Atomic Models and Electron Configuration

TL;DR

You'll learn how our understanding of atoms changed over time, from simple spheres to complex electron clouds. We'll explore how electrons are arranged within an atom using shells, subshells, and orbitals. This arrangement, called electron configuration, dictates how an atom behaves chemically.

1. The Mental Model

Imagine an atom like a tiny solar system where electrons orbit a central nucleus, but with more defined pathways. Different pathways have different energy levels, and only a certain number of electrons can fit into each.

2. The Core Material

Our understanding of the atom has evolved dramatically. Early models were simple, treating atoms as solid spheres. Then came the plum pudding model, where electrons were embedded in a positive "pudding." Rutherford's gold foil experiment showed atoms had a dense, positive nucleus with electrons orbiting. Bohr refined this by proposing electrons orbit in specific, fixed energy levels or shells.

The most current and accepted model is the quantum mechanical model. This model doesn't define exact orbits but rather regions of probability called orbitals where an electron is likely to be found.

Here's how the atomic model evolved:

graph TD
    A["Dalton's Model (Early 1800s)"] --> B["Thomson's Plum Pudding (1897)"]
    B --> C["Rutherford's Nuclear Model (1911)"]
    C --> D["Bohr's Planetary Model (1913)"]
    D --> E["Quantum Mechanical Model (1920s-present)"]

Electron Configuration Basics

Photo by Magda Ehlers on Pexels

Electrons occupy specific energy levels or shells, denoted by the principal quantum number 'n' (n=1, 2, 3, ...). Higher 'n' means higher energy and farther from the nucleus.

Within each shell, there are subshells, identified by letters: s, p, d, f. Each subshell has a different shape and can hold a specific maximum number of electrons:
* s subshell: holds up to 2 electrons (1 orbital)
* p subshell: holds up to 6 electrons (3 orbitals)
* d subshell: holds up to 10 electrons (5 orbitals)
* f subshell: holds up to 14 electrons (7 orbitals)

Each orbital can hold a maximum of two electrons, and they must have opposite spins (Pauli exclusion principle). Electrons fill orbitals in order of increasing energy (Aufbau principle), and they spread out among degenerate (same energy) orbitals before pairing up (Hund's rule).

The common order of filling orbitals is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

You write electron configurations using a notation like:
1s² 2s² 2p⁶ 3s¹
The large number is the principal energy level (shell), the letter is the subshell, and the superscript is the number of electrons in that subshell.

3. Worked Example

Let's find the electron configuration for Oxygen (O).
1. First, find Oxygen on the periodic table. Its atomic number is 8, meaning it has 8 protons and, in a neutral atom, 8 electrons.
2. Now, fill the orbitals in order:
* The first two electrons go into the 1s subshell: 1s² (2 electrons used, 6 remaining)
* The next two electrons go into the 2s subshell: 2s² (2 electrons used, 4 remaining)
* The remaining four electrons go into the 2p subshell: 2p⁴ (4 electrons used, 0 remaining)
3. Combine them: 1s² 2s² 2p⁴

You can also use noble gas shorthand. The noble gas before Oxygen is Helium (He), which has 2 electrons (1s²). So, Oxygen's configuration can be written as: [He] 2s² 2p⁴.

4. Key Takeaways

• Atomic models have evolved from simple spheres to complex probabilistic regions called orbitals.
• The quantum mechanical model describes electrons occupying specific energy levels (shells) and subshells (s, p, d, f).
• Each subshell contains a certain number of orbitals, and each orbital can hold up to two electrons with opposite spins.
• Electron configurations like 1s² 2s² 2p⁴ show how electrons are distributed within an atom.
• The Aufbau principle, Pauli exclusion principle, and Hund's rule guide how electrons fill orbitals.
• Electron configuration determines an atom's chemical behavior and reactivity.

5. Now Try It

Determine the full and noble gas shorthand electron configuration for Chlorine (Cl). Use the periodic table to find its atomic number (number of electrons). Then, fill the orbitals following the energy order. You'll know you're successful if your final configuration adds up to the correct number of electrons and follows the filling rules.