Classification Of Elements And Periodicity In Properties

Step 1: Periodic Law
Mendeleev's periodic law states that the properties of elements are periodic functions of their atomic weights, but Moseley's work later confirmed that the properties of elements are periodic functions of their atomic numbers.
The periodic table is now arranged by atomic number.
Step 2: Explanation of Other Options
Option (a) is incorrect because elements are now arranged by atomic number, not atomic weight, due to isotopes.
Option (c) is incorrect because chemical properties are related to atomic number but are not the periodic function themselves.

Option (d) is incorrect because the number of protons (atomic number) is what influences periodicity, not just the number of protons alone.

Step 3: Conclusion Thus, the attributes of corresponding elements are the periodic functions of the atomic number.

Step 1: Ionization Energy Trends
The ionization energy is the energy required to remove an electron from an atom.
Ionization energies generally increase across a period from left to right in the periodic table.
However, there are some exceptions due to the electron configurations of the elements.
Step 2: Explanation of the Trend for the Given Elements
has the lowest ionization energy because its electron configuration is stable with a fully filled orbital.

has a slightly lower ionization energy than because removing an electron from a orbital in is easier than removing one from a more stable orbital in .

has a higher ionization energy than because oxygen has paired electrons in its orbitals, which experience repulsion, making it easier to ionize.

Step 3: Conclusion Thus, the correct order is .