Answer

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**Hint**The fifth period contains 18 elements, beginning with rubidium and ending with xenon. As a rule, period 5 elements fill their 5s shells first, then their 4d, and 5p shells, in that order; however, there are exceptions, such as rhodium.

**Complete step by step answer**

The 5th period of periodic table contains 18 elements not 32

N=5, l=0, 1, 2, 3. The order in which the energy of available orbitals 4d, 5s and 5p increases is \[5s < 4d < 5p\] and the total number of orbitals available are 9 and thus 18 electrons can be accommodated.

In other words, in the fifth period, f orbitals are not filled.

Thus, both assertion and reason are correct and reason is the correct explanation for assertion.

For 5TH period, principal quantum number, n=5 and values of azimuthal quantum number, l=0, 1,2,3

According to Aufbau principle, filling of orbitals in subshells takes place in increasing order of energy of subshell .Since energy of 4f subshell is higher than 5s subshell, the sub shells available for filling are 5s, 4d, 5p.

Therefore, total number of orbitals =1 for 5s+ 5 for 4d + 3 for 5p

\[ = 1{\text{ }} + {\text{ }}5{\text{ }} + {\text{ }}3{\text{ }} = 9\]

Therefore, total number of electrons that can be accommodated in 9 orbitals\[\; = {\text{ }}2{\text{ }}x{\text{ }}9{\text{ }} = {\text{ }}18\]

Hence, the number of electrons or elements in 5th period are 18

**Hence, option A is correct.**

**Additional information**

-Principle quantum number

1.It is denoted by n

2.It tells us the main energy level or shell in which the electron may be present.

3.Its value can be an integer except zero.

4.The energy levels are also denoted by letters K, L , M, N ……

5.The energy of the various shells follow the sequence : \[1{\text{ }} < {\text{ }}2{\text{ }} < {\text{ }}3{\text{ }} < {\text{ }}4\]

6.The total number of electrons present in any main shell is given by \[2{n^2}\]

-Azimuthal quantum number

1.It is also known as subsidiary or angular momentum quantum number

2.It is denotes by l

3.It arises from the explanation that electrons I main energy level do not have the same energy in case of a multi electron system. This is because the electrons move along different paths and hence have different angular momentum.

4.It tells us the number of subshells present within the main shell.

5.In general, we can say that the number of subshells present in any main shell is equal to the number of the main shell

6.Energy sequence is \[\;s < {\text{ }}p < d{\text{ }} < {\text{ }}f\]

**Note:**

Aufbau‘s principle-

It is also known as (n+l ) rule . According to this principle, during electron filling in an atom, the orbital having lowest energy is filled first followed by the orbitals of higher energy.

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