Atomic Structure: From Bohr Model to Electron Configurations

Electrons behave less like tiny billiard balls and more like a “smear” of charge that surrounds the nucleus. The Heisenberg uncertainty principle prevents simultaneous knowledge of an electron’s exact position and momentum, so scientists describe electrons with probability functions rather than fixed paths. An orbital is often visualized by a 90 % boundary that encloses the region where the electron is most likely to be found. Within that region the electron density is highest near the centre of the orbital and tapers off toward the edges.

The Bohr Model and Energy

The Bohr model, introduced by Niels Bohr about a century ago, does not depict the true trajectories of electrons, but it remains useful for illustrating discrete energy states. When an electron absorbs a photon, it gains energy and jumps to a higher energy level—an “excited” state. In these higher states the electron resides farther from the nucleus, where the Coulomb attraction is weaker, making the electron easier to remove or to share in a bond. Electrons naturally relax back to lower energy states, often releasing a photon in the process.

Filling Orbitals

Electrons occupy the lowest‑energy orbitals first. After the 1s subshell fills with two electrons, any additional electrons must move to the next available shell (for example, 2s) because like charges repel each other. This ordering of filling mirrors the periods (rows) of the periodic table: each period corresponds to a new principal energy shell. Every subshell—defined by its geometric shape—can hold two electrons, leading to the familiar electron configurations such as 1s¹ for hydrogen, 1s² for helium, and 1s² 2s¹ for lithium.

Mechanisms in Action

• Orbital Filling: Electrostatic repulsion forces electrons to occupy the next empty, lower‑energy shell once an inner subshell is full.
• Excitation: Photon absorption raises an electron to a higher‑energy probability distribution; the electron may later emit a photon as it returns to a lower state.
• Coulomb Interaction: Greater distance from the nucleus in higher shells weakens the attractive Coulomb force, increasing the electron’s reactivity and its susceptibility to being “plucked off” during chemical bonding.