Question

For each of the following atoms or ions, state the total number of electrons, the number of valence electrons, and the number of core electrons.

a) Rn

b)${\mathbf{Sr}}^{\mathbf{+}\mathbf{1}}$

c)${\mathbf{Se}}^{\mathbf{2}\mathbf{-}}$

d)role="math" localid="1663668460405" $\mathit{s}{\mathit{b}}^{\mathbf{-}}$

Short answer

a) 86

b) 37

c) 36

d) 52

Step-by-step solution

Introduction

Atoms are tiny particles that formany matter. Atoms are neutral, asthe number of protons and electrons are equal. Atoms cannot exist independently except for inert gases.

Ions are the species that are charged and where the number of protons and electrons are not equal. In orderto form stable atoms, they must have 8 valence electrons in the outermost shell and if they do not have 8, they try to attain a complete octet either by losing electrons or by gaining electrons.

If they lose electrons, they become positively charged (cation) and if they gain electrons, they become negatively charged (anion).

Core electrons are all electrons present in the inner energy levels with completely filled d and f subshells.

 Step 2: a) Rn

The atomic number of Rn is 86,

• The total number of electrons is equal to the atomic number so the total number of electrons is 86.
• The electrons in the outermost shell are called valence electrons and the configuration of radon is 2,8,18,32,18,8, hence the number of valence electrons is 8.

$Rd=\left[\text{Xe}\right] {\text{4f}}^{\text{14}}{\text{,5d}}^{\text{10}}{\text{,6s}}^{\text{2}}{\text{,6p}}^{\text{6}}$

• The highest energy subshells are 6s and 6p, so excluding electrons from these two subshells, all the inner electrons are core electrons. So, the total number of core electrons is 78.

b) Sr+

The atomic number of Sr is 38 and the electronic configuration is 2,8,18,8,2. After losing one electron from its valence shell it becomes$S{r}^{+}$

• The total number of electrons is 37.
• The electrons in the outermost shell are called valence electrons and the configuration of$S{r}^{+}$is 2,8,18,8,1. Hence, the number of valence electrons is 1.

${\text{Sr}}^{\text{+}}=1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^{10},4p^6,5s^1$

• The highest energy subshell is 5s, so excluding electrons from the 5s subshell, all the inner electrons are core electrons. Therefore, the total number of core electrons is 36.

c) Se2-

The atomic number of Se is 34 and the electronic configuration is 2,8,18,6. After gaining two electrons it becomes${\text{Se}}^{\text{2 -}}$.

• The total number of electrons is 36.
• The electrons in the outermost shell are called valence electrons and the configuration of${\text{Se}}^{\text{2 -}}$ is 2,8,18,8. Hence the number of valence electrons is 8.

${\text{Se}}^{\text{2 -}}=1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^{10},4p^6$

• The highest energy subshells are 4s and 4p, so excluding electrons fromthese two subshells, all the inner electrons are core electrons. Therefore, the total number of core electrons is 28.

d) Sb-

The atomic number of Se is 51 and the electronic configuration is 2,8,18,18,5. After gaining one electron it becomes${\text{Sb}}^{\text{-}}$

• The total number of electrons in${\text{Sb}}^{\text{-}}$is 52.
• The electrons in the outermost shell are called valence electrons and the configuration of${\text{Sb}}^{\text{-}}$ is 2,8,18,18,6 hence the number of valence electrons is 6.

${\text{Sb}}^{\text{-}}=1s^2,2s^2,2p^6,3s^2,3p^6,4s^2,3d^{10},4p^6,4d^{10},5s^{2}5p^4$

• The highest energy subshells are 5s and 5p, so excluding electrons from these two subshells, all the inner electrons are core electrons. Hence, the total number of core electrons is 46.