HISTORY OF THE ATOM

• Mendeleev's periodic table organized all of the known elements and even predicted new elements that had not yet been discovered
• Periodic table


  Arranged in order of increasing atomic number, which represents the number of protons in the atoms of that element
• Nucleus symbol


  Contains the elemental symbol, atomic number, and mass number
• Mendeleev found that the elements displayed a pattern of chemical properties that repeated at regular intervals
• Periods


  New rows formed when the pattern of chemical properties repeated
• Groups


  Vertical columns of elements with similar chemical properties
• Group 0 is called the noble gases, and they all have full outer shells and are very unreactive
• The block of elements between Group 2 and Group 3 are the transition metals
• Group number


  Tells you the number of electrons in the outermost shell of the elements in that group
• Group 1 elements (alkali metals) react violently with water
• As you go down a group


  The elements become more reactive
• As you go down Group 7 (halogens)


  The elements become less reactive
• Zigzag line on the periodic table

  Separates the metals (left) from the nonmetals (right)
• There are more metals than nonmetals on the periodic table
• Different periodic table formats present the same information in slightly different ways, but the atomic number will always be the smaller of the two numbers shown
• Atomic theory


  The idea that everything is made up from tiny little particles that can't be broken down any further and that they're separated from each other by empty space
• Democritus proposed atomic theory

  Around 500 BC
• When atomic theory was improved upon


  1800s
• John Dalton's atomic model


  Atoms as solid spheres, different types of spheres make up different elements
• J.J. Thompson proposed the plum pudding model


  1897
• Plum pudding model


  • Atoms contain negatively charged particles (electrons)
  • Atoms have a general ball of positive charge with discrete electrons stuck in it
• Rutherford and students made a big discovery


  1909
• Rutherford's experiment


  1. Fired positively charged alpha particles at a thin sheet of gold

  2. Some alpha particles were deflected to the side or back, proving the plum pudding model wrong
• Rutherford's nuclear model


  Atoms have a compact nucleus containing all the positive charge, with a cloud of negative charge around it
• Rutherford's nuclear model had a flaw - the negative electrons should collapse into the nucleus
• Niels Bohr suggested a solution

  1913
• Bohr's model


  Electrons orbit the nucleus in shells, preventing the atom from collapsing
• Further experiments supported Bohr's model with a few small changes
• Protons


  Small discrete particles making up the positive charge in the nucleus
• Neutrons


  Neutral particles in the nucleus, discovered by James Chadwick