Atomic Structure / Theory

• Atomic Model


  The model used to describe the structure and composition of an atom
• Model


  A three-dimensional representation of an object, a proposed structure, or a person which is typically smaller than the original
• Democritus


  • Began the search for a description of matter 2400 years ago
  • Student of Leucippus of Miletus
  • Originated the concept of atoms
• Democritus: '"Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?"'
• Atomos


  Not to be cut
• Democritus's Atomic Theory


  • All matter is composed of atoms that are too small to be seen and cannot be further split into smaller portions
  • There is a void or empty space between atoms
  • Atoms were infinite in number, always moving, and capable of joining together
  • Atoms are completely solid
  • Atoms are small, hard particles that are made up of the same material but different shapes and sizes
  • Atoms are homogeneous with no internal structure
• This theory was ignored and forgotten for more than 2000 years
• Empedocles


  • Proposed that matter is made up of four elements: earth, fire, air, and water
• Aristotle


  • Rejected the idea of atomism in matter
  • Believed that there is no limit to subdividing matter
• John Dalton's Atomic Theory (1803)


  • Atoms are the smallest particles of matter and cannot be divided into smaller particles
  • All atoms of an element are identical, but they differ in mass, size, and properties
  • Atoms cannot be created or destroyed. They combine, separate, or rearrange
  • Atoms of two or more elements combine to form compounds in ratios of whole numbers
  • Atoms of the same element can unite in more than one ratio to form more than one compound
• Revisions on Dalton's theory (Modern Atomic Theory)
• Subatomic Particles

  • Protons
  • Neutrons
  • Electrons
• Isotopes


  Elements having the same atomic number but different masses
• Nuclear Charge


  Total charge of protons in the nucleus of an atom
• Three Laws of Matter

  • Law of Conservation of Mass
  • Law of Definite Proportion
  • Law of Multiple Proportion
• Law of Definite Proportion


  • A chemical compound always contains the same elements combined in the same proportion by mass
  • A compound will always have the same proportion by mass of its elements
• Law of Conservation of Mass


  The total mass of substances before the chemical process is still the same after
• Law of Multiple Proportion


  • Two or more elements can combine to form a compound
  • The mass of one element combined with the mass of the other are in a fixed ratio
• Regions of an Atom

  • Nucleus
  • Electron Cloud
• Nucleus


  The center of the atom that contains most of its mass
• Electron Cloud


  • Surrounds the nucleus and takes up most of the space of the atom
  • Electrons can be found here
• Particles in the Nucleus

  • Protons
  • Neutrons
• Protons

  Positively charged particles
• Neutrons


  Neutrally charged particles
• Electrons


  Negatively charged particles and relatively no mass
• Atomic Number


  • The number of protons in the nucleus of an atom which determines the type of element and its place in the periodic table
  • Unless otherwise stated, number of protons = number of electrons in a neutrally charged atom
• Mass Number


  Mass number of an atom is equal to the number of protons and neutrons
• Isotopes


  Atoms of an element having the same atomic number but has different mass numbers
• Atomic Mass


  • Expressed in the atomic mass unit (amu)
  • The quantity of matter contained in an atom of an element
• Atomic Mass Unit


  Mass equal to 1/12 mass of a single carbon-12 atom
• Atomic Weight


  The ratio of the average atomic mass in different isotopes of that element
• Isotope Abundance


  The relative proportions of the stable isotopes of each element
• To solve for the atomic weight

  1. [(% of isotope)(mass of isotope 1)] + [(% of isotope)(mass of isotope 2)]

  2. When a value is missing, substitute x in the formula