Historical Development of an Atom

Democritus 470-380 B.C Democritus was a Greek philosopher (470-380 B.C.) who is the father of modern atomic thought. He proposed that matter could NOT be divided into smaller pieces forever (uncuttable) He believed that matter was made of small, hard, particles, that he called “atomos” Thought atoms are made of a single material formed into different shapes and sizes Only had a mental model or the atom – nothing recorded

John Dalton 1807 He built upon Democritus’s atomic theory. Matter is made up of atoms Atoms cannot be divided into smaller pieces. All the atoms of an element are exactly the same Different elements have different kinds of atoms Compounds form by combining atoms/elements (experiments)

John Dalton 1807 Model Drawing Solid throughout Same throughout

J.J. Thompson 1898 Particles smaller than the atom exists Discovered the electron, which is negatively charged If atoms have negative particles, then atoms must also have positive particles. Cathode ray experiment “plum pudding model” – said the atom was like plum pudding with pieces of plums dispersed throughout

J.J. Thompson 1898 Model Drawing Positive and negative particles

Ernest Rutherford 1911 Rutherford performed the gold foil experiment. In his experiment a radio active beam emitted (+) alpha particles toward gold foil. Most particles went through, some were deflected, and others bounced back! This showed that atoms have a very dense (+) center.

Ernest Rutherford 1911 Rutherford showed that atoms have (+) particles in the center and are mostly empty space. He called the center of the atoms the nucleus. All the mass of an atom and its positive charge are located in the nucleus.

Ernest Rutherford 1911 Model Drawing Positive nucleus Negative particles surround nucleus

Niels Bohr 1913 Niels Bohr improved on Rutherford’s model. He proposed that electrons move around the nucleus in specific layers or shells/energy levels Every atom has a specific number of electron shells. Said electrons can jump levels

Niels Bohr 1913 Electrons move in orbits around the nucleus 1st energy level = 2 electrons 2nd energy level = 8 electrons 3rd energy level = 18 electrons 4th energy level = 32 electrons

Niels Bohr 1913 Model Drawing Electrons orbit the positive nucleus on energy levels

James Chadwick 1932 Chadwick discovered neutrons He worked with Rutherford and discovered particles with no charge He called those particles neutrons Neutrons are found in the nucleus of an atom.

Chadwick 1932 p=protons n = neutrons P n n n Pnnnnppp

Electron Cloud Theory Erwin Schrodinger - 1925 Electrons do not follow fixed orbits, but tend to occur more frequently in certain areas around the nucleus it is impossible to know where an electron is at any given time *this is the theory we use today!

Electron Cloud Model Current Model Drawing Electrons are located in clouds