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?

Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements ?

What property do all elements in the same column of the Periodic Table as fluorine have in common?

What were the criteria used by Mendeléev in creating his Periodic Table ?

Nitrogen (atomic number $7$) and phosphorus (atomic number $15$) belong to group $15$ of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?