Atomic Structure and Electron Configuration Principles

Fundamental Atomic Concepts

Atomic Number and Mass Number

Two fundamental properties of an atom are its atomic number and mass number.

• The Atomic Number (Z): This is the number of protons in the nucleus of an atom. For example, a neutral Sodium atom (Na) with atomic number 11 has 11 protons and 11 electrons.
• The Mass Number (A): This is the total number of protons and neutrons in the nucleus of an atom. For example, Sodium-23 (²³Na) has a mass number of 23.

The number of neutrons can be calculated by subtracting the atomic number from the mass number (A - Z).

The Atom, Molecule, and Orbitals

The Atom

In physics and chemistry, an atom is the smallest unit of a chemical element that retains its identity and properties. It cannot be divided further by chemical processes.

The Molecule

A molecule is the smallest particle of a substance that retains the specific chemical properties of that substance. If a molecule is divided into its constituent atoms, those parts will have a different nature than the original substance. Molecules of compounds are formed by the atoms of the elements that compose them.

Atomic Orbitals

An orbital is the region of space around the nucleus where there is the highest probability of finding an electron. Orbitals are designated by letters:

• s, p, d, and f are the most common.
• In theory, others such as g, h, i, etc., also exist.

When atoms come together to form compounds, their atomic orbitals combine to form molecular orbitals, which constitute the chemical bonds.

Principles of Electron Arrangement

Pauli Exclusion Principle

The Pauli exclusion principle describes the arrangement of electrons in energy states. For electrons, this principle states that a maximum of two electrons can occupy any single orbital, and these two electrons must have opposite spins. This means that no two electrons in an atom can have the same set of four quantum numbers (n, l, m_l, m_s), making each electron's quantum state unique. This principle is fundamental for building the electron configurations of any element and is complemented by Hund's rule.

Hund's Rule

Hund's rule states that when filling orbitals of equal energy (degenerate orbitals), such as the three p orbitals, five d orbitals, or seven f orbitals, electrons will occupy separate orbitals with parallel spins before pairing up in the same orbital. An atom is more stable (has lower energy) when it has more unpaired electrons (parallel spins) compared to when those electrons are paired (opposite or antiparallel spins).

Electron Configuration

An electron configuration is the way in which electrons are structured within an atom, molecule, or other physical structure, according to the electronic shell model. In this model, electrons can move from one energy level or orbital to another by either emitting or absorbing a quantum of energy, such as a photon. Because of the Pauli exclusion principle, no more than two electrons can occupy the same orbital; therefore, a transition can only occur if there is a vacancy in the target orbital.

Order of Orbital Filling

The standard order for filling electron orbitals is as follows:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6