Atomos: Not to Be Cut

1. Atomos: Not to Be Cut The History of Atomic Theory

2. Who are these men and what are they doing in our classroom?

3. Democritus Aristotle

4. Democritus 400 BC • began the search more than 2400 years ago. • proposed that matter could not be divided into smaller and smaller pieces forever • named the smallest piece of matter “atomos,” meaning “not to be cut.”

5. This theory was ignored and forgotten for more than 2000 years!

6. Why? Aristotle and Plato, had a more respected, but wrong, theory. Aristotle and Plato favored the earth,fire, air and waterapproach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

7. Ancient Philosophy • Who:Aristotle, Democritus • When: More than 2000 years ago • Where: Greece • What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. • Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

8. Alchemists • Who:European Scientists • When: 800 – 900 years ago • Where: Europe • What: Their work developed into what is now modern chemistry. • Why: Trying to change ordinary materials into gold.

9. Alchemic Symbols

10. John Dalton

11. Dalton’s Theory • All elements are composed of atoms. • Atoms are indivisible and indestructible particles. • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compounds are formed by the joining of atoms of two or more elements.

12. Particle Theory • Who:John Dalton • When: 1808 • Where: England • What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. • Why: Building on the ideas of Democritus in ancient Greece.

13. . • This theory became one of the foundations of modern chemistry.

14. J. J. Thompson

15. Thomson Model • “PlumPudding” model. • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding.

16. Thomson Model • Thomson studied the passage of an electric current through a gas. • As the current passed through the gas, it gave off rays of negatively charged particles.

17. Section1 Development of the Atomic Theory Chapter 11

18. Thomson Model Where did they come from? • This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

19. Discovery of Electrons • Who:J. J. Thompson • When: 1897 • Where: England • What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. • Why:Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

20. Ernest Rutherford

21. He fired a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

22. Section1 Development of the Atomic Theory Chapter 11

23. What did this mean? • all of an atom’s positively charged particles were contained in the nucleus • The negatively charged particles were scattered outside the nucleus around the atom’s edge.

24. Atomic Structure I • Who: Ernest Rutherford • When: 1911 • Where: England • What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. • Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

25. Niels Bohr

26. Bohr Model • placed each electron in a specific energy level. • electrons move in definite orbits around the nucleus • These paths are located at certaindistances from the nucleus.

27. Atomic Structure II • Who: Niels Bohr • When: 1913 • Where: England • What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus. • Why: Bohr was trying to show why the negative electrons were not drawn into the nucleus of the atom.

28. ATOMIC STRUCTURE S.MORRIS 2006

29. The Wave Model • based on the principles of wavemechanics. • electrons do not move about an atom in a definite path, like the planets around the sun.

30. The Wave Model • it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. • An atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

31. Electron Cloud: • A space in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • They are not moving around in random patterns. • Location of electrons depends upon how much energy the electron has.

32. Electron Cloud: • Depending on their energy they are locked into a certain area in the cloud. • Electrons with the lowest energy are found in the energy level closest to the nucleus • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

33. Electron Cloud Model • Electrons travel around the nucleus in random orbits. • Scientists cannot predict where they will be at any given moment. • Electrons travel so fast, they appear to form a “cloud” around the nucleus.