Question

1. write the electron configuration, for sr and sr⁺². how many electrons are in the valence energy level?

sr:_______________________
sr⁺²:_______________________
which neutral atom is isoelectronic with sr⁺²? _______________________
how many valence electrons does neutral sr have? _______________________

Explanation:

Step1: Determine Sr's electron configuration

Strontium (Sr) has an atomic number of 38. The electron configuration follows the Aufbau principle. The noble gas before Sr is Krypton (Kr, atomic number 36). So the electron configuration of Sr is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 5s^2$ or $[Kr] 5s^2$.

Step2: Determine $Sr^{2+}$'s electron configuration

When Sr loses 2 electrons (to form $Sr^{2+}$), it loses the 2 electrons from the outermost (5s) orbital. So the electron configuration of $Sr^{2+}$ is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6$ or $[Kr]$.

Step3: Find isoelectronic neutral atom with $Sr^{2+}$

Isoelectronic means having the same number of electrons. $Sr^{2+}$ has 38 - 2 = 36 electrons. A neutral atom with 36 electrons is Krypton (Kr), since Kr has an atomic number of 36.

Step4: Determine valence electrons of neutral Sr

Valence electrons are the electrons in the outermost energy level. For Sr, the outermost energy level is n = 5, and the electron configuration of the outermost level is $5s^2$. So there are 2 valence electrons.

Answer:

Sr: $\boldsymbol{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 5s^2}$ (or $\boldsymbol{[Kr] 5s^2}$)
$Sr^{2+}$: $\boldsymbol{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6}$ (or $\boldsymbol{[Kr]}$)
Isoelectronic neutral atom with $Sr^{2+}$: $\boldsymbol{Kr}$ (Krypton)
Valence electrons of neutral Sr: $\boldsymbol{2}$