (Bohr)

Electrons as Waves – Electrons as Waves – The Double Slit ExperimentThe Double Slit Experiment. . (Click to start)(Click to start)

The Atom and its PartsThe Atom and its Parts

NucleusNucleus Protons and NeutronsProtons and Neutrons

Protons and neutrons are made up of quarksProtons and neutrons are made up of quarks

Around the NucleusAround the Nucleus Electrons – # electrons equals # protons Electrons – # electrons equals # protons

when the atom has no charge.when the atom has no charge.

In Between Electrons and Nucleus – In Between Electrons and Nucleus – EMPTY SPACE !!!EMPTY SPACE !!!

Atomic numberAtomic number: : ZZ, the number of protons in , the number of protons in the atom (Unique to an element, if the atomic the atom (Unique to an element, if the atomic number is different the element is different)number is different the element is different)

Mass numberMass number: : MM, the number of protons and , the number of protons and neutrons in the atom.neutrons in the atom.

IsotopesIsotopes are atoms of the same element, ie are atoms of the same element, ie the have the same atomic number, but they the have the same atomic number, but they have a different mass number. They differ in have a different mass number. They differ in the number of neutrons. The chemical the number of neutrons. The chemical properties of the isotopes are the same, but properties of the isotopes are the same, but they have different masses.they have different masses.

NotationNotationMass Number# of protons + neutrons

Atomic Number# of protons

PracticePractice

Find the number of protons, neutrons and Find the number of protons, neutrons and electrons in the following elementselectrons in the following elements

Cl3517 Am243

95

AnswersAnswers

Cl3517

17 protons17 protons17 electrons17 electrons35-17 = 18 neutrons35-17 = 18 neutrons

95 protons95 protons95 electrons95 electrons243-95 = 148 neutrons243-95 = 148 neutrons

Am24395

The Atom in SummaryThe Atom in Summary

Particle Mass Charge Location in Atom

Electron 9.1x10-28g ≈ 1/1000 amu

-1.6022x10-19C (-1)

Outside the nucleus

Proton 1.67 x10-24g ≈ 1 amu

+1.6022x10-19C (+1)

Inside the nucleus

Neutron 1.67 x10-24g ≈ 1 amu

0 Inside the nucleus

(1amu=1.66x10-24g)

Atomic MassAtomic Mass

Definition: Mass of an atom of a particular Definition: Mass of an atom of a particular element in amu (atomic mass unit)element in amu (atomic mass unit)

1 amu = 1.67 x 101 amu = 1.67 x 10-24-24 g (mass of a proton) g (mass of a proton)

Average Atomic Mass: is the weighted Average Atomic Mass: is the weighted average of all of the atomic masses of the average of all of the atomic masses of the isotopes in a naturally occurring sample of isotopes in a naturally occurring sample of the element ON THE PERIODIC TABLEthe element ON THE PERIODIC TABLE

Some Useful Internet SitesSome Useful Internet Sites

Cathode Ray Tube: Cathode Ray Tube: http://www.chem.uiuc.edu/clcwebsite/video/Cath.aviGold Foil:Gold Foil: http://micro.magnet.fsu.edu/electromag/java/rutherford/Millikan Oil Drop, Millikan Oil Drop, http://www.physchem.co.za/Static%20Electricity/Millikan.htm

Law of Constant CompositionLaw of Constant Composition (or (or Definite Proportions):Definite Proportions):

Dalton - 1803Dalton - 1803

different samples of the same compound different samples of the same compound always contain its constituent elements in always contain its constituent elements in the same proportion by massthe same proportion by mass

Example:Example:

Water always contains hydrogen and Water always contains hydrogen and oxygen in a mass ratio of 1:8 no matter oxygen in a mass ratio of 1:8 no matter how it is formed or where it is found.how it is formed or where it is found.

Law of Multiple proportions: Law of Multiple proportions:

If two elements can combine to form more than If two elements can combine to form more than one compound, the masses of one element that one compound, the masses of one element that combine with a fixed mass of the other element combine with a fixed mass of the other element are in ratios of small whole numbers. are in ratios of small whole numbers.

Example:Example: Water and Hydrogen peroxide both contain Water and Hydrogen peroxide both contain hydrogen and oxygen: hydrogen and oxygen: Water H:O by mass is 1:8Water H:O by mass is 1:8Hydrogen peroxide is 1:16 Hydrogen peroxide is 1:16

Cathode Ray Tube ExperimentCathode Ray Tube ExperimentThomson - 1897Thomson - 1897