Atom History Timeline and Structure.ppt
- 1. Investigating Atoms and Atomic Theory Students should be able to: Describe the particle theory of matter. PS.2a Use the Bohr model to differentiate among the three basic particles in the atom (proton, neutron, and electron) and their charges, relative masses, and locations. PS.3
- 2. Atomos: Not to Be Cut The History of Atomic Theory
- 3. Atomic Models This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels.
- 4. Original Model of the Atom The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard (shooting pool) ball →
- 5. Who are these men? In this lesson, we’ll learn about the men whose quests to know what the smallest unit of matter is helped build the model of the atom as we know it today.
- 6. Democritus This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago in 400 BC. He asked: Could matter be divided into smaller and smaller pieces forever?
- 7. Atomos His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”
- 8. Atomos To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.
- 9. This theory was ignored and forgotten for more than 2000 years!
- 10. Why? The more popular philosophers, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.
- 11. Dalton’s Model In 1803, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.
- 12. Dalton’s Theory He gathered that all elements are composed of atoms. Atoms are indivisible (can’t be divided) particles. Every element has a different atom. Compounds are formed by the joining of atoms of two or more different elements.
- 13. Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.
- 14. Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.
- 15. 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.
- 16. Thomson Model This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? Where did they come from?
- 17. Thomson concluded that the negative charges came from inside the atom. A particle smaller than an atom had to exist. Today those particles are known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.
- 18. Rutherford’s Gold Foil Experiment In 1908, the English physicist Ernest Rutherford experimented by firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)
- 19. Rutherford’s Gold Foil Experiment Most of the positively charged “bullets” passed straight through the gold atoms . Some of the positively charged “bullets,” bounced away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.
- 21. This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole.
- 22. Rutherford Rutherford reasoned that all of an atom’s positively charged particles were in the center which he called the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.
- 23. Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each negatively charged particle called an electron in a specific energy level.
- 24. Bohr Model According to Bohr’s atomic model, electrons move in orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. 1st Energy Level 2nd Energy Level
- 25. Electron Cloud: Electrons whirl (spin) around the nucleus billions of times in one second Electrons whirl so fast you cannot see the individual levels so it actually looks more like a cloud.
- 26. Electron Cloud: Electrons with the lowest energy are found in the energy level closest to the nucleus
- 27. 3rd Subatomic Particle: Neutron In 1932, the English scientist James Chadwick used electrical currents to discover a neutral atomic particle with a mass close to a proton. What he discovered was the neutron.
- 28. Indivisible Electron Nucleus Orbit Neutron Greek X Dalton X Thomson X Rutherford X X Bohr X X X Chadwick X X X