Question

predicting properties of undiscovered elements
when mendeleev arranged all known elements by increasing atomic mass,
there were large gaps between some elements. he predicted that scientists
would discover elements that would fit into these spaces. mendeleev also
predicted that the properties of these elements would be similar to the known
elements in the same columns. both of his predictions turned out to be true.
changes to mendeleevs table
mendeleevs periodic table enabled scientists to relate the properties of the
known elements to their position on the table. however, the table had a
problem-some elements seemed out of place
mendeleev believed that the atomic masses of certain elements must be
incorrect because the elements appeared in the wrong place on the periodic
table. for example, mendeleev placed tellurium before iodine despite the fact
that tellurium has a greater atomic mass than iodine. he did so because
iodines properties more closely resemble those of fluorine and chlorine, just
as coppers properties are closer to those of silver and gold, as shown in the
figure below.

identify what property did mendeleev use to organize the periodic table?

Explanation:

The text states "Mendeleev arranged all known elements by increasing atomic mass." This directly indicates the property used for organization.

Answer:

Atomic mass.