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Unit 5 The Structure of Matter Chapter 16 The Atom 16A – The Atomic Model • Objectives: – Compare and contrast the modern atomic models to the atom as understood by ancient Greek philosophers – Summarize the current understanding of the atom – Describe the major discoveries leading to the modern atomic model – Show how each atomic model was more workable than its predecessor – Associate the key scientist with each advance in the atomic model • Assignments: Timeline of the Scientists, Worksheet, and Section Review page 388 Introduction • What is the universe made of? – Let’s Read… – Page 379 • Models – Physical, conceptual, mathematical – Never absolute, can be overturned at any time based on current understanding and technology Ancient Atomic Models • Discussion of the structure of matter can be recorded back to the Hindus more than 2600 years old; however, Democritus is given credit for the first term of the atom. – Atomos • Atom • Indivisible – He stated that everything is made of atoms, even thoughts and the soul – Atoms were the only things that exist, always existed, and would always exits – Consistent with Greek philosophers that believed reason was the path to all knowledge he did not perform any scientific experiments to backup his ideas – Denied the possibility of supernatural influence – Ideas were so radical that even his fellow philosophers rejected his theory • Aristotle – All matter was continuous, without tiny distinct particles – This influenced scientists for thousands of years John Dalton: The Indivisible Atom • 1790’s • Joseph Louis Proust – French chemist and botanist, an early balloonist – Discovered up with the law of definite proportions – The masses of chemical substances combine in definite, repeatable ratios when forming compounds • John Dalton – English school teacher used this law for the basis of his theory John Dalton: The Indivisible Atom • Dalton’s Core-Envelope Model – An atom cannot be created or destroyed in chemical reactions – An atom cannot be subdivided into smaller particles – Elements are made of only atoms – The atoms of an element are all alike – The atoms of one element are different from the atoms of all other elements, especially their masses • Much of Dalton’s theory held true for centuries JJ Thomson: The Divisible Atom • • • • English physicist Discovered the electron and isotopes of elements Invented the mass spectrometer Won the Nobel Prize for the discovery of the electron (corpuscles) • Known for his Plum-Pudding Model – Pictured the atom as a positively charged mass with negatively charged electrons embedded in it like plums in a plum pudding (British dessert) – The attraction of the opposite charges held things in place • Electrons don’t vary in mass or charge but are the same in every element • Electrons only were responsible for a fraction of the total mass of the atom, so scientist still wondered what else composed the rest of the atom Ernest Rutherford: The Nuclear Atom • English physicist, Discovered the nucleus, Father of Nuclear Physics, Won the Nobel Prize in Chemistry in 1908 • Known for his Gold Foil Experiment which resulted in the Nuclear Model – Lots of empty space in the atom – A very dense, tiny, positively charged region in each atom, called it the nucleus – Positive particles, protons, first discovered by Eugene Goldstein – Electrons whirled around the nucleus at high speeds so they were not drawn in – The nucleus’ mass was more than the protons which led to the discovery of the neutrons by James Chadwick • Did not explain the motions or locations of electrons, how far away from the nucleus were they, how were they arranged…. Niels Bohr: Electron Energy Levels • Danish physicist, helped develop the first atomic bobs in WWII, won the Nobel Prize in Physics • Knew that certain elements emit specific colors of light when heated to incandescence • Passed the emissions through a prism he was able to identify elements by their line spectra • He suspects these emissions were caused by the electrons in the atoms • Electrons can jump from one energy level to the next and release/absorb energy • The energy between orbits were fixed, quantized • His model is often called the Planetary Model Bohr Model Diagram The Quantum Atom • Discoveries showed that the electron acts like both a wave and a particle • Physicists refined Bohr’s model • Instead of having a defined area of where an electron would be found, they talked about the probability of finding an electron in specific regions around the nucleus • The Quantum Model – Page 388