2.4 Determination of Electronic Configuration of Atoms and Ions

This topic explains how to determine the electronic configuration of atoms and ions using proton number, electron number, and charge. It also covers the explanation of electronic configuration in terms of energy and inter-electron repulsion.

Definition

Electronic Configuration: The distribution of electrons in an atom's or ion's orbitals and subshells.

Main Idea

To determine the electronic configuration of atoms and ions, you need to consider the number of protons and electrons. In a neutral atom, the proton number (atomic number, Z) equals the number of electrons. For ions, the number of electrons is adjusted based on the charge. The configuration is then written using the Aufbau principle, Pauli's exclusion principle, and Hund's rule.

Core Concept

• Identify the neutral atom's electron configuration.
• Determine the charge of the ion.
• Write the electron configuration for the ion by adding or removing electrons.

Detailed Explanation

Steps to Determine Electronic Configuration

• Each neutral atom has a specific number of electrons, which can be found in the Periodic Table of Elements.
• Example: A neutral fluorine atom has nine electrons.

• Ions can be positively or negatively charged, depending on whether they have gained or lost electrons.
• Positive ions (cations): Have a deficit of electrons.
• Negative ions (anions): Have an excess of electrons.

• For cations: Start with the neutral atom's electron configuration and remove electrons equal to the magnitude of positive charge from the outermost shell.
• For anions: Start with the neutral atom's electron configuration and add electrons equal to the magnitude of negative charge.

Examples

Example 1: Fluorine (F) and Fluoride Ion (F⁻)

• Neutral fluorine atom: 9 electrons
• Electronic configuration: 1s² 2s² 2p⁵
• Fluoride ion (F⁻): 9 electrons + 1 electron (added) = 10 electrons
• Electronic configuration: 1s² 2s² 2p⁶

Example 2: Sodium (Na) and Sodium Ion (Na⁺)

• Neutral sodium atom: 11 electrons
• Electronic configuration: 1s² 2s² 2p⁶ 3s¹
• Sodium ion (Na⁺): 11 electrons − 1 electron (removed) = 10 electrons
• Electronic configuration: 1s² 2s² 2p⁶

Example 3: Magnesium (Mg) and Magnesium Ion (Mg²⁺)

• Neutral magnesium atom: 12 electrons
• Electronic configuration: 1s² 2s² 2p⁶ 3s²
• Magnesium ion (Mg²⁺): 12 electrons − 2 electrons = 10 electrons
• Electronic configuration: 1s² 2s² 2p⁶

Example 4: Chlorine (Cl) and Chloride Ion (Cl⁻)

• Neutral chlorine atom: 17 electrons
• Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁵
• Chloride ion (Cl⁻): 17 electrons + 1 electron = 18 electrons
• Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶

Explanation of Electronic Configuration in Terms of Energy and Inter-Electron Repulsion

• Electrons tend to minimize repulsion by occupying their own separate orbital, rather than sharing an orbital with another electron.
• This is because electrons repel each other due to their identical charges.
• The repulsion between electrons increases with the increase in the number of electrons in the same orbital, leading to a higher energy for larger orbitals.

• The energy of an orbital depends on its distance from the nucleus and the electron-electron repulsion it experiences.
• Orbitals with lower energies are filled before those with higher energies.

• Electrons with similar spin repel each other. This is called spin pair repulsion.
• Electrons therefore occupy separate orbitals in the same degenerate sub-shell to minimize this repulsion and have their spin same (Hund's rule).

Writing the Electronic Configuration

• Electronic configuration of elements can be done by filling the electrons from lower energy levels to high energy levels.
• The configuration 3p⁶ means 3p_x², 3p_y², 3p_z², with two electrons in each having opposite spins.

Example: - If an element has atomic number 25 (Manganese), its electronic configuration will be: 1s2, 2s2, 2px2, 2py2, 2pz2, 3s2, 3px2, 3py2, 3pz2, 4s2, 3dxy1, 3dyz1, 3dxz1, 3dx2-y21, 3dz21

Table: Electronic Configuration of First Eighteen Elements

Intermediate Level

Steps to Determine Electronic Configuration

• Find the atomic number from the periodic table.
• The number of electrons in a neutral atom equals the atomic number.

• Cations: positive charge → lose electrons.
• Anions: negative charge → gain electrons.

• For cations: remove electrons from the outermost shell.
• For anions: add electrons to the outermost shell.

Examples

Explanation in Terms of Energy and Repulsion

• Inter-electron repulsion: Electrons occupy separate orbitals to minimize repulsion.
• Orbital energy: Lower energy orbitals are filled first.
• Spin pairing repulsion: Electrons with similar spin repel each other; they occupy separate orbitals in the same degenerate sub-shell (Hund's rule).

Important Points

• 1. Neutral atom: electrons = atomic number (Z).
• 2. Cation: electrons = Z − charge.
• 3. Anion: electrons = Z + charge.
• 4. Fill orbitals in order of increasing energy.
• 5. Apply Pauli's exclusion principle and Hund's rule.
• 6. Inter-electron repulsion and spin pairing repulsion affect electronic configuration.
• 7. 3p⁶ means 3p_x², 3p_y², 3p_z².

Remember

• - Neutral atom: Electrons = Z
• - Cation: Electrons = Z − charge
• - Anion: Electrons = Z + charge
• - Order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < …
• - Pauli: Max 2 electrons per orbital, opposite spins
• - Hund: Singly first, then pair
• - Repulsion: Electrons minimize repulsion by occupying separate orbitals

Common Mistakes

• 1. Forgetting to adjust electrons for ions: Cations lose electrons; anions gain electrons.
• 2. Mixing up the order of orbitals: Follow the Aufbau principle and n+l rule.
• 3. Violating Pauli's exclusion principle: Two electrons in the same orbital must have opposite spins.
• 4. Violating Hund's rule: Electrons occupy degenerate orbitals singly first.
• 5. Ignoring inter-electron repulsion: Electrons occupy separate orbitals to minimize repulsion.

Quick Revision

• Neutral atom: electrons = atomic number (Z).
• Cation: electrons = Z − charge.
• Anion: electrons = Z + charge.
• Fill orbitals in order of increasing energy.
• Apply Pauli's exclusion principle and Hund's rule.
• Inter-electron repulsion and spin pairing repulsion affect electronic configuration.
• 3p⁶ means 3p_x², 3p_y², 3p_z².

60-Second Revision

• Neutral atom: Electrons = Z
• Cation: Electrons = Z − charge
• Anion: Electrons = Z + charge
• Order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < …
• Pauli: Max 2 electrons per orbital, opposite spins
• Hund: Singly first, then pair
• Repulsion: Electrons minimize repulsion by occupying separate orbitals

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