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Atomic Theory and Structure what everything is made of What is an atom? • The smallest particle into which an element can be divided and still be the same substance – The smallest unit of of a substance • And when I say small, I mean small – A penny has 20,000,000,000,000,000,000,000 atoms (20 thousand billion billion) If it’s so small, how was it discovered? • Not long after Greece finally defeated Persia, a Greek philosopher named Democritus did some thinking – If you cut something in half, and then cut that in half, and keep going, you’ll end up with something you can’t cut Over 2000 years later • John Dalton, a teacher, wondered why hydrogen and oxygen always combine in the same way to make water (a compound) – It must have something to do with how single atoms interact So, what’s in an atom? • Atoms have a nucleus – Made of protons and neutrons – The nucleus is very small, dense, and positively-charged – Contains most of the atom’s mass • Outside the nucleus, electrons zip around in electron clouds Protons • Particles in the atomic nucleus with a positive (+) charge • We use the number of protons in an atom’s nucleus to determine what element it is – Atomic number • A particle in the Neutrons nucleus with NO charge • Isotopes are atoms that have the same number of protons, but different numbers of neutrons • Mass Number is the sum of protons and neutrons in an atom • Atomic Mass is the average mass of an element’s isotopes Electrons • A particle outside the nucleus with a negative (-) charge – Found within electron clouds • Very small – 1,800 electrons are about the size of one proton • Their (-) charge cancels out the proton’s (+) charge • An atom with unequal protons and electrons is called an ion – Either (+) or (-) So what?