Lesson Objectives• Understand the experimental design and

conclusions used in the development of modern atomic theory including– Dalton’s Postulates– Thomson’s discovery of electron properties– Rutherford’s nuclear atom– Bohr’s nuclear atom

Atomic Theory

Atomic Structure

A brief look at the history of the atom.

Democritus of Abdera

Began the discussion of the possible existence of ATOMS!

ATOM: from the Greek meaning “cannot be cut”.

But, he had no empirical evidence, so for centuries people believed Aristotle.

Democritus' Ideas• Matter is composed of empty space through which atoms move.• Atoms are solid, homogeneous, indestructible, and indivisible.• Diff. kinds of atoms have different sizes and shapes• The differing properties of matter are due to the size, shape, and movement of atoms.• Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves.

The Aristotelian Approach to Knowledge

• The scientific method is still another 2000 yrs. away.

• They believed one should separate the mind from the body (emotions) in order to understand anything. Attaining knowledge by pure reason, or Rationalism.

• What’s the purpose of knowledge??

• To lead you to the truth.

The Empirical Approach

• Come to knowledge by relying on our senses . . . the beginning of experimentation.

• So, compare Rationalism to Empiricism.

• What’s better??• Rational – problems with

our reason. Plato’s Academy.

• Empirical – problems with trusting our senses.

• First credible atomic theory introduced by John Dalton in 1808

• Dalton studied the ratio in which elements combine when reacted with each other

• Based theory on experimental results gathered by other scientists (Lavosier’s conservation of mass and Proust’s law of definite proportions)

Development of Atomic Theory

• Dalton’s Postulates– Elements are made of small and indivisible

particles called atoms– Atoms of a given element are identical to one

another but differ from atoms of other elements– Chemical reactions occur when atoms are

combined, separated, or rearranged in simplified whole-number ratios

– Atoms are neither created nor destroyed in chemical reactions

• Shortcomings of Dalton’s Postulates– Atoms of an element can have masses that differ

slightly– Atoms can be divided into subatomic particles

Dalton’s Experimental Theory

• Scientists noticed that some substances decomposed when exposed to electrical current– Elements are held

together by electrical forces

• Scientists discovered the cathode ray

Atoms and Electricity

Cathode Ray Tube

• Rays in a cathode tube were attracted to a metal plate that had a positive charge– Cathode ray – stream of

negatively charged particles

• Thomson calculated the electric charge to mass ratio of the particles in cathode rays– Mass of these

particles was much less than the atom with the least mass

• Electron (e-) – negatively charged subatomic particle

Discovery of the Electron

Cathode Ray Tube

• Thomson formulated the “Plum Pudding” model – Atoms are made of a

positively charged mass and electrons are scattered throughout the mass, like plums mixed in a pudding

• Failed to correctly establish the structure of the atom

Thomson’s Atomic Model

Plum Pudding Model

• Bombarded a thin foil of gold with alpha particles– Alpha particles have a +2 charge

• Discovery of nucleus– Nucleus – positively charged, dense center of an

atom

Rutherford’s Gold Foil Experiment

• Rutherford estimated that diameter of the nucleus was only 1/10000 of the diameter of the atom

• Nucleus accounts for almost all of an atom’s mass– Nucleus contains protons and neutrons

• Proton – positively charged subatomic particle (p+)• Neutron – neutrally charged subatomic particle (n0) - J.

Chadwick

• Rutherford’s nuclear atom showed a positive nucleus orbited by electrons

Rutherford’s Nuclear Atom

By en:User:Cburnett [GFDL]

• Following discovery of the nucleus, Bohr proposed his own atomic theory– Tried to explain why the emission spectrum of

the hydrogen atom included only some light frequencies

– Proposed a diagram that explained electron movement and energy relations

Bohr’s Nuclear Atom

Hydrogen gas discharge tube Silt Pris

m

Detecting Screen

• Proposed that electrons– Are arranged in circular orbits around the

nucleus– Do not lose energy as they orbit – In each orbit have fixed amount of energy– In an orbit closest to the nucleus are in their

lowest energy state – Gain and lose energy in certain, fixed amounts

called quanta– May receive energy and move to a higher energy

orbit – Return back to lower energy orbit when they

release energy• Photon – particle of electromagnetic radiation that

possesses a quantum of energy– Energy of each photon corresponds to a certain frequency

Bohr’s Nuclear Atom

e-

Photon

• Did not explain emission spectrums of other elements

• Did not explain chemical properties of atoms• Further experimentation indicated that

electrons don’t revolve in circular orbits– Movement of electrons is much more complex

Short Comings of Bohr’s Nuclear Atom