Answers:
The true statements are:
a. It contains more elements than Mendeleev's periodic table
c. It is now arranged by increasing atomic number instead of increasing atomic mass.
Justification:
Dimitri Mendeleev (1834 - 1907) created the first useful and widely accepted periodic table. He organized the elements in increasing order of atomic masses, and found a pattern on their properties. He left blank spaces when found that some properties did not follow the patterns, and was able to predict the existence of unknwon elements along with their properties.
Let's see every proposed statement comparing the current (modern) periodic table to establish its validity:
a. It contains more elements than Mendeleev's periodic table
This is true. The current periodic table contents 118 elements (some discovered and named only few years ago), whereas Medeleev's periodic table had some 66 elements.
b. Mendeleev's periodic table didn't group the elements, but were just a long list
This is false. Mendeleev grouped the elements in ascending number of atomic masses and arranged them in rows and columns according to the similarity in their properties.
c. It is now arranged by increasing atomic number instead of increasing atomic mass.
True. As said, Meedeleev arranged by increasing atomic number. This caused that some elements appeared placed in groups with elements of different properties. It was Henry Moseley, by 1913, who discovered that the order by the number of protons (atomic number) was more appropiated.
d. The atomic masses on the modern periodic table are much different than they were on Mendeleev's periodic table
False. Except for counted exceptions (i.e. Cerium), the atomic masses on modern periodic table differ slightly of those in Mendeleev's periodic table (in the tenths units mainly). The main reason of these tiny discrepances is the change of the pattern (originally hydrogen and oxygen were used as the standards and currently the standard is the isotope carbon-12).
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