# I need some help understanding how some ions have the same electronic arrangement as noble gasses? Like why does a Li+ ion has the same electron structure as helium or a Mg2+ ion has the same electron structure as a Na+ ion and an O2- ion?

Apr 22, 2017

A noble gas electron structure where the outmost electron shell is completely filled. Elements other than VIII A elements can achieve a completely filled outer shell by gaining or losing electrons.

#### Explanation:

A completely filled outer electron shell is stable because it can not gain any more electrons, it is full. Being completely full it has no desire to lose electrons thereby losing its stable structure.

Neon for example has 10 electrons. Its electron configuration is
$1 {s}^{2} 2 {s}^{2} 2 {p}^{6}$
The $1 {s}^{2}$ is the structure of Helium a completely filled first electron shell. These two electrons are very stable.

${2}^{2} {p}^{6}$ is a completely fill second electron shell. This is a very stable electron configuration.

Oxygen starts with an electron configuration of
$1 {s}^{2} 2 {s}^{2} 2 {p}^{4}$

This is two electrons short of the stable structure of Neon. By gaining two electrons Oxygen can achieve this stable structure so
${O}^{-} 2 = 1 {s}^{2} 2 {s}^{2} 2 {p}^{6}$

Sodium starts with an electron configuration of
$1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{1}$

This is one more electron than the stable structure of Neon. By losing one electron Sodium can achieve Neon stable structure so
$N {a}^{+} 1 = 1 {s}^{2} 2 {s}^{2} 2 {p}^{6}$

Elements electron configurations are all about achieving stability.
The most stable electron configurations are those of the noble gases.

May 18, 2017

Here's what I get.

#### Explanation:

The electron configuration of helium is ${\text{1s}}^{2}$.

The electron configuration of lithium is $\text{1s"^2 "2s}$.

To form $\text{Li"^"+}$, we must remove the outermost $\text{2s}$ electron from $\text{Li}$.

The electron configuration of $\text{Li"^"+}$ becomes ${\text{1s}}^{2}$.

That's the same electron configuration as for $\text{He}$.

The electron configuration of $\text{Mg}$ is ${\text{1s"^2 "2s"^2 "2p"^6 "3s}}^{2}$.
The electron configuration of $\text{Mg"^"2+}$ is ${\text{1s"^2 "2s"^2 "2p}}^{6}$.

The electron configuration of $\text{Na}$ is $\text{1s"^2 "2s"^2 "2p"^6 "3s}$.
The electron configuration of $\text{Na"^"+}$ is ${\text{1s"^2 "2s"^2 "2p}}^{6}$.

The electron configuration of $\text{O}$ is ${\text{1s"^2 "2s"^2 "2p}}^{4}$.
The electron configuration of $\text{O"^"2-}$ is ${\text{1s"^2 "2s"^2 "2p}}^{6}$.

In each case we formed the ions by adding or removing electrons and ended up with the same ${\text{1s"^2 "2s"^2 "2p}}^{6}$ electron configuration.