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Composition of an Atom

Composition of an Atom. Models of the Atom: A Historical Perspective. Atomic Theory Timeline. 400 B.C. Democritus. 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)

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Composition of an Atom

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  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 particles that had a charge that did not change regardless of the metal they came from… • He called these charged particles electrons (negatively charged particles) • 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)

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