In the Modern Periodic Table,calcium (atomic number $20$) is surrounded by elements with atomic numbers $12$,$19$,$21$ and $38$. Which of these have physical and chemical properties resembling calcium?

Which of the following statement$(s)$ about the Modern Periodic Table are incorrect?
$(i)$ The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number.
$(ii)$ The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses.
$(iii)$ Isotopes are placed in adjoining group$(s)$ in the Periodic Table.
$(iv)$ The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number.

Which element has:
$(a)$ two shells,both of which are completely filled with electrons?
$(b)$ the electronic configuration $2, 8, 2$?

Which of the given elements $A, B, C, D$ and $E$ with atomic numbers $2, 3, 7, 10$ and $30$ respectively belong to the same period?

The elements $A, B, C, D$ and $E$ have atomic numbers $9, 11, 17, 12$ and $13$ respectively. Which pair of elements belong to the same group?

The element with atomic number $18$ belongs to which group?