Question

What is the last element in the period $4$ ?

Answer 1

The ${4^{th}}$ period is the fourth row of the modern periodic table. We can see that it has four shells. Also it will contain eighteen elements. This period starts from Potassium and will end at a noble gas. This noble gas will have fully filled shells because after that the fifth period starts.

Complete answer:
The modern periodic table consists of seven periods and eighteen groups in whole which contain different elements. They all are classified under $s -$ block elements, $p -$ block elements, $d -$ block elements and $f -$ block elements. All the elements of the modern periodic table come under these categories. The number of periods will suggest to us the number of shells it contains. Likewise its ${4^{th}}$ period will contain four shells and hence its principal quantum number is four. The very first element of every period must be a metal and it ends with a noble gas. The noble gases are inert in nature. The fourth period contains eighteen numbers of elements. As said earlier the very first element of the period is a metal. Here also the first element of the fourth period is a metal called potassium. Potassium is the member of a group called alkali metals. The atomic number of potassium is $19$ . That is the fourth period starts with atomic number $19$ . While moving from left to right in ${4^{th}}$ period we encounter transition elements. These are scandium, titanium, vanadium, chromium, manganese, iron, nickel, copper and zinc. At the last of this period there is a noble called krypton.
Thus krypton is the last element of the ${4^{th}}$ period. The atomic number of krypton is $36$ .

Note:
Noble gases are the most stable elements in the modern periodic table. This is because the noble gases have fully filled shells. The fully filled shells are the most stable in nature. Transition elements are the $d -$ block elements.