Composition of an Atom

1. Composition of an Atom Models of the Atom: A Historical Perspective

2. Atomic Theory Timeline 400 B.C. Democritus

3. Democritus • In 400 B.C. Democritus believed all existence consisted of two things: • atoms (coined the term atom from the Greek word ‘atomos’ meaning indivisible) • the void (space) • Democritus’ theory: • atoms are particles in space (the void) • atoms always in motion; make life predictable • matter could not be divided indefinitely; atoms are indivisible • all things we see differ because of the shape, size, and arrangement of atoms

4. Atomic Theory Timeline 400 B.C. 350 B.C Democritus Aristotle

5. Aristotle • In 350 B.C. Aristotle and his followers dismissed the idea of atoms • Instead modified an earlier theory that matter was made up of four ‘elements’: • earth • fire • water • air • Even though Aristotle was wrong his theory persisted for 2000 years!

6. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 0 Lavoisier Democritus Aristotle

7. Lavoisier • During his lifetime, 1743-1794, Antoine Lavoisier established the Law of Conservation of Matter • Law= in a chemical reaction, the mass of the reactants is equal to the mass of the products • Theory stated that mass is concrete and measurable, rather than ‘magical’

8. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 1800 0 Dalton Lavoisier Democritus Aristotle

9. Dalton • In 1800 John Dalton was examining gases dissolved in water and decided that matter was made of individual particles with spaces between them • Devised a chemical atomic theory: • 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

10. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 1800 0 1900 Dalton Lavoisier Thomson Democritus Aristotle

11. Thomson • In 1897, looking at charged particles, J.J Thomson found that the ratio of mass of the particles to the charge of the particles did not change regardless of the metal they came from • He called these charged particles electrons • Later he proposed a model for the composition of the atom called the ‘raisin bun’ or ‘plum pudding model’ although he was proved to be incorrect

12. Adding Electrons to the Model • Materials, when rubbed, can develop a charge difference. This electricity is called ‘cathode rays’ when passed through an evacuated tube • These rays have small mass and are negative • Thomson noted that these negative subatomic particles were a fundamental part of all atoms • 1) Dalton’s ‘Billiard Ball’ model (1800-1900): • atoms are solid and indivisible

13. 2) Thomson’s ‘Plum Pudding’ Model (1900): • negative electrons in a positive framework 3) The Rutherford Model (around 1910): • atoms are mostly empty space • negative electrons orbit a positive nucleus

14. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 1800 0 1900 Dalton Lavoisier Democritus Aristotle Thomson 1903 Rutherford

15. Rutherford • Used radiation to unlock clues to the atom; alpha particle (α) -positive charge and beta particle(β) -negative charge • Ernest Rutherford short alpha (α) particles at gold foil • Most particles passed through; therefore, atoms are mostly empty • Some positive alpha deflected or bounced back! • Thus, a ‘nucleus’ is positive and holds most of an atom’s mass

16. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 1800 0 1900 Dalton Lavoisier Democritus Aristotle Thomson 1903 1907 Millikan Rutherford

17. Millikan • In 1907 Robert Millikan found that all electrons are identical • Millikan was able to find the charge of a single electron

18. Atomic Theory Timeline 1743-1794 400 B.C. 350 B.C 1800 0 1900 Dalton Lavoisier Democritus Aristotle Thomson 1907 1903 1913 Bohr Rutherford Millikan

19. Bohr • Around 1913 Niels Bohr discovered that electrons orbit the nucleus in ‘shells’ • Electrons can be bumped up to a higher sell if hit by an electron or a photon of light • It is when electrons fall back down energy level(s) that they release a photon

20. There are two types of spectra : • continuous spectra • line spectra • These jumps down from shell to shell account for the line spectra seen in gas discharge tubes (through spectroscopes)