(a) Identify the element X. How many valence electrons does it have?
Ans. Nitrogen, valence electrons=5

(b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
Ans. Electronic dot structure is as follows. Covalent bond is formed in it.

(c) Draw the electron dot structure for ammonia and what type of bond is formed in it?
Ans. Covalent bond is formed

Inert gases could be placed in Mendeleev’s Periodic Table without disturbing the original order.

 Noble gases; like Helium, Neon and Argon had been mentioned by various scientists even before Mendeleev. But these gases could be grouped together as noble gases much later than Mendeleev’s discovery. Mendeleev used 63 elements till his periodic table because these were the only known elements till his time. However, he was bold enough to leave gaps in his periodic table. He had predicted that that more elements would be discovered in times to come.
 
 Mendeleev’s Periodic Table was the first comprehensive attempt to classify elements on the basis of their chemical properties. This is the main reason that new elements could be easily placed in Mendeleev’s Periodic Table without disturbing the original order.

During Mendeleev’s time, only 63 elements were known. Mendeleev examined the correlation between chemical properties and Atomic masses of elements. For this, he concentrated on compounds which were formed by the elements with hydrogen and oxygen. He selected hydrogen and oxygen, because these elements make compound with most of the other elements.

 Mendeleev made 63 cards and wrote the name of an element on each card; along with the chemical properties of a particular element. Then, he pinned those cards on a wall. He could observe that most of the elements could be arranged in periodic table in increasing order of their atomic masses. That is how Mendeleev arrived at “Periodic Law”.
 
Mendeleev’s Periodic Law states that properties of elements are periodic function of their atomic masses.