History of the Atom - Timeline Antoine Lavoisier makes a substantial number of contributions to the field of Chemistry J.J. Thomson discovers the electron and proposes the Plum Pudding Model in 1897 Niels Bohr proposes the Bohr Model in 1913 James Chadwick discovered the neutron in in 1932 1766 – 1844 1871 – 1937 1887 – 1961 460 – 370 BC 1700s 1800s 1900s Erwin Schrodinger describes the electron cloud in 1926 Democritus proposes the 1st atomic theory John Dalton proposes his atomic theory in 1803 Ernest Rutherford performs the Gold Foil Experiment in 1909 1891 – 1974 1743 – 1794 1885 – 1962 1856 – 1940 Click on picture for more information

Democritus (460 BC – 370 BC) Proposed an Atomic Theory (along with his mentor Leucippus) which states that all atoms are small, hard, indivisible and indestructible particles made of a single material formed into different shapes and sizes. Aristotle did not support his atomic theory Image taken from: https://reich-chemistry.wikispaces.com/T.+Glenn+Time+Line+Project

Scientists and Their Contribution to the Model of an Atom History of the Atom Scientists and Their Contribution to the Model of an Atom

Image taken from: www.ldeo.columbia.edu/.../v1001/geotime2.html Antoine Lavoisier (1743 – 1794) Known as the “Father of Modern Chemistry” Was the first person to generate a list of thirty-three elements in his textbook Devised the metric system Was married to a 13-year old Marie-Anne Pierette Paulze; she assisted him with much of his work Was a tax-collector that was consequently guillotined during the French Revolution Discovered/proposed that combustion occurs when oxygen combines with other elements Discovered/proposed the Law of Conservation of Mass (or Matter) which states, in a chemical reaction, matter is neither created nor destroyed Image taken from: www.ldeo.columbia.edu/.../v1001/geotime2.html

Image taken from: chemistry.about.com/.../John-Dalton.htm In 1803, proposed an Atomic Theory which states: All substances are made of atoms; atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different Atoms join with other atoms to make new substances Calculated the atomic weights of many various elements Was a teacher at a very young age Was color blind Image taken from: chemistry.about.com/.../John-Dalton.htm

Image taken from: www.wired.com/.../news/2008/04/dayintech_0430 J.J. Thomson (1856 – 1940) Proved that an atom can be divided into smaller parts While experimenting with cathode-ray tubes, discovered corpuscles, which were later called electrons Stated that the atom is neutral In 1897, proposed the Plum Pudding Model which states that atoms mostly consist of positively charged material with negatively charged particles (electrons) located throughout the positive material Won a Nobel Prize Image taken from: www.wired.com/.../news/2008/04/dayintech_0430

Cathode Ray Tubes A cathode ray is a ray of light traveling in a vacuum (no other particles inside) The ray travels from one metal plate to another as the plates are connected to electricity Cathode ray Metal plate (cathode) releases stream Metal plate (anode) to which stream travels

Cathode Ray Tubes & Charge In the late 1800’s, JJ Thomson put charged plates outside the tube Negatively charged plate - + Ray is deflected away from negative plate and towards positive plate Positively charged plate It made no difference what type of metal he used in the tube—all material produced this stream that curved towards the positive charge

Ernest Rutherford (1871 – 1937) In 1909, performed the Gold Foil Experiment and suggested the following characteristics of the atom: It consists of a small core, or nucleus, that contains most of the mass of the atom This nucleus is made up of particles called protons, which have a positive charge The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space Did extensive work on radioactivity (alpha & beta particles, gamma rays/waves) and was referred to as the “Father of Nuclear Physics” Won a Nobel Prize Was a student of J.J. Thomson Was on the New Zealand $100 bill Image taken from: http://www.scientific-web.com/en/Physics/Biographies/ErnestRutherford.html

What happened in the experiment?

What did he see? Most of the alpha particles passed straight through with no deflection These particles did not run into anything Some did deflect slightly These particles ran into something much smaller than themselves A few were reflected back the direction they came from These particles ran into something very dense

Image taken from: commons.wikimedia.org/wiki/File:Niels_Bohr.jpg In 1913, proposed the Bohr Model, which suggests that electrons travel around the nucleus of an atom in orbits or definite paths. Additionally, the electrons can jump from a path in one level to a path in another level (depending on their energy) Won a Nobel Prize Worked with Ernest Rutherford Image taken from: commons.wikimedia.org/wiki/File:Niels_Bohr.jpg

The Bohr Model It consists of the nucleus with protons & neutrons and electrons in concentric orbits (circles) outside the nucleus The circle closest to the nucleus contains the lowest energy electrons The first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18.

Pictures of the Bohr Models Electron Proton Neutron Hydrogen-1 Helium-4 Lithium-6

Image taken from: nobelprize.org/.../1933/schrodinger-bio.html Erwin Schrodinger (1887-1961) In 1926, he further explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found Did extensive work on the Wave formula  Schrodinger equation Won a Nobel Prize Image taken from: nobelprize.org/.../1933/schrodinger-bio.html

Image taken from: www.wired.com/.../news/2009/02/dayintech_0227 James Chadwick (1891 – 1974) Realized that the atomic mass of most elements was double the number of protons  discovery of the neutron in 1932 Worked on the Manhattan Project Worked with Ernest Rutherford Won a Nobel Prize Image taken from: www.wired.com/.../news/2009/02/dayintech_0227

Democritus & John Dalton Progression of the Atomic Model + - The structure of an atom, according to: Democritus & John Dalton James Chadwick J.J. Thomson Neils Bohr Ernest Rutherford Erwin Schrodinger