Development of the Periodic Table

• In the early 1800s, how were elements arranged in the periodic table?

  Arranged by atomic weight
• Why do we use the word ‘periodic’ in the periodic table?

  Scientists noticed a ‘periodic’ pattern in the properties of elements, i.e. a pattern that repeats
• Why were elements often placed in the wrong group in early versions of the periodic table?


  There were placed in order of atomic weight rather than atomic mass, and didn’t take into account the elements have chemical properties
• How did Mendeleev arrange his periodic table?

  He mainly ordered the elements by increasing atomic weight. He arranged elements that had similar properties into rows (periods). He left gaps
• Why did Mendeleev leave gaps in his table?

  To ensure that elements with similar properties stayed in the same group, and to leave space for elements that hadn’t been discovered yet
• How did they discovery of isotopes help to validate Mendeleev’d periodic table?

  Some isotopes had different masses but the same chemical properties - so he was correct in putting them in groups with other elements that had the same chemical properties
• How are elements arranged in the modern periodic table?

  Increasing atomic number. Patterns of chemical and physical properties (periods/groups)
• Why are elements put into groups?

  To group the elements that have similar properties
• What does the group number tell you about the atoms of an element?

  The group number tells you the number of electrons in the outer shell. E.g. all elements in group 3 will have 3 electrons in their outer shell
• What does the period tell you about the atoms of an element?

  The number of shells an atom has. E.g all elements in period 2 have energy shells
• What name is given to the elements in group 1?

  Alkali Metals
• What names is given to the elements in group 2?

  Alkaline earth metals
• What name is given to the elements in group 7?

  Halogens
• What names is given to the elements in group 0? (Remember group 0 is the group one space to the right of group 7)

  Noble gases
• Why did Mendeleev think there must be undiscovered elements?

  If the elements known about at the time were placed consecutively, some elements would be placed into groups where they had different chemical properties from the other elements in the group. This meant they would be in the ‘wrong’ group, and so Mendeleev thought some undiscovered element must go in that place instead