Models of the Atom a Historical Perspective

1. Models of the Atom a Historical Perspective

2. fire Democritus earth air water Aristotle Early Greek Theories • This led to the idea of atoms in a void. • 400 B.C. - Democritus thought matter could not be divided indefinitely. • 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. • Aristotle was wrong. However, his theory persisted for 2000 years.

3. 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. • 1800s -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 definite proportions (elements combine in fixed ratios).

4. Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube. These rays have a small mass and are negative. Thompson noted that these negative subatomic particles were a fundamental part of all atoms. Adding Electrons to the Model • Dalton’s “Billiard ball” model (1800s) Atoms are solid and indivisible. • Thompson “Plum pudding” model (1900) • Negative electrons in a positive framework. • The Rutherford model (around 1910) • Atoms are mostly empty space. • Negativeelectronsorbitapositivenucleus.

5. 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 empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

6. Electrons orbit the nucleus in “shells” Bohr’s model • Electrons can be bumped up to a higher shell (excited state) 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 (ground state) that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).

7. Wave Mechanical Model • Atom has a dense, positively charged nucleus • Instead of electrons moving in definite, fixed orbits around the nucleus (Bohr), this model portrays electrons having distinct amounts of energy moving in orbitals • Region in which an electron of a particular amount of energy is most likely to be located

9. Bright Line Spectra • When electrons are given more energy (heat, light, etc.) they jump to an unstable, excited state • Eventually they will fall back down to their ground state by emitting energy in the form of light • Each atom has its own distinct pattern of emission lines and they can be used to identify elements