In the modern periodic table, the element calcium (atomic number = $20$) is surrounded by elements with atomic number $12, 19, 21 \& 38$. Which of these elements has physical and chemical properties resembling those of calcium and why?

Explain the variation in the following property of the elements in the periodic table.

Atomic radius in a period.

Explain the variation in the following property of the elements in the periodic table.

Metallic character in a period.

Explain the variation in the following property of the elements in the periodic table.

Valency in a group.

The elements $\mathrm{Li}, \mathrm{Na}$ and $\mathrm{K}$ each having one valence electron, are in period $2, 3$ and $4$ respectively of modern periodic table.

In which group of the periodic table should they be? Give reason to justify your answer.

The elements $\mathrm{Li}, \mathrm{Na}$ and $\mathrm{K}$ each having one valence electron, are in period $2, 3$ and $4$ respectively of modern periodic table.

Which one of them is least reactive? Give reason to justify your answer.

The elements $\mathrm{Li}, \mathrm{Na}$ and $\mathrm{K}$ each having one valence electron, are in period $2, 3$ and $4$ respectively of modern periodic table.

Which one of them has the largest atomic radius? Give reason to justify your answer.