For each scientist you need to know  Approximate date of discovery  Discovery  Experiment/Theory  Atomic Structure from their perspective FOCUS

What do you know about Chemistry?

ATOMIC THEORY

Democritus – 440 BCE  Greek philosopher  Credited with the discovery of the atom  Believed that if you continue to cut matter that eventually you would end up with a particle that cannot be cut  Most people did not believe this theory

Democritus  Atom came from the word atomos which means “not able to be divided”  Thought atoms were all made of a single material formed into different sizes and shapes  Atom was a small, hard, particle

John Dalton  Created original Atomic Theory  All substances are made of atoms  Atoms are small particles that cannot be created, divided, or destroyed  Atoms of the same element are exactly alike and atoms of different elements are different  Atoms join together with other atoms to make new substances

John Dalton  No new atom picture  Was the first to organize elements according to mass

William Crookes  Developed the Crookes Tube, which helped him test Cathode Rays  Showed that rays travel in straight lines  Thought the rays were made up of ordinary particles  Doesn’t add to atomic theory, so is he important???

JJ Thomson  Continued working with Cathode Ray Tubes (developed by Crookes)  A glass tube with metal plates on each end (cathode and anode) that is hooked up to a power source  Electric beam goes through tube  When plates were not charged, the beam went straight  When plates were charged, the beam bent toward the positively charged plate  What does that tell you about the beam?

JJ Thomson  Since the beam bent towards the positively charged plate, Thomson discovered that there must be a negative charge in the atoms (neg. and pos. charges attract)  Credited with the discovery of ELECTRONS!

JJ Thomson  Plum Pudding Model  Changed model of the atom from a solid sphere to a model that had electrons mixed throughout the entire atom, like plums in a pudding  Electrons are able to come out of the atom leaving behind a positively charged area

Ernest Rutherford  Gold Foil Experiment  Beam of small, positively charged particles aimed at a thin sheet of gold foil with a coating on it that would allow him to see where the particles hit  Expected particles to pass through the foil in a straight line, and most of them did  Some particles were deflected or bounced straight back

Ernest Rutherford  Why did most of the particles go straight through the foil?  Atoms are mostly empty space, so the positive particles would simply go through that empty space  Why did some of the particles bounce back?  Rutherford discovered that there must be a dense positive charge in the middle of an atom (nucleus) and the positive charges were occasionally hitting that small, dense area  Credited with the discovery of the NUCLEUS!!

Rutherford’s Model  Center of the atom is small, dense, positive area called the nucleus with electrons surrounding in

Niels Bohr  Studied the way that atoms react to light  Proposed that electrons move around the nucleus in certain paths or energy levels

Niels Bohr  The energy levels are like a ladder  There are no paths between the energy levels, but atoms can jump from one level to another, just like you can step from one rung of the ladder to the next  Atoms cannot be in between levels, just like you cannot stand in between rungs of the ladder

Modern Atomic Theory  Electrons do not move in definite paths  The path of an electron cannot be predicted  There are regions inside of the atom where electrons are likely to be found called electron clouds

To Recap… 