Chapter 11, Problem 42A

Interpretation Introduction

Interpretation: Presence of valence electrons is expected to be in $4d$ or $5s$ orbital for rubidium and strontium atom is to be explained.

Concept introduction: 3-D space around nucleus in which probability of finding an electron of particular energy is maximum, is called orbital. Quantum numbers are group of 4 numbers which tell everything about electron present in an atom. Principal quantum numbers indicates principal energy level in which electron in present. It is denoted by $n$ .

Explanation of Solution

As per Aufbau’s rule electrons are filled in lower energy orbitals that are closer to the nucleus before they are filled in higher energy ones. The order of orbital arranged in their increasing energies is as follows:

  $\begin{array}{l}1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<\\ 6s<4f<5d<6p<7s<5f<6d<7p\end{array}$ .

Higher orbital are indicated by higher

  $n$ value prefixed to symbol of orbital. That is among $1s$ and $2s$ , $2s$ has higher energy value.

Also, while filling of electrons, in a principal energy level $\left(n+1\right)s$ orbital is always filled before $nd$ orbital. That is in $n=4$ shell, $\left(n+1\right)s$ orbital which is $5s$ is filled before $nd$ which is $4d$ orbital.

But, when removal of electrons takes place the electron will remove first from 5s followed by 4d.

Therefore, valence electrons of Rubidium with $Z=37$ and strontium atom with $Z=38$ reside in $5s$ orbital and not $4d$ orbital.