DEVELOPMENT OF CHEMISTRY IN THE MODERN WORLD

This is a presentation about the development of chemistry in the modern age.

1. Welcome to CARLO JAMES Q. SABLAN PHYSICAL SCIENCE! Science Teacher

2. Modern Theories on Development of Atom

3. “What can be asserted without evidence can be dismissed without evidence.” Christopher Hitchens

4. Objectives: Discuss key developments in the concept of the atomic structure in modern chemistry; Present a timeline of events leading to the discovery of modern ideas on atom; and, Appreciate the importance of experiments and evidence-based facts.

5. Activity:

6. “ATTRIBUTION OF ASPECTS TO CORRECT MODELS” Directions: Check which atomic model applies to each attribute. There may be more than one applicable model per item.

7. Modern Chemistry Beginnings of modern chemistry were seen in the sixteenth and seventeenth centuries , where great advances were made in metallurgy, the extraction of metals from ores. In the seventeenth century, Boyle defined an element as a substance that cannot be broken down into two or more simpler substances by chemical means.

8. Modern Chemistry Robert Boyle stated... A substance was an element unless it could be broken down to two or more simpler substances. Air, therefore, could not be an element because it could be broken down into many pure substances. Robert Boyle

9. Modern Chemistry During the eighteenth century, Priestley discovered oxygen gas and the process of combustion where carbon-containing materials burn vigorously in an oxygen atmosphere. In the late eighteenth century, Lavoisier wrote the first modern chemistry text. His most important contribution was the law of conservation of mass , which states that in any chemical reaction, the mass of the substances that react equals the mass of the products that are formed. He is known as the father of modern chemistry.

10. Modern Chemistry Joseph Priestley Antoine Lavoisier

11. John Dalton (1766 – 1844) A meteorologist Unlike Democritus, he had experimental evidence to support his theory. Dalton had four major points (postulates) to his theory.

12. John Dalton’s Atomic Theory 1804 1. All matter is made of indivisible atoms. 2. Atoms of one element are all the same. The atoms of any one element are different from those of another. 3. Chemical reactions occur when atoms separate, join, or rearrange. In a chemical reaction, atoms of one element NEVER change into another. 4. Atoms of different elements mix or combine in whole number ratios to form compounds. Example: Oxygen combines with hydrogen to form water in a 2:1 ratio.

13. Dalton’s Early Atomic Model “ Billiard Ball” Model He envisioned atoms as solid, hard spheres, like  billiard (pool)  balls ; so, he used wooden  balls  to  model  them.

14. Discovered the electron He was the first scientist to show the atom was made of even smaller things. Joseph John Thomson (1859 – 1940)

15. Used the Cathode ray tube to discover electrons Cathode Ray Tube Experiment

16. Thomson’s Experiment Voltage source + - Vacuum tube Metal Disks

17. In the tube was an inert gas, and two plates, a positive and a negative. Voltage source + - Thomson’s Experiment

18. Passing an electric current makes a beam appear to move from the negative to the positive end Voltage source + - Thomson’s Experiment

19. Voltage source Adding an electric field cause the beam to move toward the positive plate. Thomson concluded the beam was made of negative moving pieces (opposites attract). + - - Thomson’s Experiment

20. Thomson’s “Plum Pudding” Atom Model From his experimental evidence, he believed that the atom was a solid positive sphere with electrons shoved into the sides of it.

21. Eugene Goldstein (1850-1930) Using a cathode ray tube he discovered canal rays which are beams of positively charged particles. He is credited with the discovery of protons in an atom.

22. Robert Millikan’s Oil Drop Experiment Determined the charge on an electron Used Thomson’s charge-to-mass ratio to calculate the mass of an electron

23. Millikan’s oil-drop experiment to measure the charge of the electron. Small drops of oil are allowed to fall between electrically charged plates. The drops pick up extra electrons as a result of irradiation by X-rays and so became negatively charged. Millikan measured how varying the voltage between the plates affected the rate of fall. From these data he calculated the negative charge on the drops. Because the charge on any drop was always some integral multiple of 1.602 * 10-19 C, Millikan deduced this value to be the charge of a single electron.

24. Ernest Rutherford (1871 – 1937) Discovered the nucleus of a gold atom with his “gold foil” experiment

25. Gold Foil Experiment Results Most alpha particles go straight through the gold foil. A few alpha particles are sharply deflected.

26. Rutherford’s Conclusions Observations Conclusions Most of the alpha particles went. Few particles were deflected at small angles. Very rarely particles were deflected at large angles. The atom is mostly empty space The alpha particles hit a small, very dense, and positively charged center (nucleus) The alpha particle came close to something small and positive (nucleus)

27. Rutherford’s Atomic Model

28. Neils Bohr (1885 – 1962) Bohr proposed that the electrons existed only at fixed distances from the nucleus at set “ energy levels ,” or quanta.

29. The system proposed by Rutherford was unstable because, under classical physics, the spinning electrons would tend to be attracted to the positive nucleus and lose energy until they collapse into the center.

30. In the Bohr model of the atom, electrons travel in defined circular orbits around the nucleus. Electrons can jump from one orbit to another by emitting or absorbing energy.