Question

sections of the periodic table, showing electron configuration of valence electrons
sentence: the periodic law tells us how elements can be arranged by increasing atomic number and repetition of their
properties
choose the best answer.
which contributions did dmitri mendeleev make to the classification of elements?
a arranged the elements in order of increasing atomic mass
b predicted the existence and properties of undiscovered elements
c demonstrated a connection between atomic mass and elements properties
d indicated that the properties of elements were due to the number of electrons

Explanation:

Dmitri Mendeleev's key contribution to the periodic table was predicting the existence and properties of undiscovered elements. Option A is about arranging by atomic mass (he did arrange by atomic mass, but the more distinctive contribution is prediction). Option C is about atomic mass - element properties connection (not his main distinct contribution). Option D is about electron number - element properties (this is related to later work on atomic number, not Mendeleev). So the best answer is B as his prediction of undiscovered elements was a major contribution.

Answer:

B. predicted the existence and properties of undiscovered elements