History of Atomic Theory

5th Century B.C. - Present

Atom

Greek for “uncuttable”

Leucippus – Greek Philosopher and founder of Atomism

5th Century B.C.

If you divide matter into smaller and smaller parts eventually you arrive at small particles which can not be further subdivided. Leucippus called these indivisible particles atoms. His atomic theory was further developed by his disciple, Democritus.

Democritus

Democritus had no instruments to extend the reach of his senses, so all of his experiments were just "mind experiments." The ancient Greeks gave humanity tremendous gifts despite only having their minds to work with. We humans tend to be skeptical and want proof before we believe in most things, so thousands of years passed before his theories could be proved.

Democritus – 450 B.C.

• All atoms in a substance are identical

• Atoms of different substances are different

sizes and shapes

• Atoms are in constant motion

• Atoms are indivisible and eternal

John Dalton (Early 1800’s)

• Atoms are indivisible and indestructible• All atoms of a single substance are identical• Atoms of different substances differ primarily in

their weights• In chemical compounds atoms combine in small

specific ratios

Known as the Father of Atomic Theory

J.J. Thomson (1897)

• Used Cathode Ray tube in his experiments

• Atoms consist of a sphere of positive charge

• The electrons are embedded in a “+” charged sphere (like seeds in a watermelon)

                                                                                              

Thomson’s “Plum Pudding” Atomic Model

Electrons are imbedded into a sphere of positive charge

Ernest Rutherford (1908)

• All atoms have a positively charged central particle called the nucleus

• Electrons (“-” negatively charged particles) are scattered around the atoms edge

Gold Foil Experiment

Rutherford’s Model of the Atom

Niels Bohr (1913)Classical Mechanical Model

• Electrons orbit the nucleus in definite paths – like planets around the sun

• Shell – the region in an atom where the electrons travel

• Electrons in the same shell are approx. the same distance from the nucleus

•Energy Levels- electrons in the same shell have similar energy

•Electrons closest to the nucleus have the least energy

•8 electrons are needed in the last shell to stabilize the atom (Happy Atoms)

Bohr’s Model (continued)

Bohr’s Model of the Atom

Max Planck Quantum Mechanical Model

• Energy Levels – his model predicts the probability of where an electron may exist

• Energy levels have sublevels

• Sublevels are the spatial distribution of electrons in an atom (describe the cloud shape)

Sublevels in Electron Energy Levels

Name of sublevel max. # of e- orientation

s 2 spherical p 6 perpendicular d 10 diagonal f 14 fundamental

Orbitals- the spatial description of the path of an electron

Atomic Orbitals The regions in space in which electrons are likely to be found

The Neutron

• In 1920 Ernest Rutherford proposed the possible existence of a neutral particle, with the approximate mass of a proton, that could result from the capture of an electron by a proton

The Neutron (cont.)

• In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, decided to use a strong polonium alpha source, which they had developed, to further investigate penetrating radiation.

The Neutron (cont.)

• James Chadwick repeated the Joliot- Curie experiments. He bombarded the hydrogen atoms in paraffin with beryllium along with other targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932. For which he received the Nobel Prize.

James Chadwick’s Apparatus

Quarks (1960-70)

• Believed to be one of the basic building blocks of matter. Quarks were first discovered in experiments done in the late 1960's and early 1970's.

• It is believed that nucleons (protons and neutrons) are made up of quarks.

• Protons – contain two up and one down quark• Neutrons – contain two down and one up quark

Quarks

• Three families of quarks are known to exist. Each family contains two quarks. The first family consists of Up and Down quarks, the quarks that join together to form protons and neutrons.

• The second family consists of Strange and Charm quarks and only exist at high energies.

• The third family consists of Top and Bottom quarks and only exist at very high energies.