The History of Atomic Theory Mr Nelson
Democritus 400 BC The Greek philosopher Democritus began the search for a description of matter more than 2400 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?
Atomos This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”
Why? 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.
Dalton’s Model In the early 1800s, English Chemist John Dalton performed experiments that verified the existence of atoms.
Dalton’s Model 1803 Dalton’s Model was that atoms are indivisible particles.
Dalton’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.
J. J. Thomson In 1897, the English scientist J.J. Thomson proved the atom is made of even smaller particles.
Thomson Model Thomson studied the Cathode Ray Tube. As the current passed through the gas, it gave off rays of negatively charged particles.
Thomson 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?
Thomson Thomson 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.
Thomson Model “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered around
Rutherford’s Gold Foil Experiment In 1908 English physicist Ernest Rutherford, began work on his gold foil experiment.
Rutherford’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.
What Happened Most particles passed straight through the gold foil A small percentage of particles were deflected at large angles or returned to the source http://micro.magnet.fsu.edu/electromag/java/rutherford/
Rutherford’s Experiment There are 2 reasons alpha particles deflected Density of the nucleus Repelling charges
Rutherford’s Conclusion An atom has a 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
Rutherford’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.
Neils Bohr Was a Jewish Scientist in Copenhagen during the onset of WWII Hitler was interested in his research of the atom. He was moved to the US to protect his knowledge.
Bohr Model Electrons travelled in a specific orbit at a certain distance from the nucleus called an energy level Worked well for Hydrogen and Helium
Nuclear symbols In this unit we need to be familiar with this type of symbol A X Z
Hyphen Notation Includes an element name a ‘-’ and a number Example: Sulfur – 32 This sulfur atom has an atomic mass of 32 Since Sulfur has ______ protons & electrons It also has ______ neutrons
Particle Symbol Charge Mass (amu) Location Proton Neutron Electron
Average Atomic Mass Calculate the average atomic mass of magnesium given the following information: Mg, mass = 23.98 amu; percent abundance = 78.99% Mg, mass = 24.98 amu; percent abundance = 10.00% Mg, mass = 25.98 amu; percent abundance = 11.01%
Bohr Model Electrons are a HUGE deal in chemistry Responsible for light, color & chem reactions
Excited atoms
Bright-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.
Pink Floyd
Spectral Lines Light is released as photons as electrons return from different energy levels. Some of the photons are visible light Balmer Series
The Electromagnetic Spectrum
What is light? Wave Particle
Some Atomic Emission Spectra Hydrogen Mercury Argon Helium
How many wavelengths are represented in each figure below? Waves: A Warm up
Wave Comparison Red Light Low frequency Long wavelength Violet Light nm = 1 x 10-9 m Red Light Low frequency Long wavelength Violet Light High frequency Short wavelength