History of the Modern Atomic Theory

History of the Modern Atomic

Theory

Chemistry

The Atomic Theory

• What do we know about atoms?

• We know that atoms are composed of protons, neutrons, and electrons. The protons and neutrons comprise almost the entire mass of the atom and make up the nucleus. The electrons are in energy levels outside the nucleus residing in orbitals which contain a maximum of 2 electrons. The proton gives the atom its identity, but it is the electron which gives the atom its chemical properties. We know that elements consist of different isotopes, some of which are radioactive.

• But HOW do we know all this stuff?

• There have been MANY scientists studying the atom and radioactivity and their studies have cumulated in this vast knowledge of the atom and its properties.

Democritus of Abdera

• First to produce an elaborate and systematic view of the atomic theory.

• Said that all matter is composed of tiny, INDIVISIBLE particles called atomos (today, called atoms)

John Dalton• (1766 – 1844)

• A Quaker and English schoolteacher who discovered he was colorblind.

• Devised the atomic theory of matter:

1. All elements are composed of atoms.

2. All of the atoms of an element are identical and are different from the atoms of any other element.

3. Atoms are not created or destroyed in a chemical reaction.

4. Compounds always have the same numbers and kinds of atoms.

• Published the first list of atomic weights and symbols which gave chemistry a language of its own.

Joseph John Thomson

• J.J. Thomson

(1856 – 1940)

• Discovered that atoms

were made of pieces, not

“indivisible” as previously

thought.

• Used cathode ray tube

• Discovered that cathode

rays are composed of

negative particles that

called ELECTRONS.

Thomson’s Cathode Ray Tube

Robert Andrews Millikan• Born March 22, 1868 in

Morrison, Illinois

• Determined the charge on an electron in his oil drop experiment

• Showed that the electron is an extremely light particle

Millikan’s Oil-Drop Experiment

Ernest Rutherford• (1871 – 1937)

• Was a research student under J.J. Thomson at Cambridge University

• Gold Foil Experiment: all of an atom’s + charge is concentrated in a small core in the center- he called it the nucleus.

• First model of the atom with a nucleus in the center and electrons on the outside. (Planetary model of the atom)

• The atom is so small that it would take over 5 million atoms side-by-side to go across your notebook paper ; he postulated the nucleus to have almost all of the mass of the atom even though the nucleus is 1000x smaller than the atom itself.

• Also studied radioactivity

• Discovered and named alpha andbeta radiation

James Chadwick

• 1891-1974

• English physicist

• Worked with Rutherford

• Imprisoned at beginning of WWI until Geiger’s laboratory interceded for his release.

• Discovered the particle in the nucelus that has no charge- the neutron

• Determined the mass of the neutron= 1.0067.

Niels Henrik David Bohr

• Born in Copenhagen on Oct. 7, 1885 (died 1962)

• During the Nazi occupation of Denmark, Bohr escaped to Sweden where he became associated with the Atomic Energy Project with Albert Einstein.

• Added to Rutherford’s model of the atom by including energy levels, outside of the nucleus, where electrons are located.

• Said that electrons “jump” to higher energy levels when they absorb energy

Benjamin Franklin

• Franklin was well known for his

studies on electricity and as

well as his many inventions

such as the Franklin stove,

swim fins, glass harmonica,

and bifocals.

• Said there are two kinds of

charges- (+) and (-).

• Said that like charges repel

and opposite charges attract

(Law of Electrostatics)

• Said that lightning is simply

static electricity

Henri Becquerel

• 1852 – 1908

• Father of Radioactivity

• Accidentally discovered

radioactivity – he placed some

Uranium on some unexposed

film. He found an image on

the film showing the crystalline

structure of the uranium. He

concluded the uranium emitted

radiation which could

penetrate paper opaque to

light. Uranium could do this

without an external source of

energy.

Marie Curie• Born Marya Sklodowska in Warsaw,

Poland on Nov. 7, 1867; died 1934

• Dreamed of a scientific career, a concept inconceivable for a woman at that time. Lack of finances force her to become a private tutor.

• Self-taught, she passed a physics degree with flying colors. Studied for her physics doctorate attempting to further study the effects and energy of uranium’s rays.

• Married Pierre Curie, with whom she shared a laboratory.

• She discovered polonium and radium in 1898.

• She along with her husband and Henri Becquerel was awarded the Nobel Prize for Physics for their work in radioactivity. (1903)

• In 1911, she was denied entrance into the Academy of Science, yet soon after she was honored with a Nobel Prize for Chemistry for determining the atomic weight of radium.

Dmitri Ivanovich Mendeleev(1834 – 1907)

• Born in Siberia, the youngest of 14 children (actual number varies from 11 to 17 depending on the source)

• Very poor, but his mother saved money for him to attend the university.

