Atomic ModelsAtomic Models

This model of the This model of the atom may look atom may look familiar to you. This is familiar to you. This is the Bohr model. In the Bohr model. In this model, the this model, the nucleus is orbited by nucleus is orbited by electrons, which are electrons, which are in different energy in different energy levels. levels. A model uses familiar ideas to A model uses familiar ideas to

explain unfamiliar facts explain unfamiliar facts observed in nature.observed in nature.

A model can be changed as A model can be changed as new information is collected.new information is collected.

Who are these men?Who are these men?

In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

DemocritusDemocritus

This is the Greek This is the Greek philosopher Democritus philosopher Democritus who began the search for who began the search for a description of matter a description of matter more than more than 24002400 years years ago.ago. He asked: Could He asked: Could

matter be divided into matter be divided into smaller and smaller smaller and smaller pieces forever, or was pieces forever, or was there a there a limitlimit to the to the number of times a number of times a piece of matter could piece of matter could be be divideddivided? ?

400 BC

AtomosAtomos

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

AtomosAtomos

To Democritus, atoms To Democritus, atoms were were smallsmall, hard , hard particles that were all particles that were all made of the same 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 number, always moving and capable moving and capable of joining together.of joining together.

Dalton’s ModelDalton’s Model

In the early 1800s, In the early 1800s, the English the English Chemist John Chemist John Dalton,Dalton, a a mathematics mathematics teacher performed teacher performed a number of a number of experiments that experiments that eventually led to eventually led to the acceptance of the acceptance of the idea of atoms.the idea of atoms.

Dalton’s TheoryDalton’s Theory

He deduced that all He deduced that all elementselements are composed of are composed of atoms. Atoms are atoms. Atoms are indivisible and indivisible and indestructible particles.indestructible particles.

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

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

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

Dalton's ProblemsDalton's Problems

Atoms of the same element can Atoms of the same element can have different masses – isotopes.have different masses – isotopes.

Atoms contain subatomic particles Atoms contain subatomic particles – electrons, protons, neutrons.– electrons, protons, neutrons.

However,this theory became one However,this theory became one of the of the foundations of modern foundations of modern chemistry.chemistry.

Thomson’s Plum Pudding ModelThomson’s Plum Pudding Model

In In 18971897, the , the English scientist English scientist J.J. Thomson J.J. Thomson provided the first provided the first hint that an atom hint that an atom is made of even is made of even smallersmaller particles. particles.

Thomson ModelThomson Model He proposed a He proposed a

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

Atoms were made Atoms were made from a positively from a positively chargedcharged substancesubstance with negatively with negatively charged electrons charged electrons scatteredscattered about, about, like raisins in a like raisins in a pudding.pudding.

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Thomson called the negatively charged “corpuscles,” today known as electrons.

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.

But he could never find them.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

In 1908, the In 1908, the English physicist English physicist Ernest Rutherford Ernest Rutherford was hard at work was hard at work on an experiment on an experiment that seemed to that seemed to have little to do have little to do with unraveling the with unraveling the mysteries of the mysteries of the atomic structure.atomic structure.

Rutherford's Gold FoilRutherford's Gold Foil MostMost of the positively of the positively

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

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

Rutherford's ModelRutherford's Model

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 open spaceopen 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 tiny tiny compared to the atom as a compared to the atom as a

whole. whole.

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. The negatively charged The negatively charged particles were particles were scatteredscattered outside the nucleus outside the nucleus around the atom’s around the atom’s edgeedge..

The problem with The problem with Rutherford's model's was Rutherford's model's was the electrons should spin the electrons should spin into the nucleus.into the nucleus.

Bohr ModelBohr Model In 1913, the In 1913, the

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

Bohr ModelBohr Model According to Bohr’s According to Bohr’s

atomic model, electrons atomic model, electrons move in definite move in definite orbitsorbits around the nucleus, much around the nucleus, much like planets circle the sun. like planets circle the sun. These orbits, or energy These orbits, or energy levelslevels, are located at , are located at certain distances from the certain distances from the nucleus.nucleus.

Bohr's model only worked Bohr's model only worked for Hydrogen and for Hydrogen and contradicted Heisenberg's contradicted Heisenberg's Uncertainty Principle. Uncertainty Principle.

Sir James ChadwickSir James Chadwick

In 1932, Chadwick made a In 1932, Chadwick made a fundamental discovery in fundamental discovery in the domain of nuclear the domain of nuclear science: he proved the science: he proved the existence of existence of neutronsneutrons - - elementary particles devoid elementary particles devoid of any electrical charge. For of any electrical charge. For this epoch-making this epoch-making discovery he was awarded discovery he was awarded the Hughes Medal of the the Hughes Medal of the Royal Society in 1932, and Royal Society in 1932, and subsequently the Nobel subsequently the Nobel

Prize for Physics in 1935.Prize for Physics in 1935.

Column 1Column 2Column 3

Schrodinger’s Wave ModelSchrodinger’s Wave Model

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

According to the According to the theory of wave theory of wave mechanics, mechanics, electrons electrons do not do not movemove about an about an atom in a atom in a definite definite path,path, like the like the planets around 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 -location of an electron -– Heisenberg's – Heisenberg's Uncertainty Principle.Uncertainty Principle. The 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 Depending on their energy they are locked

into a certain area in the cloud. into a certain area in the cloud.

No two particles can be in one place at a No two particles can be in one place at a once – Pauli Exclusion Principle. once – Pauli Exclusion Principle.

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.