Modèle orbital d'électrons

Atomic orbitals are three-dimensional regions around the nucleus where electrons are most likely to be found. Rather than fixed circular paths, orbitals are cloud-like areas characterized by different energy levels, shapes, and three-dimensional orientations. Electrons occupy these regions based on their energy state and the atom's electron configuration.

Electrons fill atomic orbitals from lowest to highest energy to reach the most stable configuration. Each orbital can hold a maximum of two electrons with opposite spins. For example, sodium's 11 electrons fill the 1s orbital first with two electrons, then the 2s orbital, followed by the three 2p orbitals, and finally one electron enters the 3s orbital.

The first energy level contains a single spherical s orbital. The second level includes one 2s orbital and three dumbbell-shaped 2p orbitals pointing along the x-, y-, or z-axis. The third level adds five d orbitals: four with cloverleaf shapes and one with two lobes and a ring. Higher energy levels contain increasingly complex orbital shapes.

The first shell holds two electrons in its single 1s orbital. The second shell holds eight total electrons: two in the 2s orbital and two in each of the three 2p orbitals. The third shell can accommodate 18 electrons across s, p, and d subshells. Higher shells accommodate more electrons as d and f subshells become available.

Bohr's 1913 model assumed electrons orbited the nucleus in fixed circular paths and accurately predicted hydrogen's behavior. Schrödinger's 1926 quantum mechanical model expanded this by treating electrons as waves affected by the nucleus's positive charge, creating cloud-like orbital regions. Schrödinger's model accurately predicts electron configurations for elements with multiple electrons.

In orbital diagrams, boxes represent individual orbitals and arrows represent electrons. Each arrow's direction indicates electron spin: spin-up or spin-down. The diagram's layout shows relative energy levels, with lower-energy orbitals positioned below higher-energy ones, allowing visualization of how electrons distribute across an atom's orbitals.

When electrons gain energy from heat or light, they enter an excited state and jump to higher orbitals. When they lose that energy rapidly, they fall back to lower orbitals and emit a photon of light. The color of the emitted photon corresponds to a specific energy amount that can be measured using a spectroscope.