Phy 102: Fundamentals of Physics II Chapter 11: The Atomic Nature of Matter

The Atomic Hypothesis All matter is made up of small invisible particles called atoms Atoms can combine to form larger blocks of matter called molecules Atoms have 2 primary regions: –The nucleus –The electron cloud The atomic nucleus contains protons & neutrons The electrons are held to nucleus by the electric force (attractive) between the protons & the electrons

History of the Atomic Hypothesis Democritus proposes the concept of “atoms” Dalton proposes his “atomic theory of matter” & re-introduces the concept of the atom Rutherford introduces the nuclear atom hypothesis Bohr proposes the “planetary” model of the atom De Broglie suggests that electrons can act like waves –leads to the wave theory of the electron cloud Schrodinger & Heisenberg simultaneously work out the foundations for the quantum theory of the atom (referred to as quantum mechanics) Gell-Mann proposes the quark hypothesis

English Chemist Considered by many to be the “Father of Modern Chemistry” John Dalton ( ) Re-introduced the atomic hypothesis of matter –Matter is never created nor destroyed –The basic units of matter are called atoms –Atoms cannot be broken down into smaller units –Substances composed of only one type of atom are called elements –Atoms of a given element are identical –Atoms of different elements have different masses –Atoms may combine in defined whole-number ratios to form molecules

Ernest Rutherford ( ) Celebrated experimental physicist Considered the “Father of Nuclear Physics” Nobel Prize in Chemistry (1908) Guided many of his students to win Nobel Prizes Contributions included: –Proposed the model of the nuclear atom –Explained radioactivity as the disintegration of atoms –The first to split the atom (O to N) –Invented the 1 st method to measure radioactive particles by electrical means –Helped invent the modern smoke detector

Neils Bohr ( ) Danish physicist Won the Nobel Prize (1922) for his work on the structure of atoms Proposed a “planetary” model of the atom, referred to as the Bohr Model Worked on the Manhattan Project at Los Alamos Later in life promoted the peaceful use of nuclear energy

Murray Gell-Mann (1929- ) Won the Nobel Prize (1969) for his contributions to elementary particle physics (the quark theory) Numerous contributions to the field of elementary particle physics including: –Classification of elementary particles & their interaction –Proposed the existence of “quarks” –Developed the following theories: Strangeness Theory” Eightfold Way Theory Quantum Chromodynamics Considered by many to be “the greatest living theoretical physicist”

Elements Substances made of only one type of atom Matter that cannot be broken down into new substances Atoms of a particular element have the same # of protons Atomic Number = # of Protons Atoms of a particular element can have different # of neutrons Atomic Mass = (# of Protons) + (# of Neutrons)

The Electron First discovered by J.J. Thomson (1897) Small negatively charged “particle” –Mass of electron = 9.11x kg –Charge of electron = -1.60x C Determine for the chemical properties of a substance Form a “cloud” around the nucleus of the atom Establish the volume of an atom Flow of electrons in conducting material is referred to as “electrical current” (or electricity)

The Atomic Nucleus The “nuclear atom” hypothesis proposed by Earnest Rutherford (1909) Dense, positively charged “core” of the atom Contains protons & neutrons Determine the mass of the atom

The Proton First observed by Eugene Goldstein (1869) –He referred to them in his experiments as “canal rays” “heavy” positive charged particle –About 2000 times as massive as an electron Mass of Proton = 1.67x kg –Same magnitude of charge as electron (opposite sign) Charge of Proton = +1.60x C Number of protons in the nucleus is unique for each element # of protons is the same as the # of electrons (except for ions)

The Neutron Discovered by James Chadwick (1932) “heavy” neutral charge particle –Mass slightly larger than proton –Mass of Neutron = 1.67x kg Provide the “glue” that holds the protons together in the nucleus Atoms of the same element with different amounts of neutrons are called isotopes

The Periodic Table The master “menu” of matter First proposed by Dmitri Mendeleev All known elements are organized by chemical properties There are currently ~120 elements have been identified Columns are called Groups Rows are called Periods

Quarks First proposed by Murray Gell-Mann (1963) Along with Leptons (electrons, neutrinos,…) form the “fundamental particles” Combine in combinations to form protons & neutrons Two Classes of quarks Charge = - 1 / 3 Down Strange Bottom (Beauty) Charge = + 2 / 3 Up Charmed Top (Truth)

Elements, Compounds & Mixtures Matter can be classified as –Pure substances (elements & compounds) –Mixtures (homogeneous & heterogeneous) Atoms of elements combine to form compounds Molecules are the basic units of compounds