• First learned about chemistry by working in the glass factory where his mother worked.

• Gave us the modern periodic table (his was by increasing atomic masses, where ours today is arranged by increasing atomic numbers.

• Left spaces for elements which had not yet been discovered, but was able to predict what properties they would have by their location in his table.

Henry Moseley• (1887 – 1915)

• British chemist who studied under Rutherford.

• Said that atoms contain a unique (+) charge in their nucleus and named it proton.

• Said that protons give an element its identity (# of protons = Atomic #)

• With the discovery of isotopes he determine the atomic weight was not the major player in the periodic law, but it was the atomic number. This discovery made the few problems with Mendeleev’s periodic table disappear.

• The modern periodic table is now based on the atomic number of the elements.

Albert Einstein• (1879 – 1955) in Germany

• Studied violin & Judaism from age 6 – 13

• Failed an exam which would have allowed him to study as an electrical engineer

• Renounced German citizenship in 1896, became a Swiss citizen in 1901 remaining stateless for these number of years.

• In 1900 graduated as a teacher of mathematics and physics.

• In 1905 showed how mass and energy were equivalent. (E = mc2)

• Explained the photoelectric effect using Planck’s theory: light causes electrons to be ejected when it hits a piece of metal. There is a minimum frequency (energy) of light needed to eject an electron.

• Said that light is composed of photon –discrete packets of energy

• Received Nobel Prize in 1921 for his work on the photoelectric effect.

Albert Einstein

• Left Germany in Dec. 1932 as the Nazis came to power, never to return.

• In 1940 Einstein became a US citizen, retaining his Swiss citizenship.

• In 1944, he made a contribution to the war by handwriting his 1905 paper on special relativity and putting it up for auction. It raised $6,000,000. It resides today in the Library of Congress.

• In 1952, after the death of the first President of Israel, the Israeli government offered him this position, of which he refused.

Antoine-Laurent Lavoisier• Antoine Lavoisier

(1743 – 1794)

• Called the “Father of Modern Chemistry” because of his careful experiments and precise measurements.

• French chemistry responsible for the Law of Conservation of Matter: “Matter, like energy, can neither be created or destroyed, only changed from one form to another.”

• Major tool was the balance

• Recognized the role that oxygen plays in burning and rusting

• He was beheaded during the French Revolution.

Joseph Proust

• 1754-1826

• French chemist

• “steadiness” of the

composition of chemical

compounds

• Said that compounds

always contain the same

elements in the same

proportions (Law of

Definite Proportions)

Michael Faraday

• 1791-1867

• English chemist and

physicist

• Said that atoms are

related to electricity,

atoms contain

particles that have

electric charges

The Infamous Kite Experiment

Max Planck

• 1858-1947

• German physicist

• Stated the relationship between energy and frequency in his equation: E= hv.

• Frequency and energy are directly proportional

• Defined quantum: unique, fixed amount of energy absorbed and/or reflected by an object

• Founder of Quantum Theory

Colleagues

• Albert Einstein and

Niels Bohr.

Atoms in a Crystal

• Image of Silicon atoms arranged on a face of a crystal. This image was made by a Scanning Tunneling Microscope, a device that “feel” the cloud of electrons that form the outer surface of atoms, rather like a phonograph needle feels the grooves in a record.

• Einstein’s studies and papers gave one of the first convincing proofs that atoms do exist as real objects.

Einstein’s Letter to FDR

• Einstein wrote a letter

attempting to alert

President Roosevelt

of the severity of

proceeding with

nuclear reactions as a

war tool.

Arthur Compton

• 1892-1962

• Said that light consists of tiny particles called photons

• Said that photons could collide with electrons like two balls colliding with each other

Werner Heisenberg

• 1901-1976

• German physicist and one of the founders of quantum mechanics

• Worked with Neils Bohr

• Discovered Heisenberg Uncertainty Principle: position and momentum of a moving object cannot be known at the same time

• Nobel Prize in Physics in 1932

Louie de Broglie

• 1892-1987

• Said that moving

particles exhibit

wavelike behavior

Wolfgang Pauli

• (1900 – 1958)

• Studied under Niels Bohr.

• In 1924 proposed the quantum spin number for electrons and in 1925 gave the scientific world his best known work, the Pauli Exclusion Principle:

• Said that electrons can have only one of two spins.

• Said that only two electrons one orbital and that each electron in an orbital must have opposite spins.

• Received the Nobel Prize in 1945.

Group Portrait

What did I do?

• Antoine Lavoisier

What important contributions did I

make?

• Marie Curie

E = mc2

• Albert Einstein

• What do you know

about me?

What did I contribute to chemistry?

• Dmriti Mendeleev

Together we made something

Great!

What is this?

• Who used it to

discover the electron?