Atomic Theory

Let’s Take a Trip Through Time!

Democritus 460 – 370 B.C. There are various basic elements from which all matter is made Everything is composed of small atoms moving in a void Some atoms are round, pointy, oily, have hooks, etc. to account for their properties Ideas rejected by leading philosophers because void = no existence

First Concept of an Atom

John Dalton Each element is composed of extremely small particles called atoms All the atoms of a given element are identical, but they differ from those of any other element Atoms are neither created nor destroyed in any chemical reaction A given compound always has the same relative numbers and kinds of atoms

6 Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. 2. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 3. Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction (Law of Definite Proportion). 4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

Dalton’s Model

8 Law of Multiple Proportions

9 8 X 2 Y 16 X8 Y + Law of Conservation of Mass

Modern Atomic Theory “Atoms cannot be subdivided, created, or destroyed.” –Atoms can be destroyed via nuclear reactions but not by chemical reactions. “Atoms of a given element are identical* in size, mass, and other properties;” –There are different kinds of atoms (differing by their masses) within an element that are known as "isotopes“.

J.J. Thomson(Discovery of Electron) Discovered electron 1897 – Cathode Ray Experiment Plum Pudding model 1904 –Electrons in a soup of positive charges Discovered isotopes 1913

J. J. Thomson(1897) –British physicist at Cambridge University Cathode Ray Experiment Hypotheses –Cathode rays are charged particles (which he called "corpuscles"). –These corpuscles are constituents of the atom. Observation: - the rays bended with a magnet -the electrometer measured a large amount of negative charge Discovery of Electron

13 J.J. Thomson, measured mass/charge of e - (1906 Nobel Prize in Physics) Cathode Ray Tube

The Electron J.J. Thomson CRT experiment Negative Charge e - Actual Mass: 9.11 x g Relative mass information - discovered later 1/1840 the mass of proton

Plum Pudding Model

16 e - charge = x C Thomson’s charge/mass of e - = x 10 8 C/g e - mass = 9.10 x g Measured mass of e - (1923 Nobel Prize in Physics) Millikan’s Experiment

Ernest Rutherford Nucleus Theory 1910 –alpha particle gold foil experiment An atom’s mass is mostly in the nucleus The nucleus has a positive charge (Moseley) Electrons in fixed orbit

Rutherford Gold Foil Experiment Bombard the gold foil with alpha particles, positively charge particles w/ 4x of the mass of H Only expect slight deflection Evidence: Wide-angle deflection Click for Interactive Demonstration

Experiment Conclusion atoms positive charge is concentrated in the nucleus Volume of a nucleus is very small compared with the total volume of an atom The nucleus is heavier than electron Suggested that the atom might resemble a tiny solar system, with a massive, positively charged center circled by only a few electrons proton (p) has opposite(+)charge of electron (-) mass of p is 1840 x mass of e - (1.67 x g)

Rutherford Model

James Chadwick Worked with Rutherford Interpreted work of the Curies Discovered Neutron 1932 Nobel Prize in Physics 1935

22 Chadwick’s Experiment (1932) (1935 Noble Prize in Physics) H atoms - 1 p; He atoms - 2 p mass He/mass H should = 2 measured mass He/mass H = 4  + 9 Be 1 n + 12 C + energy neutron (n) is neutral (charge = 0) n mass ~ p mass = 1.67 x g

The Neutron Chadwick nuclear bombardment No charge n 0 Actual Mass: 1.67 x g Relative Mass: 1

Niels Bohr Planetary Model 1913 –Nucleus surrounded by orbiting electrons at different energy levels –Electrons have definite orbits Utilized Planck’s Quantum Energy theory Worked on the Manhattan Project (US atomic bomb)

Bohr’s Model

Bohr Model for Nitrogen

Ernst Schrödinger Quantum Mechanical Model 1926 –Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed –Electrons are particles and waves at the same time –Developed quantum numbers based on theories of Einstein and Planck Werner Heisenberg

Orbitals

Quantum Mechanical Theory Electron in a Hydrogen atom

Atomic Theories Timeline Name DemocritusDaltonThomsonRutherfordBohrSchrödinger Heisenberg Time Frame Key Points Historical Events