Question

multiple - choice question
why did the periodic table initially not follow a pattern?
because mendeleev was ordering the table by atomic mass.
because there is no pattern to the natural world.
because some elements had not been discovered yet.

Explanation:

Mendeleev ordered the periodic table by atomic mass initially. However, there were some discrepancies. For example, tellurium (Te) has a higher atomic mass than iodine (I) but was placed before it in the table based on chemical properties. This was because ordering by atomic mass alone did not account for all chemical and physical trends perfectly. There is a pattern in the natural world (the modern periodic table, ordered by atomic number, shows this). And the fact that some elements were undiscovered is not the reason for the initial non - perfect pattern. The main issue was the ordering principle (atomic mass).

Answer:

Because Mendeleev was ordering the table by atomic mass.