Atoms & Their Structure Chapter 2 Section 1 Part 2

Objectives  How have historic experiments led to the development of the modern model of the atom?  How is the modern model of the atom different from previous models?  What information is available in an element block of the periodic table?

Important Vocabulary  Atom  Atomic theory  Law of conservation of mass/matter  Law of definite proportions  Electron  Proton  Electron cloud  Isotope  Neutron  Nucleus  Atomic number  Mass number  Atomic mass unit  Atomic mass

Subatomic Particles  In the mid-1880’s, through many experiments, scientists discovered that atoms can be broken down  The smaller parts of atoms are called subatomic particles  The three most important subatomic particles are the electron, proton, & neutron  Other subatomic particles include: quarks, leptons, photons, gravitons, & neutrinos

Discovery of the Proton  Evidence for a positively charged particle was found in 1886 by Eugen Goldstein  He observed a cathode- ray tube and found rays traveling in the direction opposite to that of the cathode rays  He called these rays canal rays

The Proton  Positively charge particle  Resides in the nucleus of an atom  Has a 1 + charge  Its mass is 1.67 x g  Symbolized with a p or p +

Discovery of the Electron  Its discovery was by accident and occurred in 1897  J.J. Thomson, an English physicist, was studying current using electrodes, one positive (anode) and the other negative (cathode)  His experiment determined that the cathode ray was negatively charged  Cathode-ray tubes, are currently used in TV sets, computer monitors and radar displays

Cathode-Ray Tube

Thomson’s Results  Thomson confirmed his prediction by seeing how electric and magnetic fields affected the cathode ray  His experiments showed that a cathode ray consists of particles that have mass and a negative charge  He also developed the plum- pudding model of an atom

Discovery of the Nucleus  In 1909, Ernest Rutherford disproved Thomson’s plum- pudding model by shooting a small beam of positively charged particles at a piece of gold foil  Some of the particles in the beam were reflected back  Leading Rutherford to hypothesize that there must be a positively charged mass in the center of the gold atoms

Rutherford’s Experiment

Rutherford’s Atomic Model  Is also known as the nuclear atom  In the nuclear atom:  The protons and neutrons are located in the nucleus  The electrons are distributed around the nucleus and occupy almost all the volume of the atom  This model was better than Thomson’s but it still was incomplete

The Nucleus  It is the dense central portion of the atoms  It contains nearly all the mass of an atom and all of the positive charge  Protons and neutrons!  This part of the model of the atom is still considered true today

Bohr’s Model of the Atom  In 1913, Niels Bohr suggested that electrons in an atom move in set paths around the nucleus  “Like planets in orbit”  The path defines the electron’s energy level 1.Electrons can only be in certain energy levels 2.Electrons must gain energy to move to a higher energy level 3.Electrons must lose energy to move to a lower level

Bohr’s Model Nucleus Electron Proton

Robert A Millikan  Was a U.S. physicist  He carried out experiments to find the quantity of charge carried by an electron  Using Thomson’s charge- to-mass ratio of an electron, he calculated the mass of an electron in 1916

Quantum Mechanics Model  In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further  He used mathematical equations to describe the likelihood of finding an electron in a certain position  Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron  This model can be portrayed as a nucleus surrounded by an electron cloud

Quantum Mechanics Model

Electrons  Are negatively charged particles  Charge is 1 ‒  Symbolized by e or e ‒  Mass is 9.11 x g or 1/1840 the mass of a hydrogen atom

Discovery of the Neutron  Was discovered by the English scientist James Chadwick in 1932  Irene Joliot-Curie had discovered that when alpha particles hit a sample of beryllium, a beam that could go through almost anything was produced  Using this experiment done by Irene Joliot-Curie, Chadwick concluded that the particles in the beam had no charge

Neutrons  Particles that have no charge  Reside in the nucleus  Have a mass of 1.67 x g  Symbolized with a n or n 0

Summary of Atoms  Are the building blocks of molecules  Smallest part of an element that still has the element’s properties  Unreacted atoms have no overall charge  Atoms have 5 basic parts  Atoms are made of protons, neutrons, & electrons (subatomic particles)  The protons & neutrons are housed in the center of the atom in the nucleus  Electrons are moving around outside of the nucleus within the electron cloud

Modern Atomic Model  By 1925, Bohr’s model didn’t explain electron behavior  The new model proposed that electrons behave like waves on a vibrating string  This is known as the “wave-particle duality of nature”  This model was developed by Louis de Broglie, Albert Einstein & Max Planck