Problem 57 - Chemistry Matter and Change

Question

Explain how the iron atom can form both an iron 2+ ion and an iron 3+ ion

Step-by-Step Solution

Answer

The iron atom forms Fe 2+ and Fe 3+ ions by losing two and three electrons from its electron configuration \( 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6 \), respectively. The Fe 2+ ion is created by losing two 4s electrons, resulting in the electron configuration \( 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 \) and a charge of +2. The Fe 3+ ion forms by losing two 4s electrons and one 3d electron, leading to the electron configuration \( 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 \) and a charge of +3.

Step 1: Understanding the electron configuration of the iron atom

The iron atom has an atomic number of 26, which means it has 26 electrons. The electron configuration of an iron atom is as follows: \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6 \] Remember that: - The principal quantum number (shell) is represented by the number in front of the letters (1, 2, etc.). - The letters (s, p, d) represent the subshell types. - The superscript (exponent) indicates the number of electrons in that subshell.

Step 2: Understand ion formation and charge determination

Ions form when an atom gains or loses electrons, which results in a net electrical charge. When an atom loses electrons, it forms a positive ion called a cation; and when an atom gains electrons, it forms a negative ion called an anion. The charge of an ion is determined by the difference between the number of protons (positive charges) and electrons (negative charges). If the number of protons is greater than the number of electrons, the ion carries a positive charge (cation). If the number of electrons is greater than the number of protons, the ion carries a negative charge (anion).

Step 3: Forming the Fe 2+ ion

To form the iron 2+ ion (Fe 2+), the iron atom loses two electrons from its outermost shell. In this case, it loses the two 4s electrons, leaving the electron configuration: \[ 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 \] Since the Fe 2+ ion has lost two electrons, it has 24 electrons and 26 protons, resulting in a net electrical charge of +2: \[ Charge = Protons - Electrons = 26 - 24 = +2 \]

Step 4: Forming the Fe 3+ ion

To form the iron 3+ ion (Fe 3+), the iron atom loses three electrons. It loses the two 4s electrons and one 3d electron, resulting in the electron configuration: \[ 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 \] Since the Fe 3+ ion has lost three electrons, it has 23 electrons and 26 protons, resulting in a net electrical charge of +3: \[ Charge = Protons - Electrons = 26 - 23 = +3 \]

Step 5: Summary

The iron atom can form both an iron 2+ ion and an iron 3+ ion by losing two and three electrons, respectively. Losing two electrons (both 4s electrons) forms the Fe 2+ ion with an electron configuration of \( 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 \) while losing three electrons (two 4s and one 3d electron) forms the Fe 3+ ion with an electron configuration of \( 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 \).

Key Concepts

Electron Configuration

An electron configuration is a method of denoting the distribution of electrons in an atom's orbitals. For an iron atom, which has 26 electrons, the electron configuration is expressed as follows: \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6 \]. This notation helps to visualize how electrons are arranged across different energy levels or shells. Each number before the letter (such as 1, 2, or 3) represents a principal quantum number indicating the shell. The letters (s, p, d) represent the types of subshells, where 's' can hold up to 2 electrons, 'p' can hold 6, and 'd' can hold 10. The superscripts denote the number of electrons in these subshells. Understanding electron configurations is crucial when studying ion formation, as it shows how electrons are removed or added to create charged particles.

Ion Formation

Ion formation occurs when atoms lose or gain electrons, resulting in a net charge. This process alters the balance between the number of protons and electrons, thereby making the atom either positively or negatively charged.

• A positive ion, or cation, forms when there are more protons than electrons, suggesting a loss of electrons.
• A negative ion, or an anion, forms when there are more electrons than protons, indicating a supplement of electrons.

The charge of an ion is determined through the simple arithmetic of subtracting the number of electrons from the number of protons, giving the formula: \[ \text{Charge} = \text{Number of protons} - \text{Number of electrons} \]. Understanding this balance is critical when discussing the conversion of elements to their ionic states because it helps in predicting the type of charge an ion will have based on its electron loss or gain.

Cation Formation

In cation formation, an atom loses one or more electrons, resulting in a positive charge. Iron, for example, can become an iron 2+ ion (Fe 2+) or iron 3+ ion (Fe 3+) by losing electrons:

• To form Fe 2+, iron loses two electrons, specifically from the 4s orbital, changing its configuration to \[ 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 \]. This results in 24 electrons and a +2 charge.
• To form Fe 3+, it loses three electrons: first two from the 4s orbital and one from the 3d orbital, giving the configuration \[ 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 \]. This leaves 23 electrons and a +3 charge.

These transformations are guided by electron configurations and are crucial to understanding the behavior of metal ions in different chemical reactions. Identifying which electrons are removed helps predict the properties and reactivity of the resulting ions.