che110

• The concept of atom was first described by the scientist Democritus around 440 BCE who said everything in this earth is made up of tiny particles which he named as a Thomas
• Aristotle said the matter is made up of four elements which are art wind water and fire
• John Dalton came up with the idea of atom, showed common substances are broken down into the same elements, and said that elements cannot be created or destroyed
• JJ Thompson's chocolate chip cookie model

  Positive charged are in a sphere filled with negatively charged electrons
• Rutherford studied the effect of x-ray in gases, shot alpha particles into a gold foil, and concluded that most of the portion of the atom is empty and the positive charged particles are concentrated at the center
• Niels Bohr said that electrons orbit the nucleus in a fixed energy and distances, and they are able to jump from one orbit to another
• Heisenberg said that it is almost impossible to tell the exact position of an electron
• Law of conservation of mass


  Matter is not created or destroyed in a closed system
• Law of constant composition

  A pure compound will always have the same proportion of the same elements
• Dalton's atomic theory postulates

  • All matter is made of atoms and atoms are indivisible and indestructible
  • All atoms of a given element are identical in mass and properties
  • Compounds are formed by a combination of two or more different kinds of atoms
  • A chemical reaction is a rearrangement of atoms
• Limitations of Dalton's atomic theory: 1) Atoms can be divided into subatomic particles, 2) Atoms of same element can have different masses, 3) Atoms of different elements can have the same masses, 4) Dalton didn't show the difference between an atom and molecule