The History of Atomic Theory. The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball → The atomic.

The History of Atomic The History of Atomic TheoryTheory

The atomic model The atomic model has changed has changed throughout the throughout the centuries, starting centuries, starting in 400 BC, when it in 400 BC, when it looked like looked like a a billiard ball billiard ball →→

DemocritusDemocritus

Greek philosopher who Greek philosopher who began the search for a began the search for a description of matter description of matter more than more than 24002400 years years ago.ago.

400 BC

DemocritusDemocritus

His theory: Matter could not His theory: Matter could not be divided into smaller and be divided into smaller and smaller pieces forever, smaller pieces forever, eventually the smallest eventually the smallest possible piece would be possible piece would be obtained.obtained.

This piece would be This piece would be indivisible.indivisible.

He named the smallest He named the smallest piece of matter “atomos,” piece of matter “atomos,” meaning “not to be cut.”meaning “not to be cut.”

DemocritusDemocritus

Said atoms were Said atoms were smallsmall, , hard particles that were hard particles that were all made of the same all made of the same material but were material but were differentdifferent shapes and shapes and sizes.sizes.

Atoms were Atoms were infiniteinfinite in in number, always moving number, always moving and capable of joining and capable of joining together to make new together to make new substances.substances.

This theory was This theory was ignored and forgotten ignored and forgotten for more than for more than 2000 2000 years!years!

Why?Why?Aristotle and PlatoAristotle and Plato

Aristotle and Plato Aristotle and Plato favored the favored the earth,earth, firefire, , airair and and waterwater approach to the approach to the nature of matter. nature of matter. Their ideas held Their ideas held sway because of sway because of their eminence as their eminence as philosophers. The philosophers. The atomosatomos idea was idea was buried for buried for approximately 2000 approximately 2000 years. years.

DaltonDalton In the early 1800s, In the early 1800s,

English Chemist John English Chemist John Dalton Dalton performed a performed a number of experiments number of experiments combining elements combining elements that eventually led to that eventually led to the acceptance of the the acceptance of the idea of atoms.idea of atoms.

Dalton’s TheoryDalton’s Theory

He deduced that all He deduced that all substancessubstances are are composed of atoms. Atoms are small composed of atoms. Atoms are small particles that cannot be created, particles that cannot be created, divided, or destroyeddivided, or destroyed

Atoms of the Atoms of the samesame element are element are exactly alike.exactly alike.

Atoms of Atoms of differentdifferent elements are elements are differentdifferent..

CompoundsCompounds are formed by the joining are formed by the joining of atoms of two or more elements in of atoms of two or more elements in specific proportions of atoms.specific proportions of atoms.

..

This theory This theory became one became one of the of the foundations foundations of modern of modern chemistry.chemistry.

ThomsonThomsonIn In 18971897, the English , the English

scientist J.J. Thomson scientist J.J. Thomson provided the first hint that provided the first hint that an atom can be divided an atom can be divided into even into even smallersmaller particles particles by conducting the cathode-by conducting the cathode-ray tube experiment.ray tube experiment.

He also conducted experiments that He also conducted experiments that proved elements combine in specific proved elements combine in specific proportionsproportions

Thomson ModelThomson Model

He proposed a model of the He proposed a model of the atom that is sometimes atom that is sometimes called the “called the “PlumPlum PuddingPudding” ” model. model.

Atoms were made from a Atoms were made from a positively positively chargedcharged substancesubstance with small with small negatively charged negatively charged particles that he called particles that he called corpuscles corpuscles scatteredscattered about, about, like raisins in a pudding.like raisins in a pudding.

RutherfordRutherford In 1908, English physicist In 1908, English physicist

Ernest Rutherford was Ernest Rutherford was hard at work on an hard at work on an experiment that involved experiment that involved firing a stream of tiny firing a stream of tiny positively chargedpositively charged particles particles at a thin sheet of at a thin sheet of gold foilgold foil (2000 atoms thick).(2000 atoms thick).

