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MODELS OF THE ATOM: A HISTORICAL PERSPECTIVE EARLY GREEK THEORIES Democritus Democritus 400 B.C. - thought matter could not be divided indefinitely This led to the idea of atoms in a void Aristotle Believed that matter was made of a combination of four “elements”: earth, fire, water, air Aristotle was wrong. However, his theory persisted for 2000 years. fire earth Aristotle air water JOHN DALTON 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason • • • • All matter is made of atoms Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. • Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios). WILLIAM CROOKES Accidental discovery of the cathode ray Discovery led to the development of the television HISTORY OF THE ATOM Joseph John Thompson found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON PLUM PUDDING MODEL Robert Millikan - Developed the charge of an atom… e- ERNEST RUTHERFORD Rutherford shot alpha () particles at gold foil Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible particles • Most particles passed through • So, atoms are mostly empty space • Some positive -particles deflected or bounced back! • Thus, a “nucleus” is positive & holds most of an atom’s mass BOHR’S MODEL • Electrons orbit the nucleus in “shells” • Electrons can be bumped up to a higher shell if hit by an electron or a photon of light •There are 2 types of spectra: continuous spectra & line spectra • It’s when electrons fall back down that they release a photon. • These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes). HELIUM ATOM Shell proton + electron N N + - neutron ATOMIC STRUCTURE Particle Charge Mass proton + charge 1 neutron No charge 1 electron - charge nil ATOMIC STRUCTURE Atomic mass the number of protons and neutrons in an atom 4 Atomic number the number of protons in an atom 2 He number of electrons = number of protons ATOMIC NUMBERS, MASS NUMBERS There are 3 types of subatomic particles. You know about electrons (e–) & protons (p+) Neutrons (n0) were also shown to exist (1930s). They have: no charge, a mass similar to protons Elements are often symbolized with their mass number and atomic number 16 E.g. Oxygen: 8 O • The Periodic Table gives you all this information • For now, round the mass # to a whole number # of protons = # of electrons = atomic number mass number – atomic number = # of neutrons Nuclear Symbol Practice Atomic Ca Ar Br Mass p+ n0 e–
