ELECTRONIC CONFIGURATION
Q27: Discuss Electronic Configuration.
Arrangement of Electrons in an Atom
Electrons move around the nucleus in different layers, called shells. These shells are like energy levels that hold the electrons. The shells are labeled with numbers and letters:
- 1st energy level: K shell
- 2nd energy level: L shell
- 3rd energy level: M shell
- 4th energy level: N shell
The shells are arranged in circles around the nucleus. The K shell, with the least energy, is closest to the nucleus. The L shell has more energy and is farther away, and so on. The outermost shell holds the most energy.
Electronic Configuration
The way electrons are arranged in an atom’s shells is called its electronic configuration.
How to Write It:
- Find the atomic number (it tells the number of electrons).
- Know the maximum electrons each shell can hold.
Electron Limits per Shell:
| Shell | Max Electrons |
|---|---|
| K | 2 |
| L | 8 |
| M | 18 |
| N | 32 |
The formula to calculate maximum electrons per shell is 2n², where n is the shell number.
Concept of Sub-shell / Sub-Orbit
An energy level, or shell, is divided into smaller parts called sub-shells. Research shows that shells contain sub-shells, and each sub-shell has orbitals.
Maximum Electron Capacity:
- 2 electrons
- 8 electrons
- 18 electrons
- 32 electrons
Types of Sub-shells:
There are four sub-shells: s, p, d, and f. These stand for:
- s: Sharp - Holds up to 2 electrons
- p: Principal - Holds up to 6 electrons
- d: Diffused - Holds up to 10 electrons
- f: Fundamental - Holds up to 14 electrons
Shells & Electron Capacity:
| Shell Number (n) | Shell Name | Max Electrons | Sub-shells |
|---|---|---|---|
| 1 | K | 2 | 1s |
| 2 | L | 8 | 2s, 2p |
| 3 | M | 18 | 3s, 3p, 3d |
| 4 | N | 32 | 4s, 4p, 4d, 4f |
Electronic Configuration of First 18 Elements
| Atomic No. | Symbol | Element | Configuration |
|---|---|---|---|
| 1 | H | Hydrogen | 1s¹ |
| 2 | He | Helium | 1s² |
| 3 | Li | Lithium | 1s² 2s¹ |
| 4 | Be | Beryllium | 1s² 2s² |
| 5 | B | Boron | 1s² 2s² 2p¹ |
| 6 | C | Carbon | 1s² 2s² 2p² |
| 7 | N | Nitrogen | 1s² 2s² 2p³ |
| 8 | O | Oxygen | 1s² 2s² 2p⁴ |
| 9 | F | Fluorine | 1s² 2s² 2p⁵ |
| 10 | Ne | Neon | 1s² 2s² 2p⁶ |
| 11 | Na | Sodium | 1s² 2s² 2p⁶ 3s¹ |
| 12 | Mg | Magnesium | 1s² 2s² 2p⁶ 3s² |
| 13 | Al | Aluminum | 1s² 2s² 2p⁶ 3s² 3p¹ |
| 14 | Si | Silicon | 1s² 2s² 2p⁶ 3s² 3p² |
| 15 | P | Phosphorus | 1s² 2s² 2p⁶ 3s² 3p³ |
| 16 | S | Sulfur | 1s² 2s² 2p⁶ 3s² 3p⁴ |
| 17 | Cl | Chlorine | 1s² 2s² 2p⁶ 3s² 3p⁵ |
| 18 | Ar | Argon | 1s² 2s² 2p⁶ 3s² 3p⁶ |
Aufbau Principle
The Aufbau principle states that electrons are added to orbitals in order of increasing energy levels. The order is:
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p
This principle helps determine the electron configuration of an atom in its ground state.
3.7 Electronic Configuration
Shells
Definition
Based on Bohr's atomic theory, shells are circular paths where electrons revolve around the nucleus. Each shell has fixed energy, described by the quantum number n (n = 1, 2, 3, ...). As n increases, the distance from the nucleus and the energy of the shell increase.
Nomenclature
- n = 1: K shell
- n = 2: L shell
- n = 3: M shell
Sub-Shells
Definition and Distribution
Sub-shells are subdivisions of shells or energy levels. They are represented by combining the n value and sub-shell symbol.
- K shell (n = 1): 1s
- L shell (n = 2): 2s, 2p
- M shell (n = 3): 3s, 3p, 3d
- N shell (n = 4): 4s, 4p, 4d, 4f
Electron Capacity and Energy Order
- s sub-shell: 2 electrons
- p sub-shell: 6 electrons
- d sub-shell: 10 electrons
- f sub-shell: 14 electrons
Energy order: 1s < 2s < 2p < 3s < 3p < 4s < 3d ...
Electronic Configuration
Definition
Electronic configuration refers to the arrangement of electrons in sub-shells. It follows the Auf Bau Principle, which states that electrons fill the lowest energy sub-shells first.
Symbols for Atoms and Ions
Format: [Mass number][Element symbol][Charge][Atomic number]. For example: Mg12242+ (Magnesium with mass number 24, charge +2, atomic number 12).
Key Points
- Rutherford's planetary model: Nucleus (protons) surrounded by electrons.
- Bohr's model: Electrons in fixed circular orbits (shells).
- Isotopes: Atoms with the same number of protons but different numbers of neutrons.
- Uranium isotope: Used in nuclear reactors and atomic bombs.
- Radioactive isotopes: Various applications in science and medicine.
- Electronic configuration: Arrangement of electrons in sub-shells.
- Auf Bau Principle: Electrons fill lowest energy levels first.
References
- B. Earl and LDR Wilford, "Introduction to Advanced Chemistry"
- Iain Brand and Richard Grime, "Chemistry (11-14)"