Development of Atomic Theory

Gold Foil Experiment In the early 1900’s, Hans Geiger sent radioactive particles (alpha particles “”) through thin gold foil He expected these relatively heavy particles to go through the atoms with a small deflection (bouncing back)

What happened in the experiment? Gold foil

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 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 circular orbits 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 electrons.

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

Use of the Bohr Model now We now believe that electrons do not really travel around the nucleus in perfect circular patterns But imagining a circular pattern helps explain where electrons are located

Atomic Structure

What are atoms? Atom - smallest piece of matter that has the chemical properties of the element.

What’s in an atom? An atom is made of three sub-atomic particles Location Mass Charge Proton Nucleus 1 amu +1 Neutron Nucleus 1 amu Electron Outside the nucleus 0.00055 amu -1 1 amu (“atomic mass unit”) = 1.66  10-27 kg

What gives an atom its identity? Every atom has a different number of protons. The number of protons determines the identity of the atom The atomic number shows the number of protons. Atomic number = protons

The Nucleus & Mass The mass of the nucleus (in amu’s) is the number of protons + neutrons Since electrons have relatively no mass, we ignore them for mass Mass # = protons + neutrons

Charges Protons have a positive charge Electrons have a negative charge Neutrons have no charge Overall charge of an atom = protons - electrons Charge = protons - electrons

How do we show information about an element?

X A C Z # Element symbols Element Symbol Charge Mass number 1 or 2 letters, found on the periodic table X A C Z # Charge # protons - # electrons (assumed to be “0” if blank) Mass number # protons + # neutrons Atomic number # of protons Number How many atoms do you have?

Example: Element symbols O = Oxygen O 16 -2 8 Charge -2 Mass number 16 Atomic number 8 Number Assumed to be “1” if blank