Sec. 4.1- Development of Atomic Theory

Intro- Glowing Things Think of a list of things you see releasing light/ glowing Discuss how these things release light- what do you need to do in order for them the glow?

Glowing Algae- Ooooo, so Pretty

Glowing Cats

Glowing Rings

Explaining the phenomena of light In order to explain how things are able to glow/release light, we must understand electron structure/configuration Electron structure → atomic structure → atomic theory At the end of this unit, you will be able to determine the electron structure of any element based on its location on the periodic table

Atomic theory The idea of an atom started way back in the days of Ancient Greece Greek philosophers- Democritus/Aristotle Democritus- atomos- small undividable sphere of matter different types of atomos- smooth= liquid, rough= solid

Democritus- Isn’t he Lovely?

Atomic Theory according to Dalton John Dalton developed the first atomic theory in the early 1800s Dalton’s atomic theory has several points: All mater is made of tiny particles called atoms Atoms cannot be created, divided, destroyed, or changed into any other type of atom

3. Atoms of the same element have identical properties 4. Atoms of different elements have different properties 5. Atoms of different elements combine in whole number ratios to form compounds 6. Chemical changes join, separate, or rearrange atoms in compounds

Now remember, the points in Dalton’s atomic theory are just that- theories Out of the points in Dalton’s theory, are there any that you can point out now as not being completely correct?

Most of the points in this theory are still considered valid today- only the second and third point have been proven untrue Considering the fact that Dalton developed the points in his atomic theory in the early 1800s, and the fact that we still accept the majority of his ideas as true is pretty amazing

Dalton thought of the atom as being a small, BB like sphere that could not be separated into smaller pieces We know today that this is not true- the atom contains things we call subatomic particles What are the three subatomic particles that make up an atom?

Discovery of the first subatomic particle J.J. Thomson is credited with discovering the whole concept of a subatomic particle Thomson conducted experiments with cathode ray tubes Cathode ray tube?

When he stuck charged metal plates up to the cathode ray, the ray bent While playing around with a cathode ray tube, Thomson thought it would be fun to stick a charged metal plate next to the ray (a magnet) When he stuck charged metal plates up to the cathode ray, the ray bent What does this mean about the cathode ray?

When playing with positive plate- cathode ray (beam) bent toward plate When playing with negative plate- cathode ray bent away from plate What does this mean about the charge of the cathode ray?

Once Thomson determined that the cathode ray was composed of negatively charged particles, he performed lots of tests with different metals Found that all metals produced the same cathode ray- what does this mean about the cathode ray?

Thomson announced that the cathode ray was composed of negatively charged particles called electrons, and that every piece of matter- including atoms- contained them But atoms are overall neutral pieces of matter… how could atoms contain negative particles if they are neutral?

A Second Subatomic Particle?!?! Proton- discovered by Eugene Goldstein in the 1870s Proton- positively charged- same amount of charge as electron, just opposite- 1837 times more mass than an electron

With the discovery of the electron and proton, scientists developed a model of the atom called the “Plum Pudding Model” in which positive and negative charged particles are evenly distributed within the atom

Plum pudding= British dessert, not familiar to us We can think of the “Plum pudding” model as the Choco chip ice cream model Ice cream= positive charge Choco chips= bits of negative charge, held in place by the “positive” ice cream

Gold Foil Experiment Shortly after the development of the Plum Pudding model, Hans Greiger performed an experiment known as the “Gold Foil Experiment”

Gold Foil Setup: Thin piece of gold foil placed in sample holder, with a sort of “gun” that contained radioactive material in it- this “gun” shot alpha particles at the gold foil Alpha particle= small, positively charged particles The gold foil was surrounded by detectors that would light up when hit by an alpha particle

Gold Foil expected result: most alpha particles would pass straight through foil, with some being only slightly deflected Remember: model of atom at this time was the plum pudding model- they didn’t think any part of the atom was very “dense”

Gold Foil actual result: Most alpha particles were shown to pass straight through with no deflection at all some passed through with slight deflection (totally makes sense- they expected this)… BUT, some of the alpha particles were reflected almost straight back at the alpha particle gun- this was very strange

Ernst Rutherford- looked at the gold foil experiment data and began developing additional theories to explain the unexpected results Alpha particles that passed straight through foil- didn’t hit anything Alpha particles only slightly deflected- hit something small, like an electron Alpha particles that were reflected- hit something very dense, with positive charge

Rutherford concluded that: Most of the atom is empty space Atoms have a nucleus- this nucleus is dense and contains most of the mass of an atom; also positively charged New model of atom: Densely packed nucleus containing protons located in small center of atom, with the electrons floating around in the space outside of the nucleus

Third subatomic particle Existence of protons/electrons explained how an atom could contain charges and still be neutral, but they did not explain the mass of an atom- masses of the protons and electrons in an atom did not add up to the actual mass To explain this, the existence of a third neutrally charged subatomic particle was proposed

The existence of this subatomic particle was proven in 1932 by James Chadwick- called it a neutron Neutron- no charge (neutral), located in nucleus, has a mass almost equal to a proton

The Bohr Model of the atom Niels Bohr entered the atomic model arena in 1913- performed many experiments with hydrogen and the release of light to determine the location of electrons in an atom

Bohr determined that electrons can be located in different “levels” according to their energy Bohr thought that electrons had to orbit the nucleus in locked orbits- the size of these orbits were determined by the energy the electrons had

Bohr Model- a drawing

Modern atomic theory Bohr’s model paved the way for the modern theory Bohr- correct in assigning energies to electrons, but incorrect in stating that they HAD to orbit the nucleus in a specific orbit Modern theory is based on quantum mechanics Boils down to probability- where are the electrons most likely located?

Atomic orbitals- regions of probability of finding an electron Electron cloud model- dots represent areas where an electron could be located- more dots= higher probability of finding an electron Rather than rings surrounding a nucleus, electrons are located in “clouds” of probability

Electron Cloud Model