3Democritus400 BCThe Greek philosopher Democritus began the search for a description of matter more than years ago.He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?
4Atomos This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”
5Why?The eminent philosophers of the time, Aristotle and Plato, had a more respected, theory.Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter.
6Dalton’s ModelIn the early 1800s, English Chemist John Dalton performed experiments that verified the existence of atoms.
7Dalton’s Model1803Dalton’s Model was that atoms are indivisible particles.
8Dalton’s Theory All matter is composed of atoms. Atoms of the same element are exactly alike.Atoms of different elements are different.Atoms are indestructible and cannot be divided.
9J. J. ThomsonIn 1897, the English scientist J.J. Thomson proved the atom is made of even smaller particles.
10Thomson Model Thomson studied the Cathode Ray Tube. As the current passed through the gas, it gave off rays of negatively charged particles.
11Thomson Model the atoms of the gas were uncharged. Where did they come from?the atoms of the gas were uncharged.Where had the negative charges come from?
12ThomsonThomson concluded that the negative charges came from within the atom.Thomson called the negatively charged electrons.Since the gas was known to be neutral, he reasoned that there must be positively charged particles in the atom.But he could never find them.
13Thomson Model “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered around
14Rutherford’s Gold Foil Experiment In 1908English physicist Ernest Rutherford, began work on his gold foil experiment.
15Rutherford’s Hypothesis Rutherford was trying to verify Thomson’s model.He expected positively charged particles to go straight through a piece of very thin gold.
16What Happened Most particles passed straight through the gold foil A small percentage of particles were deflected at large angles or returned to the source
18Rutherford’s Experiment There are 2 reasons alpha particles deflectedDensity of the nucleusRepelling charges
19Rutherford’s Conclusion An atom hasa small, dense, positively charged center that repelled the positively charged particles.Named the center of the atom the “nucleus”The nucleus is tiny compared to the atom as a whole.The atom is mostly empty space
20Rutherford’s Nuclear Model An atom’s positively charged particles were contained in the nucleus.The negatively charged particles were scattered outside the nucleus around the atom’s edge.
21Neils BohrWas a Jewish Scientist in Copenhagen during the onset of WWIIHitler was interested in his research of the atom.He was moved to the US to protect his knowledge.
22Bohr ModelElectrons travelled in a specific orbit at a certain distance from the nucleus called an energy levelWorked well for Hydrogen and Helium
23Nuclear symbolsIn this unit we need to be familiar with this type of symbolAXZ
24Hyphen Notation Includes an element name a ‘-’ and a number Example: Sulfur – 32This sulfur atom has an atomic mass of 32Since Sulfur has ______ protons & electronsIt also has ______ neutrons
26Average Atomic MassCalculate the average atomic mass of magnesium given the following information:Mg, mass = amu; percent abundance = 78.99%Mg, mass = amu; percent abundance = 10.00%Mg, mass = amu; percent abundance = 11.01%
27Bohr Model Electrons are a HUGE deal in chemistry Responsible for light, color & chem reactions
29Bright-line Spectra Electrons in atoms: when an atom absorbs energy - electrons jump to higher energy levels.An “EXCITED” electron jumps from its ground state to a higher energy level.When the electron returns it releases the same amount of energy that it absorbed.