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Democritus Contribution to The Development of
Atomic Model
According to Democritus' atomic theory,
everything is made up of atoms, which are
physically indivisible: atoms are indestructible,
eternal and invisible, small and unable to be
diminished !e posited that atoms occupy space
and di"er in si#e, shape, magnitude, position and
arrangement $n his model, atoms are
homogeneous
%sing this as a basis to the physical world,
Democritus was able to e&plain all changes in the
world as changes in motion of atoms, or changesin the way the atoms were paced together The
theory e&plained physics and combined
mathematics, since its structure was (uantitative
and sub)ect to mathematical laws
The Democritus theory elaborated further on how
nature behaves According to the theory, nature
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is a highly comple& mechanism *hen it comes to
(ualities such as warmth and taste, atoms di"er
only in (uantity and the properties of these
(ualities are only by convention According toDemocritus, atoms and the +oid are the only
things that actually e&ist
$n his theory, he e&plained that by atoms moving
randomly and colliding to form larger bodies was
how the universe began According to
Democritus, the world always e&isted, will forevere&ist and is lled with atoms moving about
randomly
-ohn Daltons Contribution to The Development of
Atomic Model
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!is atomic theory said that elements consisted of
tiny particles called atoms $t states an element is
one of a ind .aa pure/ because all atoms of an
element are identical All the atoms that mae up
the element have the same mass All elementsare di"erent from each other due to di"ering
masses
A compound is a pure .one of a ind/ substance
due to di"erent elements bonded together They
are not easily separated from one another
Compounds have a &ed ratio of atoms 0ach
atom has its own characteristic weight, creating a
weight ratio between elements Also, his theory
said that chemical reactions involve the
rearrangement of combinations of atoms !is
atomic theory stated the general information on
the atom 0verything is made of atoms All atoms
of di"erent substances have di"erent weights
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-- Thomsons Contribution to The Development of
Atomic Model
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$n 1234 the 5ritish physicist -oseph -ohn .- -/
Thomson .12678139/ discovered the electron in
a series of e&periments designed to study thenature of electric discharge in a high;vacuum
cathode;ray tube, an area being investigated by
numerous scientists at the time Thomson
interpreted the de
that he calculated as having a very large value
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for the charge;to;mass ratio ?ater he estimated
the value of the charge itself
$n 139 Thomson suggested a model of the atom
as a sphere of positive matter in which electrons
are positioned by electrostatic forces !is e"orts
to estimate the number of electrons in an atom
from measurements of the scattering of light, @,
beta, and gamma rays initiated the research
tra)ectory along which his student 0rnestutherford moved Thomsons last important
e&perimental program focused on determining
the nature of positively charged particles !ere
his techni(ues led to the development of the
mass spectrograph !is assistant, Brancis Aston,
developed Thomson's instrument further and withthe improved version was able to discover
isotopesatoms of the same element with
di"erent atomic weightsin a large number of
nonradioactive elements
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0rnest utherfords Contribution to TheDevelopment of Atomic Model
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5y 1311 the components of the atom had beendiscovered The atom consisted of subatomic particlescalled protons and electrons !owever, it was not clearhow these protons and electrons were arranged within
the atom -- Thomson suggested theplum puddingmodel $n this model the electrons and protons areuniformly mi&ed throughout the atom:utherford tested Thomson's hypothesis by devising hisgold foil e&periment utherford reasoned that if
Thomson's model was correct then the mass of theatom was spread out throughout the atom Then, if he
shot high velocity alpha particles .helium nuclei/ at anatom then there would be very little to de
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alpha particles could be de
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that energy is transferred only in certain well dened(uantities 0lectrons should move around the nucleusbut only in prescribed orbits *hen )umping from oneorbit to another with lower energy, a light (uantum isemitted 5ohr's theory could e&plain why atoms emittedlight in &ed wavelengths
-ames Chadwics Contribution to The Development ofAtomic Model
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-ames Chadwic discovered the neutron using evidencecollected by $rene -oliot;Curie, who discovered thatwhen beryllium was bombarded with positively chargedalpha particles a beam with a high penetrating powerwas created -ames Chadwic discoverd that this beamwas not de
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