Problem 160 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of d electrons retained in \(\mathrm{Fe}^{2+}\) (At. number of \(\mathrm{Fe}=26\) ) ions is (a) 3 (b) 4 (c) 5 (d) 6

Step-by-Step Solution

Answer

The number of d electrons retained in \(\mathrm{Fe}^{2+}\) is 6.

Step 1: Understanding Electron Configuration

The atomic number of iron (Fe) is 26, which means a neutral iron atom has 26 electrons. The electron configuration for a neutral iron atom is \([\text{Ar}] \, 3d^6 \, 4s^2\). This means that the 4s orbital is filled before the 3d orbital despite having a lower principal quantum number.

Step 2: Determining Iron Ion Configuration

For \(\mathrm{Fe}^{2+}\), the atom has lost two electrons. Electrons are removed first from the outermost shell (highest energy level), which is the 4s orbital. Therefore, the electron configuration for \(\mathrm{Fe}^{2+}\) becomes \([\text{Ar}] \, 3d^6\) since the two 4s electrons are removed.

Step 3: Counting d Electrons

In the configuration \([\text{Ar}] \, 3d^6\), all six electrons in the 3d subshell are retained in \(\mathrm{Fe}^{2+}\). Thus, the number of d electrons in \(\mathrm{Fe}^{2+}\) is 6.

Key Concepts

Iron Atom

The iron atom is a crucial component of many biological and geological processes on Earth. Iron, denoted by the symbol Fe, is a transition metal and is the 26th element on the periodic table.

• Iron's position on the periodic table tells us a lot about its properties and behavior.
• Its atomic number, which is 26, indicates that a neutral iron atom has 26 protons in its nucleus and 26 electrons orbiting around it.

These electrons are distributed among several energy levels and orbitals. For iron, the electron configuration is particularly important when it comes to understanding how it interacts chemically. The configuration notation for a neutral iron atom is \[ \text{[Ar]} \, 3d^6 \, 4s^2 \]. This means:

• The electrons fill the lower energy levels like the core levels associated with Argon, \[ \text{[Ar]} \].
• Then, electrons fill the 3d subshell before the 4s, a unique characteristic of transition metals.

This specific distribution of electrons is what gives iron its magnetic properties and makes it a key player in building strong alloys and playing roles in various enzymes.

Fe2+ Ion

When neutral iron loses electrons, it can become an ion, specifically the \( \mathrm{Fe}^{2+} \) ion, which is quite common. The loss of electrons occurs because it is energetically favorable for iron to reach a more stable electron configuration.

• When an iron atom becomes \( \mathrm{Fe}^{2+} \), it loses two electrons.
• The electrons are first removed from the outermost shell, which is the 4s orbital in this case.

This is because although the 4s orbital is filled before the 3d, it actually has a higher energy level once filled, causing it to lose electrons first. As such, the electron configuration for an \( \mathrm{Fe}^{2+} \) ion is \[ \text{[Ar]} \, 3d^6 \]. With this configuration, the 3d subshell retains all six of its electrons, playing a pivotal role in iron’s chemistry and magnetic properties.

d Electrons

d electrons are those residing in the d subshell of an atom's electron configuration. For transition metals like iron, d electrons are significant as they largely define the metal's properties.

• In a neutral iron atom, order of filling leads to a \[ 3d^6 \, 4s^2 \] configuration.
• During the formation of \( \mathrm{Fe}^{2+} \), electrons are removed from the 4s subshell, leaving the 3d subshell full.

The retention of d electrons in \( \mathrm{Fe}^{2+} \) is crucial for its magnetic properties and ability to form various coordination compounds. These electrons, being in the d subshell, enable iron to have multiple oxidation states, participate in redox reactions, and form complex ions, enhancing its utility in numerous chemical processes.

Atomic Number

The atomic number is one of the most fundamental descriptors of an element. It is defined as the number of protons in the nucleus of an atom, and for iron, the atomic number is 26.

• This defines not only the identity of the element but also its electron configuration.
• For an atom, electrons equal the number of protons when it is neutral.

In essence, the atomic number determines where the element resides on the periodic table and indicates the chemical behavior the element will likely exhibit. In the case of iron, its atomic number places it in the middle of the d-block, characterizing it as a transition metal with unique conducts among metals such as magnetism and multifarious oxidation states.