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Periodic Table Mystery

By Chuck Roser, Retired Chemistry Instructor
North Carolina School of Science and Mathematics

Lab Safety

National Science Education Standards

Grades 9–12

(HU2—Evidence and Models, HB2—Structure and Properties of Matter)


Using coded symbols for the main group elements in the first 4 periods of the periodic table, students will generate a periodic table from pertinent clues. They will use their knowledge of physical properties and periodic properties to predict the missing properties of several elements based on the elements’ locations in the table.


The periodic table used today is a product of the 1st periodic table published by Russian chemist Dmitri Mendeleev in l869. The amazing accuracy of his predictions for as-yet unknown elements, using trends within groups and periods, has been very significant to chemists. The bases of his periodic table were the physical properties, the chemical properties, and the atomic masses of the elements rather than atomic numbers. Henry Moseley rearranged the Mendeleev Periodic Table based on atomic numbers of the elements. In accordance with this modification, the periodic law states that the properties of the elements are periodic functions of their atomic number.

Each known element possesses a complete set of properties that fully characterizes it and distinguishes it from all other elements. The arrangement of the elements into groups or families (vertically) and into periods (horizontally) reflects the periodic or repeating nature of many of the properties of the elements.

Pre-activity questions

  1. List 4 properties that distinguish metals from nonmetals.
  2. The elements on the periodic table are essentially in order of increasing atomic mass but are in exact order of increasing atomic number. What pairs of elements are exceptions to the increasing order of average atomic mass?


Place the unknown main group elements on the blank periodic table using the clues provided. See if you understand the patterns. The following sets of elements belong together in groups but not necessarily in this order:



  • U has a total of 6 electrons in its neutral atom
  • I2A is the simple formula of an oxide
  • P is less dense than S
  • S is an alkali metal
  • E is a noble gas
  • W is a liquid at room temperature
  • Z has the highest ionization energy in its group
  • B has 10 protons
  • O has a lower chemical reactivity and lower ionization energy than V
  • D has the lowest electron affinity of its group
  • C has 5 electrons in its outer energy level
  • F is a colorless gas
  • X has an atomic number one higher than F
  • Y is a metalloid
  • O is a halogen
  • T has a higher chemical reactivity than that of H
  • J has an atomic number 3 times that of T
  • Q has an atomic mass 2 times that of A
  • Atoms of I are larger than those of S
  • M has a larger atomic radius than that of A
  • The electrons of atom N are distributed over 3 major energy levels
  • The atomic radius of K is the largest of the group

Post-activity questions

  1. Is L a metal, nonmetal, or metalloid? Support your answer.
  2. What is the chemical formula of the chloride of G? Support your answer.
  3. Does R conduct electricity as a solid? Support your answer.

Pre-activity answers

  1. Metals have metallic luster (shine) and nonmetals do not.
    Metals are malleable and ductile and nonmetals are not.
    Metals are electrical conductors and nonmetals are nonconductors (except for graphite).
    Metals are good heat conductors and nonmetals are poor heat conductors.
  2. The pairs are Ar and K, Co and Ni, Te and I. Higher percentages of heavier isotopes account for these exceptions.

Post-activity answers

  1. L is a metal. All Group 2 elements are metals.
  2. The formula is GCl4. G has 4 valence electrons and chlorine has 7 valence electrons.
  3. R conducts electricity as a solid since it is a metal.
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