Development of the Atom Chapter

JOHN DALTON All matter is composed of atoms All atoms of a given element are identical Atoms of specific elements are different from any other atoms Atoms cannot be created, divided, or destroyed Different atoms combine in simple whole number rations to form compounds In chemical reactions atoms are separated, combined, or rearranged.

Dalton’s atomic theory proves the Law of Conservation of mass

Development of the modern atoms Thomson – He proved that atoms of any element are composed of different particles. – ALL atoms must contain these negative particles.(electrons) – atoms were not negative so there must be a positive charge as well – Cathode ray tube experiment Rutherford – Learned physics in J.J. Thomson’ lab. – Noticed that ‘alpha’ particles were sometime deflected by something in the air. – Gold-foil experiment

Thomson Used cathode ray tubes to determine there were negative particles called ELECTRONS in atoms

Cathode Rays Cathode ray = electron Electrons have a negative charge Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, pages High voltage cathode source of high voltage yellow-green fluorescence shadow (A) The effect of an obstruction on cathode rays (B) The effect of an electric field on cathode rays High voltage cathode source of high voltage positive plate negative plate anode source of low voltage + -

William Thomson (Lord Kelvin) From his experiments with the cathode ray tube… In 1910 proposed the Plum Pudding model – Negative electrons were embedded into a positively charged spherical cloud. Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 56 Spherical cloud of Positive charge Electrons

Milliken Expanded on Thomson and determined the charge of an electron. – Used his oil drop experiment to determine that an electron has a mass of 1/1840 of a hydrogen atom – Verified that the electron is negative

Ernest Rutherford ( ) Learned physics in J.J. Thomson’ lab. Noticed that ‘alpha’ particles were sometime deflected by something in the air. Gold-foil experiment

Rutherford ‘Scattering’ In 1909 Rutherford undertook a series of experiments He fired  (alpha) particles at a very thin sample of gold foil According to the Thomson model the  particles would only be slightly deflected Rutherford discovered that they were deflected through large angles and could even be reflected straight back to the source particle source Lead collimator Gold foil  

What he expected… California WEB

Because, he thought the mass was evenly distributed in the atom

What he got…

Density and the Atom Since most of the particles went through, the atom was mostly empty. Because the alpha rays were deflected so much, the positive pieces it was striking were heavy. Small volume and big mass = big density This small dense positive area is the nucleus California WEB

Explanation of Alpha-Scattering Results Plum-pudding atom Alpha particles Nuclear atom Nucleus Thomson’s modelRutherford’s model

Rutherford’s Gold-Leaf Experiment Conclusions: Atom is mostly empty space Nucleus has (+) charge Electrons float around nucleus Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 120

Discovery of the 3 sub atomic particles Rutherford – Proton Chadwick- Neutron Thomson/Milliken- Electron ParticleSymbolLocationChargeRelative Mass Protonp+p+ Nucleus+11 amu Neutronnono nucleus+01 amu Electrone-e- Electron Cloud 1/1840 amu

Bohr Model of Atom The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation. Increasing energy of orbits n = 1 n = 2 n = 3 A photon is emitted with energy E = hf e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e-

Models of the Atom 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, 3 rd Edition, 1990, page 125 Greek model (400 B.C.) John Dalton pictures atoms as tiny, indestructible particles, with no internal structure 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 Hantaro Nagaoka, a Japanese physicist, suggests that an atom has a central nucleus. Electrons move in orbits like the rings around Saturn New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus 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 Erwin Schrodinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

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). Niels Bohr & Albert Einstein