Atoms:Atoms:Development of Development of

the Atomic the Atomic TheoryTheory

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DemocritusDemocritus

460 BC - Greek philosopher proposes 460 BC - Greek philosopher proposes the existence of the the existence of the atom atom

His theory:His theory: all atoms are small hard particles all atoms are small hard particles made of a single material formed into made of a single material formed into

different shapes and sizesdifferent shapes and sizes always moving, and that they form always moving, and that they form

different materials by joining togetherdifferent materials by joining together

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John DaltonJohn Dalton 1803 - British chemist; elements 1803 - British chemist; elements

combined in specific proportions to combined in specific proportions to form compounds.form compounds.

His theory:His theory: all substances are made of atoms that all substances are made of atoms that

cannot be created, divided, or destroyedcannot be created, divided, or destroyed atoms join with other atoms to make new atoms join with other atoms to make new

substancessubstances atoms of the same element are exactly atoms of the same element are exactly

alike, and atoms of different elements are alike, and atoms of different elements are different in mass and size (different in mass and size (elementselements)) 3

Edward FranklandEdward Frankland

1852 - English chemist developed the 1852 - English chemist developed the valence valence theorytheory

His theory:His theory: every atom has a fixed number of bonds every atom has a fixed number of bonds

(chemical links) that it can form(chemical links) that it can form for the atom to be stable, all of these for the atom to be stable, all of these

bonds must be used.bonds must be used.

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J.J. ThomsonJ.J. Thomson

1897 - English chemist and physicist 1897 - English chemist and physicist discovered 1discovered 1stst subatomic particles subatomic particles

His theory:His theory: negatively charged particles called negatively charged particles called

electrons electrons andand positively charged matterpositively charged matter created a model to describe the atom as created a model to describe the atom as

a sphere filled with positive matter with a sphere filled with positive matter with negative particles mixed innegative particles mixed in

Referred to it as the plum pudding modelReferred to it as the plum pudding model

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Ernest RutherfordErnest Rutherford 1912 - New Zealand physicist 1912 - New Zealand physicist

discovered the nucleusdiscovered the nucleus

His theory:His theory: small, dense, positively charged particle small, dense, positively charged particle

present in present in nucleusnucleus called a called a protonproton electronselectrons travel around the travel around the nucleus,nucleus, but but

their exact places cannot be describedtheir exact places cannot be described

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Niels BohrNiels Bohr

1913 - Danish physicist discovered 1913 - Danish physicist discovered energy levelsenergy levels

His theory:His theory: electronselectrons travel around the nucleus in travel around the nucleus in

definite paths and fixed distancesdefinite paths and fixed distances electrons can jump from one level to a electrons can jump from one level to a

path in another levelpath in another level

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Erwin ShrodingerErwin Shrodinger

1924 - Austrian physicist developed 1924 - Austrian physicist developed the the electron cloud electron cloud modelmodel

His theory:His theory: electrons exact path cannot be electrons exact path cannot be

predictedpredicted regions, referred to as the electron regions, referred to as the electron

cloud, are areas where electrons can cloud, are areas where electrons can likely be found. likely be found.

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James ChadwickJames Chadwick

1932 - English physicist discovered 1932 - English physicist discovered neutronsneutrons

His theory:His theory: neutronsneutrons have no electrical charge have no electrical charge neutrons have a mass nearly equal to neutrons have a mass nearly equal to

the mass of a protonthe mass of a proton unit of measurement for subatomic unit of measurement for subatomic

particles is the particles is the atomic mass unit atomic mass unit ((amuamu))

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Modern Theory of the Modern Theory of the AtomAtom

AtomsAtoms are composed of three main are composed of three main subatomicsubatomic particles: the particles: the electronelectron, the , the proton,proton, and the and the neutronneutron. .

Most of the mass of the atom is Most of the mass of the atom is concentrated in the concentrated in the nucleusnucleus of the atom. of the atom.

The The protons protons and and neutrons neutrons are located are located within the within the nucleusnucleus while the while the electronselectrons exist outside of the nucleus. exist outside of the nucleus.

In stable atoms, the number of In stable atoms, the number of protonsprotons is is equal to the number of equal to the number of electronselectrons. .

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Modern Theory of the Modern Theory of the AtomAtom

The type of atom is determined by the The type of atom is determined by the number of number of protons protons it has. it has.

The number of protonsThe number of protons in an atom is equal in an atom is equal to the to the atomic number atomic number

The sum of the number of protonsThe sum of the number of protons and and neutrons in a particular atom is called the neutrons in a particular atom is called the atomicatomic massmass

Valence electrons Valence electrons are the outermost are the outermost electrons and are where bonding takes electrons and are where bonding takes placeplace

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