Atomic Structure

• Defining the Atom

  • The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)

  - He believed that atoms were indivisible and indestructible

  - His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy
• Dalton’s Atomic Theory (experiment based!)

  1. All elements are composed of tiny indivisible particles called atoms
  2. Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
  3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds
  4. In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.
• Sizing up the Atom

  • Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element

  - If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long

  - Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes
• Structure of the Nuclear Atom

  • One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:

  - Electrons, protons, and neutrons are examples of these fundamental particles

  - There are many other types of particles, but we will study these three
• Discovery of the Electron

  In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron
• Modern Cathode Ray Tubes

  Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.
• Mass of the Electron

  1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge
• Conclusions from the Study of the Electron:

  • Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
  • Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
  • Electrons have so little mass that atoms must contain other particles that account for most of the mass
• Conclusions from the Study of the Electron:

  • Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
  • 1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton
• Thomson’s Atomic Model

  Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.
• Ernest Rutherford’s Gold Foil Experiment - 1911

  • Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
  • Particles that hit on the detecting screen (film) are recorded
• Rutherford’s Findings

  • Most of the particles passed right through
  • A few particles were deflected
  • VERY FEW were greatly deflected

  Conclusions:

  • The nucleus is small
  • The nucleus is dense
  • The nucleus is positively charged
• The Rutherford Atomic Model

  Based on his experimental evidence:

  • The atom is mostly empty space
  • All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
  • The nucleus is composed of protons and neutrons (they make the nucleus!)
  • The electrons distributed around the nucleus, and occupy most of the volume
  • His model was called a “nuclear model”
• Atomic Number

  • Atoms are composed of identical protons, neutrons, and electrons

  - How then are atoms of one element different from another element?

  • Elements are different because they contain different numbers of PROTONS
  • The “atomic number” of an element is the number of protons in the nucleus
  • # protons in an atom = # electrons
• Isotopes

  • Dalton was wrong about all elements of the same type being identical
  • Atoms of the same element can have different numbers of neutrons.
  • Thus, different mass numbers.
  • These are called isotopes.
• Isotopes

  • Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912
  • Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
  • Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.
• Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
• Isotopes

  • Elements occur in nature as mixtures of isotopes.
  • Isotopes are atoms of the same element that differ in the number of neutrons.
• Measuring Atomic Mass

  • Instead of grams, the unit we use is the Atomic Mass Unit (amu)
  • It is defined as one-twelfth the mass of a carbon-12 atom.

  - Carbon-12 chosen because of its isotope purity.

  • Each isotope has its own atomic mass, thus we determine the average from percent abundance.
• To calculate the average:

  • Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.
  • If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)
• Atomic mass is the average of all the naturally occurring isotopes of that element.
• The Periodic Table: A Preview

  • A “periodic table” is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties

  - The periodic table allows you to easily compare the properties of one element to another
• The Periodic Table: A Preview

  • Each horizontal row (there are 7 of them) is called a period
  • Each vertical column is called a group, or family

  - Elements in a group have similar chemical and physical properties

  - Identified with a number and either an “A” or “B”