1 Development of Atomic Theory & Atomic Structure

2 Atomic Theory u Many models/theories u hypothesis and experiments u Models and theories are based off each other and further developed

3 Early Theories u Democritus ( B.C.) u Greek philosopher (scientist)  There are various basic elements from which all matter is made  Proposed – eventually end up with “uncuttable” particle  Named that particle ‘atom’ (atomos = “indivisible”)  Atom = small, hard particles – made of same material, different shapes  Some atoms are round, pointy, oily, have hooks, etc. to account for their properties

First concept of an atom 4

5 Early Theories u Aristotle ( B.C.) u Greek philosopher  Disagreed w/ Democritus  NEVER end up with indivisible particle – they can be divided forever  More popular at the time than Democritus so ……….  Democritus' ideas rejected by leading philosophers/scientists for thousands of years

6 John Dalton 1766 – 1844 British chemist & teacher u Late 1700’s – scientists learned elements combined specific ratios (based on mass) to form compounds u Dalton used actual experiments u Introduced his ideas in 1803 u All substances made of atoms u All the atoms of a given element are identical, but they differ from those of any other element u Atoms are neither created nor destroyed u A given compound always has the same relative numbers and kinds of atoms

7 J.J. Thomson 1856 – 1940 British Scientist u Discovered electron 1897 – Cathode Ray Tube (CRT) Experiment u Discovered small particle inside the atom u Called them corpuscles u Now called electrons u Found in all atoms u All were identical u All had negative charge

8 J.J. Thomson 1856 – 1940 British Scientist u Knew atoms had neutral charge u Must also be a + charge u Didn’t know location of + or - particles u Plum Pudding model 1904 u Electrons in a soup of positive charges u He also discovered isotopes in 1913

9 J.J. Thomson’s Plum Pudding Model

10 Ernest Rutherford 1871 – 1937 he was one of J.J. Thomson’s students u In 1909 and 1910 he did his gold foil experiment u Discovered the center of atom u small, dense, with a + charge u Called that the nucleus u An atom’s mass is mostly in the nucleus

11 Ernest Rutherford’s Gold Foil Experiment

12 Ernest Rutherford u Calculated that the diameter of nucleus is 100,000 times smaller than the diameter of the gold atom u Electrons in fixed orbit u Nucleus Theory

13 Rutherford’s Model of the Atom

Niels Bohr 1885 – 1962 Danish Scientist u Proposed Planetary Model in 1913 u Nucleus surrounded by orbiting electrons at different energy levels (different distances from nucleus) u Electrons have definite orbits u No paths in between u Electrons can “jump” from level to level u Showed why different atoms interact and why they are reactive or not 14

The Bohr Model of the Atom u Bohr later worked on the Manhattan Project 15

Bohr Model of Nitrogen Atom 16

17 Manhattan Project u Developed the atomic bomb u Trinity test (July 16, 1945) u 1 st human-engineered atomic explosion u Little Boy u 1 st nuclear bomb used – Hiroshima u Fat Boy u 2 nd (last) nuclear bomb used - Nagasaki

18 Modern Atomic Theory u Erwin Schrödinger (1887 – 1961) u Werner Heisenberg (1901 – 1976)

19 Modern Atomic Theory u Schrödinger and Heisenberg u Explained nature of electrons u Electron Cloud Model u Electrons don’t travel in specific paths but in a general area u Can only predict probability of an electrons location

Atomic Theory Timeline

21 Next Atomic Theory? u Which one of you will develop a better atomic theory and win the Nobel Prize?

Atomic Structure composed of 3 subatomic particles 22 Electron Proton Neutron

23 The Atom u Made up of three subatomic particles u Proton u Neutron u Electron 23 Proton Neutron Electron

24 The Nucleus uCuCenter of the atom u9u99.9% of mass u2u2 particles uPuProton uPuPositive charge u1u1.7 x g u1u1 amu uNuNeutron uNuNeutron uNuNeutral charge u1u1 amu uSuSlightly more mass than proton

25 The Electron u Electron u Negative charge u ~0 amu (1/1836 amu) (takes 1836 electrons to = 1 amu) u Orbit around nucleus billions times/sec u Location of electron depends on amount of energy the electron has u Properties of elements depend on the # of electrons in various energy levels u # of electrons in outer energy level determines an elements ability to bond (combine) with other elements

26 Subatomic Particles u All protons are identical to all other protons u All neutrons are identical to all other neutrons u All electrons are identical to all other neutrons

27 Electron Cloud u Region around the nucleus where electrons are likely to be found u Located outside the nucleus u Like bees around a beehive u The further away from the nucleus an electron is, the more energy it has

28 Electron Cloud u 1 st ring - 2 electrons u 2 nd ring - 8 electrons u 3 rd ring - 8 electrons

Atomic Mass & Atomic Number u Atomic number = number of protons u Atomic Mass = average mass of isotopes of an element = the total number of protons & neutrons in the nucleus 29 Atomic Mass Atomic Number

Isotopes u an atom with the same number of protons but different number of neutrons u Same atomic number, different atomic mass 30

Isotopes u Atomic number = total # of protons u Mass number = sum of protons and neutrons u Isotopes have different mass numbers but the same atomic numbers u Isotopes share most of same physical and chemical properties 31

Radioactive Isotopes u Unique property of some isotopes u They are unstable u Called radioactive decay u Nucleus changes composition u Become different elements! u Not random – follow specific path of different elements 32

33 Drawing atoms – Carbon (we will always use the Bohr model to draw atoms) Steps: 1. Find number of protons  Number of Protons = ? 6 Atomic number

34 Drawing an atom Steps: 2. Find number of electrons  same as number of protons 3. Find number of neutrons  atomic mass – atomic number =6

Final Drawing of a Carbon atom 35 P=6 N=6