Atomic Structure History Dalton-Bohr

5/26/2018 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.

Atomic Structure History Dalton-Bohr

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