When did Mendeleev create a new row in his periodic table?

• A. After every five elements
• B. When the atomic mass increased by ten units
• C. Whenever he discovered a new element
• D. When the properties of the elements began to repeat periodically

Answer: (D)

Verified step by step guidance

1. Understand that Mendeleev's periodic table was organized based on the periodicity of element properties rather than a fixed number of elements or atomic mass increments.
2. Recall that Mendeleev arranged elements in order of increasing atomic mass but noticed that properties of elements repeated at regular intervals.
3. Recognize that a 'new row' or period in the periodic table was created whenever the properties of the elements began to repeat, indicating a new cycle of chemical behavior.
4. This means the length of each row was not fixed by a number of elements or a specific mass increase, but by the recurrence of similar chemical properties.
5. Therefore, Mendeleev's creation of a new row was guided by the periodic repetition of element properties, which is the fundamental principle behind the periodic table.