atomic structure history dalton-bohr
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Atomic Structure
History of Atomic Theory
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Democritus (460 - 370 BC) Was the first person to come up with the idea
of atom
Believed that all matter was composed ofindivisible particles he called ATOMS Which is derived from the Greek word
Atomos meaning indivisible
He also believed that different atoms: Are different sizes Have different properties
Other philosophers of that time did not agree
with his theories.
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John Dalton (1766-1844) Dalton is the Father ofAtomic Theory
Daltons ideas were sobrilliantthat they haveremainedessentially
intact up to the presenttime and has only beenslightly corrected.
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Daltons Atomic Theory (1803)
aka: 5 Postulates1. All matter is composed of extremely small
particles called atoms. (I agree with Democritus!)
2. All atoms of a given element areidentical, having the same:- size- mass- chemical properties.
3. All atoms of different elements aredifferent.
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4. Atoms cannot be created, divided intosmaller particles, or destroyed.
**In a chemical reaction, atoms of different elementsare separated, joined or rearranged. They are
never changed into the atoms of anotherelement.We will learn more later**
5.Atoms combine in definite wholenumber ratiosto make compounds(you cant have a of a Carbon bonding withOxygen; its a whole atom or no atom)
Daltons Atomic Theory (1803)
aka: 5 Postulates
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Daltons Atomic ModelBased on Daltons AtomicTheory (5 postulates), most
scientists in the 1800s believedthat the atom was like a tinysolid ballthat could notbebroken up into parts.
Dalton was credited for thethreeAtomic Lawsthat wereproven after his time.
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Daltons Atomic Laws1. Law of Conservation of Mass
Matter cannot be createdor
destroyedin any physical orchemical process, just transferred.
2. Law of Constant Composition
When atoms combine to form molecules,the ratio of atoms is constant.
ExampleH2O will always have 2 timesas many Hydrogen atoms as Oxygen.
http://hydrogenatom.net/wp-content/uploads/2010/04/atom1.gif -
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Daltons Atomic Laws3. Law of Multiple Proportionsif two
elements can combine to form more than one
compound, then the ratio of the second elementcombined with a certain mass of the first elementis always a ratio of small whole numbers.
Formula
Ratio of N:O
Example:CO vs. CO2
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When a chemical reaction occurs, massis neither created nor destroyed but only
changed.
The Law of Conservation of Mass
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JJ Thomson (1856-1940)Used cathode rays toprove that Daltons Solid-
ball model could bebroken into smallerparticles
Thomson is creditedwith discoveringelectrons
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Cathode Ray Tubes
Cathode rays had been used forsome time before Thompsonsexperiments.
A cathode rayis a tube that hasa piece of metal, called anelectrode, at each end. Eachelectrode is connected to a power
source (battery).
When the power is turned on, the electrodesbecome charged and produce a stream of chargedparticles. They travel from cathode, across the tube
to the anode.
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Cathode Ray Tubes Thomson put the tube
in a magnetic field.He predicted that the
stream would travel ina straight path.
Instead, he found thatthe path curved away
from a negativelycharged plate andtoward a positivelycharged plate
Why?
Likecharges repeleach other,
and objects with unlike chargesattract each other, Thomsonconcluded that the stream ofcharged particles had electronsin them.
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Cathode Ray Tube Experiment
Video Clip
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Cathode Ray Tube Experiment
Thompson Concluded:
Cathode rays are made up of
invisible, negatively charged
particles called Electrons.
These electrons had to come from the matter (atoms)
of the negative electrode.
Since the electrodes could be made from a variety ofmetals,then all atoms must contain electrons!
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Thomsons Plum Pudding ModelThomsons Plum Pudding modelis a +charge sphere that has (- )chargedelectrons scattered inside, like raisins in
plum pudding.Overall, the atom is neutral atombecause the atom had the same numberof positiveand negativecharges.
From Thomsons experiments, scientists concluded thatatoms were not just neutral spheres, but somehow werecomposed of electrically charged particles.
The balance of positiveand negativecharge supports the
neutral atom.
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Rutherford (1871-1937) Took Thomsons Plum
Pudding Model and addedto it
Used the Gold FoilExperiment to discoverthe existence of:An atomic Nucleus
Protons (in later experiments) You must be able to
explain the Gold FoilExperimentit will be on
the CST
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Gold Foil Experiment Rutherford directed a narrow
beam of alpha particles (+charges) at a thin piece of
gold foil.
Based on observations fromother experiments involving
alpha particles, he predictedthat the (+) charges wouldgo through the foil
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Results from Gold Foil Experiment
Rutherford found that everyonce and a while, a + particle
was deflected bounced back.(about 1% of the time)
Why?
Because the + charge hit acentral mass of positive chargeand was repelled.
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The Gold Foil Experiment
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The Nuclear Model of the Atom
To explain the results of the experiment,Rutherfords team proposed a new model ofthe atom.
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Conclusions from Rutherfords
Gold Foil Experiment (memorize this!)The atom contains a positively charged
nucleus
This nucleuscontains almost all of the massofthe atom, but occupies a very small volumeofthe atom.
The negatively charged electronsoccupied mostof the volume of the atom.
The atom is mostly emptyspace.
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Rutherfords Planetary ModelTo explain hisobservations,
Rutherford developed anew model
The electronsorbitthe nucleuslike theplanets revolve aroundthe sun.
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Bohr (1885-1962) Worked in Rutherfords lab Wondered whyelectrons
are not attracted to the +
nucleus and cluster aroundit
Disproved RutherfordsPlanetary Model
Experimented with light andits interaction with matterto develop a new model.
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Bohrs Energy Level ModelEnergy Level Model: Electrons arearranged in circles around thenucleus. Each circle has a different
energy.Electrons are in constant motion,traveling around the circle at thespeed of light.
Electrons can jump from one circleto the next
But they cant go to the nucleus they
traveling too fast to be fully attracted.
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Bohrs Energy Level Model He proposed the following:
4. The electrons must absorbor emita fixedamount of energy to travel between theseenergy levels
3. The electrons orbit the nucleus at fixed
energy levels.
2. Electrons can only be certain distancesfromthe nucleus.
1. Protons and neutrons are in the nucleus
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ReviewWho is the father of atomic theory?
Dalton
What was the first model of the atom?
Daltons Tiny Ball Model
What are Daltons 3 Laws?Law of Conservation of Mass, Law ofConstant Composition, Law of MultiplePorportion
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ReviewHow were Thomsons and Daltons model
different?
Daltons model was 1 sphere that cannotbe divided, Thomson had the plum puddingwhere electrons are randomly spread
throughout a positively charged sphere.What did Thomson find out?
Atoms have electrons, they have a - charge
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ReviewWhat were Rutherfords conclusions from the
Gold Foil Experiment?
Atom has a positively charged nucleus electrons are outside,
atoms are mostly empty
Nucleus contains most of the mass.