Discovering the Atom

Greek Model Democritus Greek philosopher Idea of ‘democracy’ “To understand the very large, we must understand the very small.” Democritus Greek philosopher (460 B.C.- 370 B.C.) Idea of ‘democracy’ Idea of ‘atomos’ Atomos = ‘indivisible’ ‘Atom’ is derived No experiments to support idea Democritus’ model of atom No protons, electrons, or neutrons Solid and INDESTRUCTABLE

Dalton’s Atomic Theory 1803 1. All matter is made of tiny indivisible particles called atoms. 2. Atoms of the same element are identical, those of different atoms are different. 3. Atoms of different elements combine in whole number ratios to form compounds 4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Radioactivity One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876 - 1934). She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces.

Thomson 1897 – discovered the electron using a cathode-ray tube

Cathode-ray Tube Animation http://www.youtube.com/watch?v=1iw0Plrk51Y

Thomson’s Plum Pudding Model of an Atom

Millikan http://www.youtube.com/watch?v=XMfYHag7Liw Determined the charge of an electron Oil Drop Experiment

Rutherford’s Gold Foil Experiment in 1909

Rutherford’s Gold Foil Exp http://www.youtube.com/watch?v=5pZj0u_XMbc

Results of foil experiment if plum pudding model had been correct

Actual results of foil experiment Rutherford’s conclusion: atoms are mostly empty space with a positive nucleus

Bohr Model of Atom Increasing energy of orbits n = 3 e- n = 2 n = 1 e- e- A photon is emitted with energy E = hf 1913 - Bohr proposed that electrons are in specific circular orbits with specific energy levels around the nucleus

Electrons as Waves Louis de Broglie (1924) Applied wave-particle theory to electrons electrons exhibit wave properties Fundamental mode Second Harmonic or First Overtone Standing Wave 200 150 100 50 - 50 -100 -150 -200 200 150 100 50 - 50 -100 -150 -200 200 150 100 50 - 50 -100 -150 -200 0 50 100 150 200 0 50 100 150 200 0 50 100 150 200

Quantum Mechanics Heisenberg Uncertainty Principle Impossible to know both the velocity and position of an electron at the same time g Microscope Electron

Quantum Mechanics Schrödinger Wave Equation (1926) finite # of solutions  quantized energy levels defines probability of finding an electron Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Mechanics Orbital (“electron cloud”) Region in space where there is 90% probability of finding an electron 90% probability of finding the electron Orbital Electron Probability vs. Distance 40 30 Electron Probability (%) 20 10 50 100 150 200 250 Distance from the Nucleus (pm) Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Quantum Mechanical Model Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

Confirmed existence of neutron Chadwick http://www.youtube.com/watch?v=ZK-yeuu_p9k Confirmed existence of neutron Start @ 3min

Modern View The atom is mostly empty space Two regions Nucleus protons and neutrons Electron cloud region where you might find an electron

Models of the Atom - Greek model (400 B.C.) Thomson’s plum-pudding "In science, a wrong theory can be valuable and better than no theory at all." - Sir William L. Bragg + - e + Greek model (400 B.C.) Dalton’s model (1803) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Models of the Atom - Dalton’s model (1803) Greek model (400 B.C.) + + - Dalton’s model (1803) Greek model (400 B.C.) Thomson’s plum-pudding model (1897) Rutherford’s model (1909) Bohr’s model (1913) Charge-cloud model (present) 1803 John Dalton pictures atoms as tiny, indestructible particles, with no internal structure. 1897 J.J. Thomson, a British scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a sphere of positive electric charge. 1911 New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus. 1926 Erwin Schrodinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model. 1913 In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus. 1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945 1924 Frenchman Louis de Broglie proposes that moving particles like electrons have some properties of waves. Within a few years evidence is collected to support his idea. 1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons. 1904 Hantaro Nagaoka, a Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around Saturn. Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

+ - e + A Dalton’s model B C D E