History of the AtomHistory of the Atom

A.A. AncientsAncientsSocrates Socrates

All substances were made up of earth, air, fire and waterAll substances were made up of earth, air, fire and water

DemocritusDemocritus Matter is composed of small particles called “atoms”Matter is composed of small particles called “atoms”

B.B. DaltonDalton

All substances were made up of atomsAll substances were made up of atoms• Small individual and indestructible partsSmall individual and indestructible parts

MODEL—marble conceptMODEL—marble concept• Atom was a small, dense sphereAtom was a small, dense sphere

C.C. ThomsonThomson Cathode-ray tube experimentCathode-ray tube experiment

• Proved that atoms had negative charges inside Proved that atoms had negative charges inside themthem

Called these charges “electrons”Called these charges “electrons”

MODEL—”Plum Pudding Model”MODEL—”Plum Pudding Model”• Better known as the Chocolate Chip Cookie ModelBetter known as the Chocolate Chip Cookie Model

Cookie is the atomCookie is the atom Chips are the electronsChips are the electrons http://www.youtube.com/watch?v=O9Goyscbazkhttp://www.youtube.com/watch?v=O9Goyscbazk

D.D. RutherfordRutherford Gold-Foil ExperimentGold-Foil Experiment

• When alpha particles passed through gold foilWhen alpha particles passed through gold foil Proved the atom was mostly empty spaceProved the atom was mostly empty space

• When alpha particles were deflectedWhen alpha particles were deflected Proved atoms had a small, dense coreProved atoms had a small, dense core

• Called this area a nucleus which contained positive Called this area a nucleus which contained positive particles called protonsparticles called protons

• http://www.youtube.com/watch?v=U83swkcL32whttp://www.youtube.com/watch?v=U83swkcL32w

E.E. BohrBohr Planetary ModelPlanetary Model

• Electrons orbited the nucleus like planets orbit the Electrons orbited the nucleus like planets orbit the sunsun

F.F. Modern TheoryModern Theory Wave-Mechanical ModelWave-Mechanical Model

• Electrons exist in regions (clouds) around the Electrons exist in regions (clouds) around the nucleusnucleus

Regions are called orbitalsRegions are called orbitals

II Atomic StructureII Atomic Structure

A.A. Subatomic ParticlesSubatomic Particles ProtonsProtons

• Mass = 1 uMass = 1 u• Charge = +1Charge = +1• Location = in the nucleusLocation = in the nucleus

NeutronsNeutrons• Mass = 1 uMass = 1 u• Charge = 0Charge = 0• Location = in the nucleusLocation = in the nucleus

ElectronsElectrons• Mass = 1/1836Mass = 1/1836thth u u• Charge = -1Charge = -1• Location = outside the nucleusLocation = outside the nucleus

B.B. Atomic NumberAtomic Number Represents the number of protons in an atomRepresents the number of protons in an atom Identifies the elementIdentifies the element

• Change the atomic number—change the elementChange the atomic number—change the element Can also be used to determine the number of Can also be used to determine the number of

electronselectrons• AtomAtom

Protons = electronsProtons = electrons

• IonIon Changes the number of electronsChanges the number of electrons

• (+) ion loses electrons(+) ion loses electrons• (-) ions gains electrons(-) ions gains electrons

C.C. Mass NumberMass Number Represents the number of protons and Represents the number of protons and

neutrons in the nucleusneutrons in the nucleus• Represents the mass of an isotope of the atomRepresents the mass of an isotope of the atom

Always a whole numberAlways a whole number

• Written two different waysWritten two different ways Co-60 or Co-60 or 6060CoCo

• Mass number is NOT found on the Reference Mass number is NOT found on the Reference TableTable

1.1. IsotopeIsotope An isotope is a different form of the same An isotope is a different form of the same

elementelement• Neutrons changeNeutrons change• Mass number changesMass number changes

Example C-12 and C-14Example C-12 and C-14• C-12 protons = 6 neutrons = 6C-12 protons = 6 neutrons = 6• C-14 protons = 6 neutrons = 8C-14 protons = 6 neutrons = 8• C-12 is stable while C-14 is radioactiveC-12 is stable while C-14 is radioactive

D.D. Atomic MassAtomic Mass Weighted average of all isotopes of an Weighted average of all isotopes of an

elementelement• ExampleExample

In nature C-12 exists in 93% of a sample of carbon while In nature C-12 exists in 93% of a sample of carbon while C-14 exists in 7% of the sample. What is the atomic C-14 exists in 7% of the sample. What is the atomic mass of carbon?mass of carbon?

• 12 x 0.93 = 11.1612 x 0.93 = 11.16• 14 x 0.07 =14 x 0.07 = 0.98 0.98• Atomic Mass = 12.14 uAtomic Mass = 12.14 u