MostMost of the positively charged of the positively charged “bullets” passed right through “bullets” passed right through the gold atoms in the sheet of the gold atoms in the sheet of gold foilgold foil without changing without changing course at all.course at all.

SomeSome of the positively of the positively charged “bullets,” however, charged “bullets,” however, did bounce away from the did bounce away from the gold sheet as if they had hit gold sheet as if they had hit something something solidsolid. He knew . He knew that positive charges that positive charges repelrepel positive charges.positive charges.

This could only mean that the gold atoms in the This could only mean that the gold atoms in the sheet were mostly sheet were mostly empty spaceempty space. Atoms were . Atoms were notnot a pudding filled with a positively charged a pudding filled with a positively charged material.material.

Rutherford concluded that an atom had a Rutherford concluded that an atom had a small, small, dense, positively charged centerdense, positively charged center that that repelledrepelled his positively charged “bullets.”his positively charged “bullets.”

He called the center of the atom the “He called the center of the atom the “nucleusnucleus”” The nucleus is The nucleus is tinytiny compared to the atom as a compared to the atom as a

whole however most of the atom’s mass is there. whole however most of the atom’s mass is there.

RutherfordRutherford

Rutherford reasoned that Rutherford reasoned that all of an atom’s positively all of an atom’s positively charged particles were charged particles were containedcontained in the nucleus in the nucleus and it is surrounded by and it is surrounded by empty space in which the empty space in which the negatively charged negatively charged particles (electrons) move particles (electrons) move around.around.

BohrBohrIn 1913, the Danish In 1913, the Danish scientist Niels Bohr scientist Niels Bohr proposed an proposed an improvement. In his improvement. In his model, he placed model, he placed each electron in a each electron in a specificspecific energy level. energy level.

Bohr ModelBohr Model

According to Bohr’s atomic model:According to Bohr’s atomic model:Electrons move in definite Electrons move in definite orbitsorbits around the around the nucleus, much like planets circle the sun. nucleus, much like planets circle the sun. Orbits, or energy Orbits, or energy levelslevels, are located only at , are located only at certaincertain distances from the nucleus.distances from the nucleus.Electrons can jump between these energy Electrons can jump between these energy levels.levels.

Wave Model

Erwin Schrödinger

The Wave ModelThe Wave Model

Today’s atomic model is Today’s atomic model is based on the principles of based on the principles of wavewave mechanicsmechanics..

According to the theory of According to the theory of wave mechanics, electrons wave mechanics, electrons do not movedo not move about an about an atom in a atom in a definite path,definite path, like like the planets around the the planets around the sun.sun.

The Wave ModelThe Wave Model

In fact, it is In fact, it is impossibleimpossible to determine the exact to determine the exact location of an electron. The location of an electron. The probableprobable location of location of an electron is based on how much an electron is based on how much energyenergy the the electron has.electron has.

According to the modern atomic model, at atom According to the modern atomic model, at atom has a has a small positively charged nucleussmall positively charged nucleus surrounded by a large region in which there are surrounded by a large region in which there are enough electrons to make an atom neutral.enough electrons to make an atom neutral.

Electron Cloud:Electron Cloud: A space in which A space in which

electrons are likely to be electrons are likely to be found.found.

Electrons Electrons whirlwhirl about the about the nucleus billions of times nucleus billions of times in one secondin one second

They are not moving They are not moving around in around in randomrandom patterns.patterns.

Location of electrons Location of electrons depends upon how much depends upon how much energyenergy the electron has. the electron has.

Electron Cloud:Electron Cloud:

Depending on their energy they are locked into a Depending on their energy they are locked into a certain area in the cloud.certain area in the cloud.

Electrons with the Electrons with the lowestlowest energy are found in energy are found in the energy level the energy level closestclosest to the nucleus to the nucleus

Electrons with the Electrons with the highesthighest energy are found energy are found in the in the outermostoutermost energy levels, farther from energy levels, farther from the nucleus.the nucleus.