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Average: 27.2/40 = 68% Top grade: 38/40 = 95% I “curve”: roughly, the corresponding letter grades are: A+ >92, A 85—92, A- 80—84, B+ 75--79 , B 65—74, B- 60—64, C+ 55—63, C 50—54, D 45—49, F < 45 score 0 5 10 15 20 25 30 35 40 Series1 Make sure you go through your test and solutions carefully and understand where you went wrong. Ask me at office hours if needed.

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Chapter 11: The Atomic Nature of Matter The atomic hypothesis: (dates back to Democritus, 460-371 BC) All things are made of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. If, in some cataclysm, all scientific knowledge were to be destroyed, and only one sentence could be passed on to the next generation of creatures, what statement conveys the most information in the fewest words? The atomic hypothesis. Richard Feynman, 1918-1988 Note: the idea of matter consisting ultimately of indivisible units dates back to 5 th BC, but really only established with Einstein in 1905. Crucial observation: Brownian motion of botanist Robert Brown, 1827.
The elements Atoms make up all the matter around us, but there are only 118 distinct types of atoms (to date). These are called elements . The elements combine in an infinite # of different ways in order to yield huge variety of substances. Actually, only 88 of the 118 discovered, are found naturally. Others are unstable, and made in nuclear reactors. Atom consists of some number of protons and neutrons , bound together in a nucleus, surrounded by a cloud of electrons . Simple model: electrons orbit nucleus like a tiny version of planets around the sun. This is a very simplified model, but ok for many purposes. Most of the volume is empty.

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More about atoms Simplest element is hydrogen: one proton, no neutrons, one electron (see more later). First element to form after the Big Bang. All other (naturally-occurring) elements were formed by thermonuclear fusion in (large) stars, and are remnants of stars that previously exploded. Most common elements on earth: hydrogen (H), carbon (C), oxygen (O), nitrogen (N). “chemical symbols” Atoms are tiny! Ratio of diameters of atom : apple equals that of apple : earth Numerous : in 1g of water, there are 10 23 atoms !
More on atoms continued… Continually recycled: Eg. Many atoms in your body are nearly as old as universe itself. Eg. When you breathe in, some atoms inhaled become part of your body; later will be part of someone else’s body, or a plant, or a building… Eg. Each breath you breathe, contains atoms that were once part of everyone who ever lived! Constantly moving:

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