Chapter 2 Atomic Structure

Early Models of an Atom Have you ever been asked to believe in something you could not see? You cannot see the tiny fundamental particles that make up matter Yet, all matter is composed of such particles called atoms An atom is the smallest particle of an element that retains its identity in a chemical reaction

Early Models of an Atom Concept of an atom intrigued a number of early scholars Scholars were unable to see the atom but this did not stop them from propose ideas on the structure of atoms Democritus was the first!

Democritus’s Atomic Philosophy Democritus was among the first to suggest the existence of atoms Democritus believed that atoms were indivisible and indestructible Democritus’s ideas agreed with later scientific theory, but they did not explain chemical behavior They also lacked experimental support b/c his approach was not based on the scientific method

Dalton’s Atomic Theory The real nature of atoms and the connection between observable changes and events at the atomic level were not established for more than 2000 years after Democritus The modern process of discovery regarding atoms began with John Dalton Dalton transformed Democritus’s ideas on atoms into a scientific theory

Subatomic Particles Most of Dalton’s atomic theory is still accepted today except it is now known that atoms are now divisible Atoms can be broken down into even smaller, more fundamental particles, called subatomic particles Three kinds of subatomic particles Electrons Protons Neutrons

Electrons 1897 JJ Thomson discovered the electron Thomson found that electrons are negatively charged subatomic particles Electrons are really small in comparison to the other two subatomic particles (1/1840 of a proton or neutron) Thomson created the Cathode Ray Tube Experiment Symbol=e-

Thomson’s Cathode Ray Tube Experiment Thomson passed electric current through gasses at low pressure He sealed the gases in glass tubes fitted at both ends with metal disks called electrodes The electrodes were connected to a source of energy and one electrode (anode) became positively charged and one electrode (cathode) became negatively charged Result: A glowing beam, or cathode ray, that traveled from the cathode to the anode

Thomsons Model

Protons Eugen Goldstein observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays Called them canal rays and concluded that they were composed of positive particles He went on to call these particles protons Proton=positively charged subatomic particle Protons have a mass 1840 times greater than the electron Symbol= p+

Neutrons James Chadwick confirmed the existence of the last subatomic particle: the neutron Neutrons are subatomic particles with no charge They have a mass that is equal to that of a proton

Rutherford’s Gold-Foil Experiment Rutherford shot alpha particles (helium atoms that have lost their two electrons and have a double positive charge) into a piece of gold foil If previous model was correct, alpha particles should have passed easily through the gold with only slight deflection Result: Most particles passed straight through without deflection, small fraction bounced off the gold at very large angles, some bounced straight back

Rutherford’s Atomic Model Rutherford proposed atom is mostly empty space Concluded that all positive charge and almost all the mass are concentrated in small region in middle Called the nucleus Nucleus is the tiny central core of an atom and is composed of protons and neutrons Also called the Nuclear Atom Nuclear atom consists of protons and neutrons that are located in the nucleus with electrons distributed around the nucleus

Nuclear Atom