2 Gold Foil ExperimentIn the early 1900’s, Hans Geiger sent radioactive particles (alpha particles “”) through thin gold foilHe expected these relatively heavy particles to go through the atoms with a small deflection (bouncing back)
4 What did he see?Most of the alpha particles passed straight through with no deflection (they did not bounce back)These particles did not run into anythingSome did deflect slightlyThese particles ran into something much smaller than themselvesA few were reflected back the direction they came fromThese particles ran into something very dense
5 What did that mean? Atoms are mostly empty space Electrons (the smaller particles) were the cause of the small deflectionsThere must be a small area of the atom with most of its mass (the protons) that caused the reflections.He called this small, dense area the nucleus
6 A third particleThe protons and electrons could explain the charges of the various parts of the atomThey could not explain the total mass of the atomsNeutrons were proposed in 1920’s but not confirmed until 1932 by James ChadwickNeutrons had mass similar to protons and no charge. They were located in the nucleus
7 More changes to the theory Niels Bohr performed experiments with hydrogen atoms & lightHe determined that electrons are in levels according to how much energy they have and that only certain energy amounts were allowed.
8 The Bohr ModelIt consists of the nucleus with protons & neutrons and electrons in circular orbits outside the nucleusThe circle closest to the nucleus contains the lowest energy electronsThe first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18 electrons.
9 Pictures of the Bohr Models ElectronProtonNeutronHydrogen-1Helium-4Lithium-6
10 Use of the Bohr Model now We now believe that electrons do not really travel around the nucleus in perfect circular patternsBut imagining a circular pattern helps explain where electrons are located
12 What are atoms?Atom - smallest piece of matter that has the chemical properties of the element.
13 What’s in an atom? An atom is made of three sub-atomic particles LocationMassChargeProtonNucleus1 amu+1NeutronNucleus1 amuElectronOutside the nucleusamu-11 amu (“atomic mass unit”) = 1.66 kg
14 What gives an atom its identity? Every atom has a different number of protons.The number of protons determines the identity of the atomThe atomic number shows the number of protons.Atomic number = protons
15 The Nucleus & MassThe mass of the nucleus (in amu’s) is the number of protons + neutronsSince electrons have relatively no mass, we ignore them for massMass # = protons + neutrons
16 Charges Protons have a positive charge Electrons have a negative chargeNeutrons have no chargeOverall charge of an atom =protons - electronsCharge = protons - electrons
18 X A C Z # Element symbols Element Symbol Charge Mass number 1 or 2 letters, found on the periodic tableXACZ#Charge# protons - # electrons(assumed to be “0” if blank)Mass number# protons + # neutronsAtomic number # of protonsNumberHow many atoms do you have?
19 Example: Element symbols O = OxygenO16-28Charge-2Mass number16Atomic number 8NumberAssumed to be “1” if blank