HISTORY OF CHEMISTRY A BRIEF OVERVIEW What have we learned? Experimentation? How scientists work? Some contributions?

What Have We Learned? Evidence for the existence of atoms is indirect Matter can be analyzed and synthesized indicating a constant combining property of atoms Matter is conserved in analysis and synthesis Atoms can not be destroyed Crystals of the same substance have the same properties and are deposited in a pattern to build a crystal

EXPERIMENTATION! We have been doing a lot of different experiments that have mimicked what past scientists have done But not all science is done by experimentation

HOW SCIENTISTS WORK Science is a product of human imagination Data must be organized, pondered, and molded into theory Scientists do not work alone

SOME CONTRIBUTORS The literature of chemistry holds a wealth of information that we can draw upon to help us learn We will learn how these researchers used information from one another to advance their own ideas We will also see how some of the best ideas came from “mistakes”

DEMOCRITUS Around 430 BC “atomos” : uncuttable Ancient Greeks did not prove the existence of atoms because they DID NOT DO ANY EXPERIMENTS His idea was not really written about until John Dalton

John Dalton Model of the atom-with only a few changes his atomic theory is still accepted today

John Dalton All elements composed of atoms that can not be divided Atoms of the same element are exactly the same and have the same mass Atoms of one element can’t be changed into an atom of a different element;can’t be created or destroyed in any chemical change, only rearranged Compounds are composed of atoms of different elements, combined in ratios “BILLIARD BALL MODEL”

JJ THOMSON 1st to hypothesize that there are particles inside of atoms

JJ THOMSON 1897 Worked with cathode ray tubes and passed electricity inside empty glass tubes. He used magnets to pull the rays apart. This mysterious glow “cathode rays” in the tube, he theorized were made of electrons Found that atoms contained negative charged particles (electrons), but scientists knew that atoms had not electrical charge, so therefore atoms must also have a positive charge In his model the electrons are scattered throughout a ball of positive charge “PLUM PUDDING OR RAISIN MUFFIN MODEL”

Ernest Rutherford 1911 Thomson’s student Wanted to collect more evidence to support Thomson’s theory Famous “Gold Foil Experiment”

Earnest Rutherford Aimed a beam of “+” charges at a sheet of thin gold foil If Thomson’s theory was correct, the “+” particles would pass right through in a straight line (the gold atoms would not have a strong enough charge in any region to repel the charged particles……..BUT……

Earnest Rutherford Most particles passed through as expected but a few were strongly deflected….WHY? He hypothesized that an tom’s “+” charge must be clustered in a tiny region…NUCLEUS and the the “+” were being deflected by the nucleus

Earnest Rutherford They also knew that the electron had little or no mass, therefore he reasoned that all of an atom’s mass is in the positively charged nucleus. He called these positively charged particles in the nucleus, PROTONS “PEACH PIT MODEL” an atom is mostly empty space with a “+” charged center. Electrons move around a small nucleus found in the center.

Neils Bohr 1913 Danish student of Rutherford and Thomson Set out to support Rutherford’s theory He predicted if Rutherford was correct, the electrons should accelerate and give off energy as they travel in a circle. As they travel they should lose energy and fall into the nucleus, but this doesn’t happen……………………….

Neils Bohr Bohr theorized that electrons must have only a specific amount of energy. This energy leads them to move in certain orbits around the nucleus. This is similar to the way planets move around the sun. They can still circle the nucleus without losing energy if they stayed in a certain orbit “PLANETARY MODEL” Similar to the lanes on a track. If you want to use less energy…… what lane would you use? Bohr’s model was essentially correct but he could not explain his observations of the actual behavior or atoms and electrons.

Electron Cloud Bohr had the right idea but it was not until the 1920’s when “Quantum Mechanics” came into the picture. Albert Einstein and Max Planck said that energy can be divided into particles

Electron Cloud Werner Heisenberg and Erwin Schrodinger developed a theory that matter could also act both like waves and particles. They then applied this to the motion of electrons Electrons do not orbit like planets, they can go anywhere within a cloud-like region, these are called ENERGY LEVELS. The electrons within a same energy level all have the same amount of energy Lowest energy levels are found closest to the nucleus, these can only hold 2 electrons, higher energy electrons are in larger energy levels (they can hold 8 or more)

James Chadwick 1932, British Worked with Ernest Rutherford Credited with discovering the Neutron His discovery completed the atomic model The neutron was very hard to detect because it has no charge. He was studying atomic mass and atomic number and found differences. If the nucleus only had “+” and “-” inside these would be the same but they are not, they were double. There must be something else inside. Even though it is the same mass as a proton it is electrically neutral, hence the name neutron