History of Atomic History of Atomic ModelsModels

The AtomThe Atom

Early GreeksEarly Greeks

Believed matter Believed matter could be broken could be broken into four into four categories:categories:

Fire (Plasma)Fire (Plasma) Earth (Solid)Earth (Solid) Water (Liquid)Water (Liquid) Air (Gas)Air (Gas)

DemocritisDemocritis

More than 2400 years ago.More than 2400 years ago. Philosopher- did no experiments.Philosopher- did no experiments. ““can matter be broken down forever can matter be broken down forever

or is there a limit”or is there a limit”

Called the smallest pieces Called the smallest pieces ATOMOS ATOMOS (atom) meaning “not to be cut”.(atom) meaning “not to be cut”.

DemocritisDemocritis

Conclusions:Conclusions:

Atoms are small, hard particlesAtoms are small, hard particles Atoms are made of the same Atoms are made of the same

material that differs in size and material that differs in size and shape.shape.

Atoms are infinite in number, always Atoms are infinite in number, always moving, and able to join.moving, and able to join.

John DaltonJohn DaltonEarly 1800’sEarly 1800’s

Found that gases Found that gases combine as if combine as if individual particlesindividual particles

Chemist who did Chemist who did experiments, had experiments, had an interest an interest meteorology and meteorology and studied air and studied air and properties of properties of gases.gases.

John DaltonJohn DaltonEarly 1800’sEarly 1800’s

Conclusions:Conclusions:

All elements are composed of atoms All elements are composed of atoms which are indivisible particles.which are indivisible particles.

Elements contain atoms that are all Elements contain atoms that are all the same for that element.the same for that element.

Atoms differ for different elements.Atoms differ for different elements. Compounds are formed by joining Compounds are formed by joining

the atoms of two or more elements.the atoms of two or more elements.

J J Thomson J J Thomson 18971897

First hint that the atom is made of smaller First hint that the atom is made of smaller particles.particles.

He sent an electric current through a gas. He sent an electric current through a gas. The gas gave off rays that Thomson The gas gave off rays that Thomson showed were negatively charged.showed were negatively charged.

To this point in time, the uncharged atom To this point in time, the uncharged atom was the only particle known.was the only particle known.

Thomson concluded that a particle smaller Thomson concluded that a particle smaller than the atom had to exist.than the atom had to exist.

J J ThomsonJ J Thomson18971897

Thomson called the negatively charge Thomson called the negatively charge particles “corpuscles” now called particles “corpuscles” now called electrons.electrons.

Thomson model had a problem. The atom Thomson model had a problem. The atom is known to have a neutral charge and now is known to have a neutral charge and now a negative particle is known to exist. The a negative particle is known to exist. The solution was that a positive particle has to solution was that a positive particle has to exist to balance the negative charge. exist to balance the negative charge. Thomson was unable to prove that the Thomson was unable to prove that the positive particle existed.positive particle existed.

JJ ThomsonJJ Thomson18971897

J J ThomsonJ J Thomson18971897

‘‘Plum Pudding‘ Plum Pudding‘ model. Plum model. Plum pudding is a rich, pudding is a rich, boiled or steamed boiled or steamed pudding made with pudding made with flour, suet, raisins, flour, suet, raisins, currants, citron and currants, citron and spices.spices.

Earnest RutherfordEarnest Rutherford19081908

Rutherford was a Rutherford was a British physicist British physicist who tested who tested Thomson’s Model.Thomson’s Model.

Gold Foil Gold Foil experiment.experiment.

Proposed that the Proposed that the atom has a small, atom has a small, dense, positively dense, positively charged center charged center called the called the nucleus.nucleus.

Rutherford 1908Rutherford 1908

RutherfordRutherford19081908

The nucleus is tiny as The nucleus is tiny as compared to the compared to the whole atom. If a whole atom. If a marble sits in the marble sits in the center of Heinz Field, center of Heinz Field, the marble represents the marble represents the nucleus while the the nucleus while the stadium represents stadium represents the whole atom.the whole atom.

The atom is mostly The atom is mostly empty space.empty space.

Neils BohrNeils Bohr19131913

Bohr improved the Bohr improved the Rutherford model.Rutherford model.

Definite orbits like Definite orbits like planets around the planets around the sun.sun.

Orbits are energy Orbits are energy levels, of which levels, of which there are 7.there are 7.

Werner HeisenbergWerner Heisenberg19261926

Heisenberg was a Heisenberg was a German Physicist who German Physicist who proved that the proved that the position of the position of the electron can never be electron can never be exactly known. exactly known.

Regions are known as Regions are known as orbitals where orbitals where electrons might be electrons might be found.found.

Electrons can jump Electrons can jump from orbital to orbital.from orbital to orbital.

Electron Cloud.Electron Cloud.

electron

neutron

proton