history of an atom

• Types of radioactive decay

  • Alpha-decay
  • Beta-decay
  • Gamma-decay
• Alpha-decay


  Nucleus emits alpha particle (+), with 2 PROTONS and 2 NEUTRONS
• Beta-decay


  Nucleus emits one electron (-), BETA MINUS or BETA PLUS (positron)
• Gamma-decay


  Nucleus emits the highest energy protons, gamma rays (0)
• Democritus (Greek philosopher) was the first person to use the term atom (atomos : meaning indivisible)


  400 BC
• Democritus thought that if you take a piece of matter and divide it and continue to divide it you will eventually come to a point where you could not divide it anymore. This fundamental or basic unit was what Democritus called an atom.
• Democritus' theory of the universe

  • All matter consists of atoms, which are bits of matter too small to be seen
  • There is an empty space between atoms
  • Atoms are completely solid
  • Each atom (of a different substance) is different in size, weight and shape
• John Dalton was the first to adapt Democritus' theory into the first modern atomic model


  1800's
• John Dalton's Atomic Model


  • All matter consists of tiny particles called atoms
  • Atoms are indestructible and unchangeable
  • Elements are characterized by the weight of their atoms
  • When elements react, it is their atoms that have combined to form new compounds
• J.J Thomson was a physicist who is credited for discovering the electron


  1890's
• Cathode Ray Tube


  1. Nearly empty tube (air has been sucked out)

  2. An electric charge is passed through the tube. Travels from cathode to an anode

  3. The charge is invisible, so to see where it travelled a fluorescent screen is laced at back of tube. Where the beam hits, a dot will appear on screen. You coils also use a fluorescent gas and the whole tube will light up

  4. This beam will always travel straight if not interfered with

  5. The deflection coils each have a specific charge. One is positive and the other is positive

  6. Thomson showed that the charge would deflect away from the negative coil he then stated that this charge was thus a negative charge
• Thomson found out that this charge was 1000 times lighter that a hydrogen atom. He made a bold statement saying that this negative charge must be inside an atom. This negative charge (he called corpuscles) became known as the electron.
• Thomson's Atomic Model


  • Each atom is a sphere filled with positively charged 'fluid'. This resembles the sticky jam part of a pudding
  • Corpuscles (later called electrons), are the negatively charged particles suspended in this 'fluid'. This resembles the plums in the pudding
  • He did not predict the movement of these electrons
• Ernest Rutherford was not convinced about the model of the atoms proposed by Thomson. He thus set up his now famous Gold Foil Experiment


  1910's
• Rutherford's Gold Foil Experiment

  1. He fired alpha particles (positively charged) at a gold foil

  2. He measured the deflections as the particles came out the other side

  3. Most of the particles did not deflect at all. Every now and then a particle would deflect all the way back

  4. He said that there must be a positive centre of the foil. He called this centre the nucleus
• Rutherford's Atomic Model (aka the Planet Model)


  • The nucleus of the atoms is a dense mass of positively charged particles
  • The electrons orbit the nucleus
  • A problem raised was : Why are the negatively charged particles not attracted by the positively charged nucleus
  • Rutherford stated that the atom was like a mini solar system and that the electrons orbited the nucleus in a wide orbit
• Niels Bohr agreed with the planetary model of the atom, but also knew that it had a few flaws. Using his knowledge of energy and quantum physics he was able to perfect Rutherford's model.

  1910's
• Bohr's Atomic Model (aka the Rutherford-Bohr Model)


  • Electrons orbit the nucleus in orbits that have a set size and energy
  • The lower the energy of the electron, the lower the orbit
  • This means that as electrons fill up the orbitals, they will fill the lower energy level first
  • If that energy level is fill (or at capacity), a new energy level will begin
  • Radiation is when an electron moves from one level to another
  • Problems with this theory: Electrons do not travel on a specific orbit
• Erwin Schrodinger was a revolutionary physicist who used Heisenberg's uncertainty principle to come up with the atomic model that we still use today


  1920's
• Schrodinger's Atomic Model (aka the Cloud Model)


  • An electron does not travel in an exact orbit
  • We can predict where will it be
  • We cannot say for certain where it is, but only here it ought to be. The type of probability orbit is dependent on the energy level described by Bohr
  • The type of probability orbit is dependent on the energy level described by Bohr
• Summary of Atom


  • The smallest part of an element is called an atom
  • Each atom (of an element) is different in structure from other atoms (of other elements)
  • An atom can be divided in smaller subatomic particles : Protons, Neutrons, and Electrons
  • The nucleus is the center of an atom. It contains protons and neutrons
  • Electrons orbit the nucleus
  • As we go up the periodic table, an electron and proton is added
  • Electrons occupy a certain energy level (of a certain size)
  • Once the energy level is full, a new level begins
  • Within each of these levels are specials types
  • Each orbital can contain two electrons
• Valence Shell Electron Pair Repulsion Theory

  • Proposed by SEDGWIK AND POWELL in 1940, they developed: Nyham and Gillespie
  • The direction of the bond around the central atom depends upon the total number of electron pair in its valence shell
• Bonded Atoms


  • Bond or share electrons
• Central Atoms


  Atoms at the center, with the lowest electronegativity value except H
• Valence Electrons


  Electrons found out at the outermost valence shell, determine the number of electrons that will bond with another atom
• Valence Shell


  Where electrons are FOUND/ATTACHED
• Types of Chemical Bonds

  • Polar Covalent Bond
  • Non-Polar Covalent Bond
  • Ionic Bond
• Non-Polar Covalent Bond

  Electrons are equally shared
• Polar Covalent Bond

  Electrons are unequally shared
• Ionic Bond


  The more electronegativity, atom is strong
• Electronegativity


  Measures the tendency of an atom to attract electrons to itself when chemically bonded
• Geometry of Molecules or Molecular Geometry


  The definite arrangement of the BONDED ATOMS with the central atom in a molecule
• Polarity of Molecules


  Tells us whether electrons are equally distributed within atom
• Factors that may Determine the Polarity of Molecules

  • Electronegativity difference of elements
  • Geometrical shape of molecule predicted
• Valence Shell Electron Pair Repulsion (VSEPR)


  Used to predict the geometrical shape of molecules
• Lewis Dot Structure


  Used to predict the geometrical shape of molecules