Atomic Theory

Atomic Theory Science is based off of observations. A Scientific Law is a summary of what is seen in observations. A Scientific Theory is an explanation of why these observations are occurring. Both laws and theories are tested by using them to predict what would happen in certain situations.

Atomic Theory -Explains why all matter acts as it does because it is composed of tiny particles called atoms, the basic building blocks of all matter First theorized by Democritus ~400 BC in ancient Greece. He thought that matter could NOT be divided infinitely. You had to reach a smallest possible piece. He named this piece an atom, which meant indivisible or can’t be cut.

More Democritus He and his followers, atomists, said atoms were small hard particles all made of the same material, but different sizes and shapes. They were also always moving and capable of joining together. Since this was ancient Greece, He and the atomists had no way of ever proving his theories and few people believed it.

Law of definite proportions All pure compounds have exactly the same proportions by mass of elements regardless of size Water is always 2 hydrogen atoms to every 1 oxygen atom By mass that is 2 (g) H : 16 (g) O

Law of multiple proportions The same elements may combine to form several different compounds For example H and O can combine form H2O (water) and H2O2 (hydrogen peroxide) C H and O may form alcohol, formaldehyde, ether (starting fluid) or many other things. basically if you see a couple of elements don’t assume they can only go together one way.

Law of conservation of mass ~ mass is neither created nor destroyed in a reaction. The mass of the products must equal the mass of the reactants.

Dalton’s Model (1803) people started to accept the idea of atoms because of his experiments He worked with gases and found that they acted as though they were made of solid microscopic particles all elements are made of atoms (indivisible and indestructible) atoms of the same element are exactly alike atoms of different elements are different compounds are formed by joining two or more atoms

John Dalton

Thomson’s Model (1897) found negative particles could come from neutral elements atom is made of smaller things (+ & -), and is divisible successfully separated negative particles (electrons) but could not separate the positive particle (protons) “plum pudding model” negative particles floating in a positively charged gel like material

Plum Pudding Model- Thomson Positive Gel Negative Particles

Thomson’s Cathode Ray

Sir J. J. Thomson The sir means he was knighted

Rutherford’s Model (1911) fired protons at a sheet of gold foil most went through unaffected, some bounced away there is a small dense area of positive particles at the center of the atom- the nucleus electrons are scattered near the outside of the atom with mostly empty space between the nucleus and the electrons

Gold Foil Experiment Gold foil Radioactive source

Rutherford’s Model Empty Space nucleus (small dense positive area) electrons

Ernest Rutherford

Bohr Model (1913) electrons move in definite orbits around the nucleus these orbits or energy levels are located at certain distances from the nucleus

Bohr’s Model nucleus Electrons

Neils Bohr

Wave Model (present day) based on complex math equations orbits are more complex than originally thought de Broglie stated that electrons (particles) have wave properties, and he viewed these as standing waves, like those produce when a guitar string is plucked (classical physics.) Schrodinger assumed that the electron in Hydrogen behaves as a standing wave.

Wave Model (continued) When Schrodinger’s equation is analyzed, many solutions are found. Each solution represents an atomic orbital. An atomic orbital is the most probable location for finding an electron.

What is an orbital? It is not a Bohr orbit (not moving in a circular path) How is the electron moving? We don’t know! There is a fundamental limitation to just how precisely we can know both the position and momentum of a particle at a given time

This is kind of how we assume an electron travels

Heisenberg Uncertainty Principle The more accurately we know the particle’s position, the less accurately we can know its momentum and vice versa. We can’t know the exact motion of the electron around the nucleus. The area that an electron orbits is called an “electron cloud”

Louis de Broglie Erwin Schrodinger

Werner Heisenberg