democritus this is the greek philosopher democritus who began the search for a description of matter...
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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 ago. years ago. He asked: Could matter He asked: Could matter
be divided into smaller be divided into smaller and smaller pieces and smaller pieces forever, or was there a forever, or was there a limitlimit to the number of to the number of times a piece of matter times a piece of matter could be could be divideddivided? ?
400 BC
AtomsAtoms
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 of moving and capable of joining together.joining together.
DaltonDalton’’s Models Model
In the early 1800s, In the early 1800s, the English the English Chemist John Chemist John Dalton Dalton performed a performed a number of 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.
DaltonDalton’’s Theorys 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.
ThomsonThomson’’s Plum Pudding s Plum Pudding ModelModel
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
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 ModelThomson Model
Thomson studied Thomson studied the the passage passage of an of an electric current electric current through a gas.through a gas.
As the current As the current passed through passed through the gas, it gave off the gas, it gave off rays of rays of negatively negatively charged particles.charged particles.
Thomson ModelThomson Model
This surprised This surprised Thomson, Thomson, because the because the atoms of the gas atoms of the gas were uncharged. were uncharged. Where had the Where had the negative charges negative charges come from?come from?
Where did they come from?
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.
RutherfordRutherford’’s Gold Foil s Gold Foil ExperimentExperiment
In 1908, the English In 1908, the English physicist Ernest physicist Ernest Rutherford was hard Rutherford was hard at work on an at work on an experiment that experiment that seemed to have little seemed to have little to do with unraveling to do with unraveling the mysteries of the the mysteries of the atomic structure.atomic structure.
RutherfordRutherford’’s experiment Involved firing s experiment Involved firing a stream of tiny a stream of tiny positively chargedpositively charged particles at a thin sheet of particles at a thin sheet of gold foilgold foil (2000 atoms thick)(2000 atoms thick)
MostMost of the positively of the positively charged charged ““bulletsbullets”” passed passed right through the gold right through the gold atoms in the sheet of atoms in the sheet of gold gold foilfoil without changing without changing course at all.course at all.
SomeSome of the positively of the positively charged charged ““bullets,bullets,”” however, 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 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 his positively charged ““bullets.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 Rutherford reasoned that all of an atomthat all of an atom’’s s positively charged positively charged particles were particles were containedcontained in the in the nucleus. The nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the outside the nucleus around the nucleus around the atomatom’’s s edgeedge..
Bohr ModelBohr ModelIn 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 energy level.level.
Bohr ModelBohr Model According to BohrAccording to Bohr’’s s
atomic model, atomic model, electrons move in electrons move in definite definite orbitsorbits around around the nucleus, much the nucleus, much like planets circle the like planets circle the sun. These orbits, or sun. These orbits, or energy energy levelslevels, are , are located at certainlocated at certain distances from the distances from the nucleus.nucleus.
Wave Model
The Wave ModelThe Wave Model TodayToday’’s atomic 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, electrons mechanics, electrons do not movedo not move about about an atom in a an atom in a definite definite path,path, like the planets like the planets around the sun.around the 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.