Models of the Atom: A Historical perspective

1. Models of the Atom:A Historical perspective

2. Democritus fire earth air water Aristotle • 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. Early Greek Theories

3. 1800 -Dalton proposed a modern atomic model based on experimentation not on pure reason • 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). John Dalton • 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.

4. Accidental discovery of the cathode ray Discovery led to the development of the television William Crookes

5. Joseph John Thompson • found that atoms could sometimes eject a far smaller negative particle which he called an History of the atom ELECTRON PLUM PUDDING MODEL Robert Millikan - Developed the charge of an atom… e-

6. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Ernest Rutherford • Rutherford shot alpha () particles at gold foil • Most particles passed through • So, atoms are mostly emptyspace • Some positive -particles deflected or bounced back! • Thus, a “nucleus” is positive & holds most of an atom’s mass

7. 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 downthat they release a photon. • These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).

8. HELIUM ATOM Shell proton N + - + N - neutron electron

9. ATOMIC STRUCTURE Particle Charge Mass proton + charge 1 neutron No charge 1 electron - charge nil

10. ATOMIC STRUCTURE He Atomic mass 4 the number of protons and neutrons in an atom Atomic number 2 the number of protons in an atom number of electrons = number of protons

11. 16 E.g. Oxygen: O 8 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 Nuclear Symbol • 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

12. Practice Atomic Mass p+ n0 e– Ca Ar Br