development of periodic table

• Why is the periodic table called "periodic"?

  Elements occur at regular intervals
• What are the groups in the periodic table?

  Columns of elements with similar properties
• Why do elements in a group have similar chemical properties?

  They have the same number of outer electrons
• Who first noticed elements with similar properties occurred in threes?

  Johan Dober
• What did Dober call the groups of three elements?

  Triads
• What did John Newlands notice about the elements he arranged?

  Every eighth element reacted similarly
• What is Newlands' law of octaves?

  Every eighth element has similar properties
• What was a problem with Newlands' system?

  Elements were grouped with different properties
• Who developed the first modern periodic table?

  Dmitri Mendeleev
• How did Mendeleev arrange the elements?

  In order of increasing atomic weight
• What two things did Mendeleev do that were innovative?

  Switched element order and left gaps
• Why did Mendeleev leave gaps in his periodic table?

  He predicted undiscovered elements' existence
• What happened when the missing elements were discovered?

  Their properties matched Mendeleev's predictions
• How are elements arranged in the modern periodic table?

  By atomic number, or number of protons
• What is a problem with ordering elements by atomic weight?

  Elements can appear in the wrong order
• What did Mendeleev do to address the problem of isotopes?

  He switched the order of elements as needed
• What group was added to the modern periodic table that Mendeleev did not include?

  Group zero, the noble gases
• Why were noble gases not included in Mendeleev's table?

  They had not been fully discovered yet
• What are the key differences between Mendeleev's periodic table and the modern periodic table?

  • Mendeleev arranged by atomic weight; modern by atomic number
  • Modern table includes group zero (noble gases)
  • Mendeleev left gaps for undiscovered elements
• What are the historical attempts to arrange elements before Mendeleev?

  1. Dober's triads: elements in groups of three
  2. Newlands' law of octaves: every eighth element similar
  3. Issues with grouping elements with different properties
• what is atomic weight

  an average of the atomic mass of isotopes