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

Neutrons A particle in the 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?