atomic thoery

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The Atomic

Theory


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The Atomic TheoryScientifc knowledge builds on past research and experimentation.

The study o the atom is at the core o chemistry because it enables us tounderstand the physical, chemical and structural properties ocompounds.Over the years, the concept o the atom has undergone many changesdue to the discovery o new technology. Prior to the scientifc revolutionand the development o the scientifc method starting in the !th

century, ideas about the atom were mainly speculative. "t wasn#t untilthe very end o the $th century that technology became advancedenough to allow scientists a glimpse o the atom#s constituent parts% theelectron, nucleus, proton, and neutron. &ere is a timeline, ollowing thedevelopment'changes o the atomic theory throughout the years and othe various scientists who were involved.


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The frst idea o the atomic theory was said to have frst started withemocritus and his teacher, 1eucippus, a 2reek philosopher.emocritus, also a 2reek philosopher, came up with his teacher, theidea that all matter is made up o tiny, indivisible particles, or atoms.emocritus 3uestioned whether matter could be divided into smallerand smaller pieces orever until eventually the smallest possible piecewould be obtained. &e believed that the smallest possible piece o

matter was indivisible. &e named the smallest piece o matter4atomos,5 meaning 4not to be cut or indivisible.5 To emocritus, atomswere small, hard particles that were all made o the same material, butwere ormed into di6erent shapes and si7es. +olor, taste, and otherintangible 3ualities were also thought to be composed o atoms. Theconcept o an ultimate particle was maintained or about 8))) years

without any scientifc experiments being conducted to prove ordisprove it.


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300 B.C: ARISTOTLE

Though the idea o the atom was supported by somelater 2reek philosophers, it was fercely attacked byothers, including 9ristotle, who argued against theexistence o such particles. 9ristotle declared that allmatter was made o only our elements% fre, air, waterand earth. &e also believed that matter had :ust ourproperties% hot, cold, dry and wet. uring the iddle 9gesin -urope, /oman +atholic theologians were heavilyin;uenced by 9ristotle#s ideas, and so atomic philosophywas largely dismissed or centuries.


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EARLY 1800s : JOHN ALTON

"n


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9toms can chemically combine with each other in simple whole numberratios. @ot all o these ideas were newA the 2reeks had already introduced the

idea that elements were composed o atoms and that atoms o di6erentelements had di6erent physical properties. alton#s particularcontribution, which distinguished his work rom what had been done

be ore, was his method or actually determining atomic weight. "n anessay published in


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18!": J.J. THO#$SON

"n


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Ghen Thompson investigated the Hcathode raysC urther andound that they could be de;ected by electric and magneticfelds, he showed that they were made up o very lightparticles. The results o his experiments enabled him tocalculate their ratio o charge to mass. Thompson urther

estimated that the mass o the electron was approximately'


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$))% -. 2O1 ST-"@

"n $)) 2oldstein discovered protons a terobserving the rays in a cathodeDray tube thatwere travelling in the direction opposite tothat o the cathode rays. 9ccording to2oldstein, since atoms are electrically neutralthey must contain e3ual number o positivelycharged particles as negatively chargedparticles.


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1!0%: ALBERT EINSTEIN

&e postulated that light was made up o di6erentparticles that, in addition to wavelike behavior,demonstrate certain properties uni3ue to particles.&e also brought orth the theory o relativity.


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$)$% -/@-ST /0T&-/IO/

"n $)$, the -nglish physicist -rnest /uther ordper ormed an experiment using positively chargedparticles fred at gold oil. Through his experiment, heproved that atoms are not a 4pudding5 flled with apositively charged material. &e theori7ed that atomshave a small, dense, positively charged center, whichhe called the 4nucleus5. &e said that nucleus is tinycompared to the atom as a whole, because the atom ismostly open spaceJ &e concluded that the negativelycharged particles are scattered outside the nucleus ata distance.


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-rnest /uther ord#s experiments with alpharays led him to describe the atom as asmall, heavy nucleus with electrons in orbitaround it. This nuclear model o the atombecame the basis or the one that is stillaccepted today.


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$ K% @-"1S *O&/

"n $ K, the anish scientist @iels *ohr proposed

an improvement. &e built on the concept that themass o an atom is contained mostly in the nucleus.&e also theori7ed that electrons move in defniteorbits around the nucleus, much like planets circlethe sun. These orbits, or energy levels, are locatedat certain distances rom the nucleus.


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"n $ K, anish physicist @iels *ohr, who had studied underboth Thomson and /uther ord, urther refned the nuclearmodel by proposing that electrons moved only in restricted,successive orbital shells and that the outer, higherDenergyorbits determined the chemical properties o the di6erent

elements. Iurthermore, *ohr was able to explain the spectrallines o the di6erent elements by suggesting that as electrons :umped rom higher to lower orbits, they emitted energy inthe orm o light. "n the $8)s, *ohr#s theory became thebasis or 3uantum mechanics, which explained in greater

detail the complex structure and behavior o atoms.


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$ K% &-@/L OS-1L

"n $ K, osely suggested that i atoms are

neutral, then the number o electrons must bethe same as the number o electrons. Sinceelectrons have negligible mass, and atomicmasses are greater than the mass o protons in

the atom then the extra mass must be theexistence o a particle with mass but no charge./uther ord also put orward this idea and calledthe particle a neutron, thus the search o theneutron began.


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0sing xDray tubes, determined the charges on thenuclei o most atoms. &e wrote MThe atomic numbero an element is e3ual to the number o protons inthe nucleusM. This work was used to reorgani7e theperiodic table based upon atomic number instead oatomic mass.


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$8(% GO1I29@2 P901"Gol gang Pauli was an 9ustrian theoretical physicist noted or his workon spin theory and 3uantum theory, and or the important discovery othe Pauli exclusion principle, which underpins the structureo matter and the whole o chemistry. "n $8(, he had proposed a3uantum number or the MspinM o electrons, with two possible values,MupM and MdownM. &e then extended this to ormulate the Pauli exclusion principle in$8B, perhaps his best known and most important work, which was arule explaining the behavior o electrons orbit the nucleus atoms. "tstates that no two electrons in an atom can have the same 3uantumnumbers. 9n electron in an atom has our such numbers. They defnethe energy o the electron in terms o the distance o its orbit rom the

nucleus, the orbit#s shape, the orientation o the axis o the orbit, andthe electron#s s in on its own axis.


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Pauli also became the frst to propose theexistence o the subatomic particle theneutrino. &e was also one o the frst to ullyunderstand the theory o general relativity.


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$K8% =9 -S +&9 G"+

"n $K8, +hadwick bombarded the element *e with alpha

particles. This produced a highly penetrating stream oparticles, which could pass through many centimeters osolid lead and which was not de;ected by electric ormagnetic felds. &e had detected the neutron.

=ames +hadwick discovers the neutron component o theatomic nucleus, explaining the nuclear fssion o uranium8KB. This also made it possible to produce elementsheavier than uranium in the lab.


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$ICT&RES


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This has brought you to the end o 2roup CsPresentation on The 9tomic Theory.2roup embers are ohamed +hand -sther 1i =ayashri ara: and =ulia Persaud

atomic thoery

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