ATOMIC THEORY

WHAT IS AN ATOM?

DEMOCRITUS

• 460 B.C. - 370 B.C.
• Atoms existed
• Indivisible & indestructible
• Atomic philosophy

DALTON

• 1766-1844
• Experimentation
• Ratios of elements in chemical reactions
• 1st atomic theory to explain observations

DALTONS ATOMIC THEORY

• All elements are composed of tiny indivisible particles called atoms
• Atoms of the same element are identical. All Elenents are different
• Atoms can mix/combine in simple whole number ratios to form compounds
• Reactions occur when atoms change, but atoms of each element stay disrinct

"SEEING ATOMS"

• Atoms are TINY
• Observable with a scanning electron microscope
• Beam of e- is focused on sample
• We can even move them around & arrange them
• Nanoscale technology

BASIC ATOMIC STRUCTURE

INSIDE THE ATOM

• Protons
• Neutrons
• Electrons

J.J. THOMPSON

• 1897 Discovered electron (e-)
• Cathode ray
• Measured ratio of e- charge to mass
• Same for all, must be in all atoms

ROBERT MILLIKAN

• 1909
• Quantity of e- charge
• Oil drop experiment, calculate e- mass
• Similar to what's accepted today
• 1 unit, - charge & 1/1840 mass of H atom

PROTONS & NEUTRONS

• 1886 discovered proton (cathode)
• Chadwick, 1932 discovered neutron
• P mass = 1, N mass =1
• Inside them are subnuclear particles called quarks

PLUM PUDDING MODEL

• J.J. Thompson
• e- spread out in positively charged material
• Short-lived, due to his student

RUTHERFORDS GOLD FOIL EXPERIMENT

• 1910
• Most went thru some deflected
• 15" shell at tissue - hit you
• Suggested new model
• Mostly empty space w/small dense + nucleus