Question

In the modern long form of periodic table, 4" period contains only 18 elements. It is due to filling of
electrons in the:
a) 4s, 4p and 4d sub-shells
b) 4s, 3d and 4p sub-shells c) 4p, 4d and 4f sub-shells
d) 3s, 3p and 3d sub-shells​

Answer 1

Answer:

the long form of the periodic table, each period starts with the filling up of a new principal shell. Clearly, the 4th period will begin with filling of 4th shell i.e. N shell. Thus, fourth period starts with the filling up of 4s orbtial (n = 4). After filling 4s orbital, the filling of 3rd and then 4p takes place. It is so because enthalpy of 3d sub level in between 4s and 4p sub levels. As,

4s = 2 electrons

3d = 10 electrons open square brackets because space 4 straight s space less than space 3 straight d space less than space 4 straight p close square brackets

4p = 6 electrons

_______________

18 electrons

Therefore, 18 elements (not eight) are present in the 4th period. The 4d and 4f sublevels are higher in enthalpy than 5s and hence are filled up in the next periods.

Answer 2

The correct answer to this question is option b) 4s, 3d, and 4p sub-shells.

Explanation:

• The Fourth period agrees to the packing of electrons in the fourth energy level, n=4. It begins with Potassium (Z=19) and ends with calcium (Z= 20).  

• Once the filling of 4s orbital is done, the filling of five 3d orbitals starts since the energy of 3d orbital is lesser than those of 4p orbitals but greater than that of 4s orbital.  

• The filling of 4d and 4f orbital doesn’t take place in this period since their energies are higher than that of 5s orbital.  

• The filling of only 9 orbitals (one 4s, five 3d, and three 4p) happens which can accommodate at most 18 electrons. Therefore the 4th period comprises 18 electrons from Potassium to Krypton.