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The History of The History of Atomic TheoryAtomic Theory
Atomic ModelsAtomic Models
A model is a A model is a schematic description schematic description of a system, theory, or of a system, theory, or phenomenon thatphenomenon that accounts for its accounts for its known or inferred known or inferred properties, andproperties, and may be used for may be used for further study of its further study of its characteristics.characteristics.
Bohr Model
The first atomic The first atomic model was model was proposed in 400 proposed in 400 BC and looked BC and looked looked like looked like a a billiard ball billiard ball →→
Models change as more evidence is discovered
Democritus – 400 B.C.Democritus – 400 B.C. Matter could not be divided Matter could not be divided
into smaller and smaller into smaller and smaller pieces forever, eventually pieces forever, eventually the smallest possible piece the smallest possible piece would be obtained.would be obtained.
This piece would be This piece would be indivisible.indivisible.
He named the smallest He named the smallest piece of matter piece of matter ““atomosatomos,,”” meaning meaning ““not to be cut.not to be cut.””
The The philosophers of philosophers of the time, the time, AristotleAristotle and and Plato, had a Plato, had a more respected, more respected, (and ultimately (and ultimately wrongwrong) idea. ) idea. The The atomosatomos idea idea was forgotten was forgotten for 2000 years.for 2000 years.
Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers.
DaltonDalton’’s Model – 1803 s Model – 1803
In the early 1800s, In the early 1800s, the English Chemist the English Chemist John John Dalton Dalton performed performed experiments that led experiments that led to the acceptance of to the acceptance of the idea of atoms.the idea of atoms.
DaltonDalton’’s Atomic Theorys Atomic Theory1.1. He deduced that all He deduced that all
elementselements are composed of are composed of atoms. atoms.
2.2. Atoms are indivisible and Atoms are indivisible and indestructible particles.indestructible particles.
3.3. Atoms of the Atoms of the samesame element are exactly element are exactly alikealike..
4.4. Atoms of Atoms of differentdifferent elements are elements are differentdifferent..
5.5. 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 Model - 1897s Model - 1897
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.
Cathode Ray ExperimentCathode Ray ExperimentThomson studied Thomson studied
the passage of an the passage 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 the gas, it gave off rays of off rays of negatively negatively charged particles.charged particles.
cathode ray tube
Thomson concluded that:• the negative charges came from within the atom.•a particle smaller than an atom had to exist. •The atom was divisible!
• the negatively charged particles are electrons.
• Since the gas was neutral, there must be positively charged particles in the atom. But he could never find them.
He proposed a model of He proposed a model of the atom that is the atom that is sometimes called the sometimes called the ““PlumPlum PuddingPudding”” model. model.
Atoms were made from Atoms were made from
a positively a positively chargedcharged substancesubstance with with negatively charged negatively charged electrons electrons scatteredscattered about, like raisins in a about, like raisins in a pudding.pudding.
RutherfordRutherford’’s Model - 1911s Model - 1911
English physicist English physicist Ernest Rutherford Ernest Rutherford used radioactive used radioactive particles to search for particles to search for the positive charged the positive charged particles in the atom.particles in the atom.
RutherfordRutherford’’s experiment Involved s experiment Involved firing a stream of tiny firing a stream of tiny positively positively chargedcharged particles (alpha particles) at particles (alpha particles) at a thin sheet of a thin sheet of gold foilgold foil (2000 atoms (2000 atoms thick).thick).
If Thompson’s model was correct, the If Thompson’s model was correct, the alpha particles should pass through alpha particles should pass through the foil.the foil.
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 gold atoms in the sheet of gold foil.foil.
But But somesome of the of the ““bullets”bullets” bounced away from the bounced away from the gold sheet as if they had gold sheet as if they had hit something hit something solidsolid..
He knew that positive He knew that positive
charges charges repelrepel positive positive charges.charges.
Experiment Results
This could only mean that This could only mean that the gold atoms in the the gold atoms in the sheet were mostly sheet were mostly open open spacespace. .
Alpha particles were Alpha particles were repelled by a repelled by a small, small, dense, positively charged dense, positively charged centercenter..
Rutherford called the Rutherford called the center of the atom the center of the atom the ““nucleusnucleus””
The nucleus is The nucleus is tiny tiny compared to the atom as compared to the atom as a whole. a whole.
Rutherford ModelRutherford Model
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 nucleus. The in the nucleus. The negatively charged negatively charged particles were particles were scatteredscattered outside the nucleus outside the nucleus around the atomaround the atom’’s s edgeedge..
Bohr Model - 1922Bohr Model - 1922Danish scientist Danish scientist
Niels Bohr Niels Bohr proposed an proposed an improvement. improvement.
In his model, each In his model, each electron occupied electron occupied a a specificspecific energy energy level.level.
Bohr ModelBohr ModelIn this model, In this model,
electrons move in electrons move in definite definite orbitsorbits around around the nucleus, like the nucleus, like planets orbit the sun. planets orbit the sun.
These orbits, are These orbits, are located at specific located at specific distances from the distances from the nucleus.nucleus.
James Chadwick - 1932James Chadwick - 1932
Noticed the Noticed the difference between difference between mass and atomic mass and atomic number (+ charge)number (+ charge)
Used radioactive Used radioactive particles to find a particles to find a neutral particle, neutral particle, “neutron”, in the “neutron”, in the nucleus.nucleus.
Chadwick ModelChadwick Model
Modern Wave Model
This model arose from work by physicists Louis de Broglie, Erwin Schrödinger and Werner Heisenberg.
It is also called the quantum mechanical model.
The Wave ModelThe Wave Model
In this model, electrons In this model, electrons are not particles that are not particles that move about an atom in move about an atom in a a definite path,definite path, like the like the planets around the sunplanets around the sun
They are waves with They are waves with definite energy levels.definite energy levels.
The Wave ModelThe Wave Model
It is It is impossibleimpossible to determine the exact location of to determine the exact location of an electron. The an electron. The probableprobable location of an electron location of an electron is based on how much is based on how much energyenergy the electron has. the electron has.
According to the modern atomic model, an atom According to the modern atomic model, an 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 CloudElectron Cloud (def.) A space in which electrons are likely to be (def.) A space in which electrons are likely to be
found.found.
Depending on their energy, electrons are Depending on their energy, electrons are located in a certain area in the cloud.located in a certain area in the cloud.
Electrons with the Electrons with the lowestlowest energy are found in the energy are found in the energy level energy level closestclosest to the nucleus. to the nucleus.
Electrons with the Electrons with the highesthighest energy are found in energy are found in the the outermostoutermost energy levels, farther from the energy levels, farther from the nucleus.nucleus.
