What is the usual number of valence electrons and valency of group $18$ elements of the periodic table?

What happens to the number of valence electrons in the atoms of elements as we go down in a group of the periodic table?

What is the main characteristics of the last elements in the periods of the periodic table? What is the general name of the such elements?

What is the number of elements in the $1^{\mathrm{st}}$ and $3^{\mathrm{rd}}$ period of the modern periodic table?

How does atomic size vary on going down from top to bottom in a group of the periodic table? Why does it vary this why?

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

How does the tendency to lose electrons change as we go down in group $1$ of the periodic table? Why does it change this way?

How does the tendency to gain electrons change as we go down in group $17$ of the periodic table? Why does it change this way?

Why does the size of the atom progressively become smaller when we move from sodium $\left(\mathrm{Na}\right)$ to chlorine $\left(\mathrm{Cl}\right)$ in the third period of the periodic table?