Atomic Theory and Structure [Autosaved] (3)

7/29/2019 Atomic Theory and Structure [Autosaved] (3)

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Democritus (460-370 BC)

To understand the very large,

we must understand the very small.

proposed that all matterwas made of tinyindivisible particles,which he named

atomos meaningindivisible, indestructibleand unchangeable.

did not carry out any

experiments to see if histheory was correct, hebased his theories on hisobservations


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John Dalton(1766-1844)

We might as well attempt to introduce a new

planet into the solar system, or to annihilate one

already in existence, as to create or destroy a

particle of hydrogen

Dalton stated that elementsconsisted of tiny particlescalled atoms

He was able to supplyexperimental results toforcefully revive the idea ofthe atom

He also called the elementspure substances because allatoms of an element were

identical and that inparticular they had thesame mass.
http://www.chem.ualberta.ca/~plambeck/che/p101/p01021.htmhttp://www.chem.ualberta.ca/~plambeck/che/p101/p01021.htm


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formulated the Law of

Conservation of Matter:

"Matter is neither

gained nor lost during achemical reaction."

- the total mass of the

reactants is always

equal to the total mass

of the products

Antoine Lavoisier (1743-1794)
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formulated the Law of

Constant Proportions:

"In a compound, the

constituent elementsare always present in a

definite proportion by

weight."

Joseph Louis Proust(1754-1826)


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J.J. Thomson (1856 - 1940)

In 1897, he used a gas

discharged tube to study

cathode rays and was ableto show that cathode rays

are usually made up of

negatively charged particles

called electrons


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Thomson realized that two factors affected thedeflection of the electrons:

1. Mass (m) of the electrons

The greater the mass, the less the

deflection

2. Charge (e) on the electrons

The greater the charge, the greater the

deflection

Thomson was not able to measure either thecharge or mass separately, but he was able to

determine the ratio between the two


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+-

voltage

sourceOFF

ON

+

-

By adding an electric field

he found that the moving pieces were negative.


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The Plum Pudding Model

Proposed by Lord

Kelvin and J.J.

Thomson

Electrons were seen

as being randomly

distributed in a

sphere of positivecharge

Spherical cloud of

Positive charge

Electrons


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In 1909 Rutherford undertooka series of experiments

He fired a (alpha) particles ata very thin sample of gold foil

According to the Thomsonmodel the a particles wouldonly be slightly deflected

Rutherford discovered thatthey were deflected throughlarge angles and could even bereflected straight back to thesource

Ernest Rutherford (1871-1937)


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Most particles flew right through the foil as

if there were nothing there

The foil was mostly empty space

A small number of particles were bouncedback to their source

There must be a small, dense nucleus with a

positive charge


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n+

dense, positively-charged nucleus

negatively-charged electrons orbiting the nucleus


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After Rutherfords

discovery, Bohr proposedthat electrons travel indefinite orbits around

the nucleus.

studied how atoms reactto light; developed atheory of how electronsmoved around thenucleus in certain pathsor energy levels.

Niels Bohr (1885 - 1962)


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Planetary

model


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Louis de Broglie ( 1892-1987) extended to matter theconcept that like light,matter must be both aparticle and a wave

confirmed inexperiments thatshowed electron beamscould be diffracted orbent as they passedthrough a slit much likelight could
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developed an equation

that relates the

wavelength of an

electron to its energywhich describes the

probability that an

electron will be at a

certain point in space.

Erwin Schrdinger (1887-1961)
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pointed out that it isimpossible to know boththe exact position and theexact momentum of anobject at the same time(Uncertainty Principle)

effectively destroys the ideaof electrons travelingaround in neat orbits. ;any

electron that is subjected tophotons will have itsmomentum and positionaffected.

Werner Heisenberg (1901-1976)


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Daltons model

(1803)

Thomsons plum-puddingmodel (1897) Rutherfords model

(1909)

Bohrs model

(1913)Charge-cloud model

(present)


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An atom is a minute electrically neutralparticle with a massive positive core callednucleus and is surrounded by revolving

electrons Atoms have extremely small masses. Because

it would be cumbersome to continually haveto express such small masses in grams, weinstead use a unit called amu (atomic massunit)


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PARTICLE ABSOLUTECHARGE ABSOLUTEMASS DISCOVERERelectron -1.602 x 10-19 C 9.109 x 10-28 g Joseph John

Thomsonproton +1.602 x 10-19 C 1.67266 x10-24 g Ernest

Rutherfordneutron 0 1.67493 x 10-24 g James

Chadwick

1 amu = 1.66054 X 10-24 g= 1/12 of the mass of C-12 atom

An atom is represented as:

A-mass numberZ-atomic numberX- symbol of element


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The isotopic composition of an element is

always expressed on a percentage basis in

terms of the relative number of atoms of the

various isotopes present

% abundance = x 100


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The boron isotopes 10B and 11B have %abundances of 19.91 and 80.09

respectively. This means that if you couldcount 10,000 boron atoms from anaverage natural sample, 1991 of them

would have a mass of 10.0129 amu and8009 would have a mass of 11.0093 amu.

Isotopic abundances can be determined

by mass spectrometer


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Orbital a region of space where there is aprobability of finding an electron

MAIN ENERGYLEVEL SUBLEVEL NUMBER OFORBITAL MAXIMUMNUMBER OF

ELECTRON/SHELLK 1 s 1 2L 2 s

p 1 43 8M 3 s

pd

13 95

18

N 4 spdf

135 167

32


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Aufbau Principle filling up an orbital in the order ofincreasing energy

Electron Configuration Mnemonics


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shows how electrons are distributed among various atomic

orbitals

Ways of Writing Electron Configuration:

1. orbital method (spectroscopic notation) use electron

configuration mnemonics

2. rectanglearrow or arrow-box method ( orbital -box

diagram) uses boxes or rectangles to represent orbitals and

arrows as electrons.

HUNDS RULE:

Each orbital is filled up by a single electron before any pairing

can occur.

3. core method uses the noble gas as core


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Using the three methods , show the electron

configuration of14N7

1.) orbital method(spectroscopic notation):

1s2 2s2 2p3

2.) orbital box diagram:

3.) core method :

[ He ] 2s2 2p3

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Atomic Theory and Structure [Autosaved] (3)