What is the number of elements in : (a) 1^st period and (b) 3^rd period, of the modern 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 characteristic of the last elements in the periods of the periodic table ? What is the general name of such elements ?

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

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