When atoms become ions, they change because they either gain or lose electrons. An ion is simply a charged particle that forms when an atom either loses or adds electrons.

What is Electron Configuration?

Electron configuration shows how electrons are arranged around an atom's nucleus in different energy levels, which are like shells. Each shell is numbered, starting from 1, with 1 being the closest to the center of the atom. The formula to find out how many electrons each shell can hold is $2n^2$, where "n" is the shell number. Here’s a quick look:

• First shell (n=1): Can hold up to 2 electrons
• Second shell (n=2): Can hold up to 8 electrons
• Third shell (n=3): Can hold up to 18 electrons

How Cations Form (Positively Charged Ions)

When an atom loses one or more electrons, it turns into a cation. The way the electrons are arranged changes based on how many electrons are lost.

For example:

• Sodium (Na) has an atomic number of 11, with an electron setup of $1s^2 2s^2 2p^6 3s^1$.
• If sodium loses one electron, it becomes a $\text{Na}^+$ ion and its electron setup changes to $1s^2 2s^2 2p^6$. This shows that the outer shell, which is the third shell, is now empty.

Cations usually form from metals because they easily lose electrons. Sodium, for instance, doesn’t need too much energy to lose an electron, making it easier for it to become positive.

How Anions Form (Negatively Charged Ions)

On the flip side, when an atom gains electrons, it becomes an anion. The electron arrangement changes based on the number of electrons gained.

For example:

• Chlorine (Cl) has an atomic number of 17, with an electron setup of $1s^2 2s^2 2p^6 3s^2 3p^5$.
• When chlorine gains an electron, it turns into a $\text{Cl}^-$ ion. Now, its electron setup is $1s^2 2s^2 2p^6 3s^2 3p^6$. This completes its outer shell and makes it stable.

Anions often form from nonmetals that have a stronger ability to attract and hold onto electrons. For example, chlorine has a high electronegativity of 3.16, which means it really likes to gain electrons.

Quick Review of Ion Formation

1. Cations (positive ions)

   • Form by losing electrons.
   • Commonly found in metals.
   • Example: Sodium ($\text{Na} \rightarrow \text{Na}^+ + e^-$).

2. Anions (negative ions)

   • Form by gaining electrons.
   • Commonly found in nonmetals.
   • Example: Chlorine ($\text{Cl} + e^- \rightarrow \text{Cl}^-$).

Key Facts

• Most elements want to have a full outer shell of eight electrons. This is called the octet rule.
• About 75% of the elements on the periodic table can form ions.
• The energy needed to remove electrons varies a lot, with alkali metals having the lowest energy needed (like lithium at 520 kJ/mol) and noble gases needing a lot more (like neon at 2080 kJ/mol).

Understanding how electron arrangements change when ions form helps us to learn about chemical bonds and how different substances react in chemical reactions.