Why Fe3+ is more stable than Fe2+?

You are going to solve this puzzle about why Fe³⁺ is more stable than Fe²⁺. Fe²⁺ and Fe³⁺ may differ in their oxidation state and physical and chemical properties. They may also differ in their names. Fe²⁺ is ferrous and Fe³⁺ is ferric. The compounds of Fe2+ and Fe³⁺ are ionic in nature.

Reason Why Fe3+ is more stable?

If we imagine the electronic configuration of  Fe²⁺ and Fe³⁺, we should observe that Fe³⁺ is more stable. In the configuration of Fe²⁺, the 2 electrons are removed from the 4s orbital because the nucleus attracts other prior orbitals with more attraction so it is easy to remove. on the other hand, the half-filled and completely-filled orbitals are more stable than that of partially filled. so in a configuration of Fe³⁺, there is half filled d orbital and in Fe²⁺ there is a partially filled d orbital.

Fe2+ (Ferrous):

• It is pale yellow and turns violet when reacting with water. 
• Fe²⁺ shows the paramagnetic property. If it forms a low-spin complex it may be diamagnetic. Paramagnetic property is the property that shows that it has an unpaired electron in its outermost shell.
• Fe²⁺ oxidizes to the Fe³⁺ ion in the presence of KMnO4, and HNO3. In addition to KMnO4, the pink color it decolorizes shows the exact oxidation of Fe2+.
• Another colored complexes test to distinguish between Fe²⁺ and Fe³⁺ is that Fe²⁺ forms a red-orange color when reacting with amine ligands.

Fe3+ (Ferric):

• It is yellow-brown when reacting with the water.
• Fe³⁺ shows paramagnetic behavior. 
• Fe³⁺ gives the read solution when reacting with the thiocyanate ions. This method is used to flake blood in movies and films.
• The basic difference between Fe²⁺ and Fe³⁺ is the number of electrons. 

General configuration of Fe:

Iron is a chemical element with the atomic number 26. Its oxidation states range from -2 to +6. But now here I will illustrate the configuration of Fe²⁺ and Fe³⁺. The general configuration of Fe is [Ar] 3d⁶, 4s².

Variable Electrovalency:

Some elements show more than one valency is known as variable valency. For example, in ferric sulfate Fe₂(SO₄)₃, the valency of Fe is 3. In ferrous sulfate FeSO₄, the valency of Fe is 2. Variable electrovalency depends upon the stability of the core. If all the valence electrons are removed by an atom, the residue obtained is called the core. The core obtained from the normal element is more stable if it has 2 or 8 electrons. The electronic configuration of the sodium atom is:

1s², 2s², 2p⁶, 3s¹

When sodium removes its 3s¹ electron and attains the electronic configuration of noble gas (neon), the residue obtained is called the core. The remaining residue is stable if it has two or eight electrons or vice versa. The d-block elements show variable valency.

Example Along Period

Fe₂₆

Fe₂₆ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁶

When two electrons are removed from Fe, then we get;

Fe²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁶

We remove 4s² in Fe₂₆ because the fourth shell is completely removed and the nucleus attracts the remaining three shells with greater force. When three electrons are removed from Fe₂₆ than we get;

Fe³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁵

This Fe³⁺ is more balanced than Fe²⁺ because half-filled and completely filled is more stable than partially-filled.

Co₂₇

Co₂₇ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁷

When two electrons are removed, then we get;

Co²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁷

When three electrons are removed from Co₂₇, then we get;

Co³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁶

Co²⁺ is more stable than Co. When we compare Co²⁺ and Co³⁺, both are equally stable but Co³⁺ is slightly more stable than Co²⁺.

Ni₂₈

Ni₂₈ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁸

Ni²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁸

Ni³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁷

Ni²⁺ is more stable than Ni. Ni²⁺ is more common and more stable than Ni³⁺ because it is only two steps away from completion and completes the valence shell.