Atomic Numbers and Electron Configurations

Question 13 • *26-27 CFISD TX-Chemistry A CR

Why did Mendeleev leave blank spaces in his version of the periodic table?

Answer

Mendeleev’s periodic table arranged elements in order of increasing atomic mass; spaces were left for elements not having the correct atomic mass.

Mendeleev’s periodic table arranged elements in groups of similar chemical properties; spaces were left for elements not having a similar chemical property.

Mendeleev’s periodic table arranged elements in order of increasing atomic mass; it was noticed that chemical properties repeated. Mendeleev predicted an element had not been discovered to fit the space.

Explanation

Mendeleev arranged the known elements mainly by increasing atomic mass and noticed that their chemical properties repeated in a pattern. When the pattern indicated a missing element, he left a space rather than disrupting the arrangement. He predicted that an undiscovered element would eventually be found to fill it, so choice C is correct.

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