Section 4.1—Development of Atomic Theory

Dalton’s Atomic Theory John Dalton’s theory based on experiments in early 1800’s All matter is made of tiny particles “atoms” Atoms cannot be created, divided, destroyed or changed into other types of atoms Atoms of the same element have identical properties Atoms of different elements have different properties Atoms of different elements combine in whole-number ratios to form compounds Chemical changes join, separate or rearrange atoms in compounds

Cathode Ray Tubes A cathode ray is a ray of light traveling in a vacuum (no other particles inside) The ray travels from one metal plate to another as the plates are connected to electricity Cathode ray Metal plate (cathode) releases stream Metal plate (anode) to which stream travels

Cathode Ray Tubes & Charge In the late 1800’s, JJ Thomson put charged plates outside the tube Negatively charged plate - + Ray is deflected away from negative plate and towards positive plate Positively charged plate It made no difference what type of metal he used in the tube—all material produced this stream that curved towards the positive charge

Thomson’s conclusions The evidence from Thomson’s work showed that there was something negatively charged in atoms Since all types of metal produced the same result, the negative charge is in all types of atoms Since atoms were overall neutral, if there was a negative charge there had to also be a positive charge In 1897, Thomson announced that the rays were electrons and they had a negative charge

Theories change Thomson’s evidence showed Dalton’s idea of solid, uniform atoms was incorrect. Eugene Goldstein conducted experiments to label the positive part “protons” and determined it has the same charge as the electron (with opposite sign) but is 1837 times heavier! Thomson developed the “plum pudding” model. Since most of us aren’t familiar with plum pudding, you can think of it as a chocolate cookie dough theory

Thomson’s Theory The “chips” are the negative electrons. The “dough” is the positive portion The “chips” are stationary and don’t move within the “dough” Remember, officially this theory is called “plum pudding” but it’s the same idea!

Gold Foil Experiment Hans Geiger performed experiments in the early 1900’s where he bombarded very thin gold foil with radioactive particles (alpha particles “”) They expected these relatively heavy particles to go through the atoms with a small deflection

What happened in the experiment? Gold foil

What did he see? Most of the alpha particles passed straight through with no deflection These particles did not run into anything Some did deflect slightly These particles ran into something much smaller than themselves A few were reflected back the direction they came from These particles ran into something very dense

What did that mean? Atoms are mostly empty space Electrons (the smaller particles) were the cause of the small deflections There 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

A third particle The protons and electrons could explain the charges of the various parts of the atom They could not explain the total mass of the atoms Neutrons were proposed in 1920’s but not confirmed until 1932 by James Chadwick Neutrons had mass similar to protons and no charge. They were located in the nucleus

More changes to the theory Niels Bohr performed experiments with hydrogen atoms & light He determined that electrons are in levels according to how much energy they have and that only certain energy amounts were allowed.

The Bohr Model It consists of the nucleus with protons & neutrons and electrons in concentric orbits (circles) outside the nucleus The circle closest to the nucleus contains the lowest energy electrons The first level can hold 2 electron, then the next two levels can each hold 8 and then levels farther out can hold 18.

Pictures of the Bohr Models Electron Proton Neutron Hydrogen-1 Helium-4 Lithium-6

Use of the Bohr Model now We no longer believe electrons are in concentric circles, but this is still a convenient way to show energy levels on 2-dimensional paper

Modern Atomic Theory In the 1920’s, Bohr’s research lead the way for the study of quantum mechanics (the study of tiny particles) Modern atomic theory uses calculus equations to show how the subatomic particles act as both particles and waves These equations show the most probable location of electrons in the atom (known as atomic orbitals)