Electron configuration describes the arrangement of electrons within an atom’s orbital structure. This system uses numbers and letters to describe the energy levels and sublevels occupied by electrons orbiting the nucleus. Understanding this arrangement is fundamental because the distribution of electrons determines an atom’s size, its potential for forming chemical bonds, and its overall chemical behavior. The configuration provides a detailed picture of the ground state of an atom, representing the lowest possible energy state for all its electrons.
The Principles Governing Electron Placement
The precise placement of electrons within an atom is guided by three fundamental quantum mechanical rules.
The Aufbau principle dictates the order in which orbitals are filled, starting with those that possess the lowest energy. This ensures that energy sublevels like \(1s\) are filled completely before electrons move on to higher energy sublevels like \(2s\) or \(2p\).
The Pauli Exclusion Principle sets a limit on the number of electrons that can occupy any single orbital. This principle states that no two electrons in an atom can share the exact same set of quantum numbers. Consequently, each atomic orbital can hold a maximum of two electrons, and those two electrons must have opposite spins.
Hund’s Rule governs how electrons distribute themselves among orbitals that share the same energy level, called degenerate orbitals. Within the three \(p\) orbitals, electrons will first occupy each orbital singly before any orbital receives a second, paired electron. This arrangement maximizes the total spin and leads to a more stable electron configuration.
Deriving Krypton’s Complete Electron Configuration
Krypton (Kr) is the thirty-sixth element on the periodic table, meaning a neutral atom contains 36 electrons that must be placed into orbitals. The full configuration requires sequentially filling the orbitals according to the energy hierarchy established by the Aufbau principle. The process begins by filling the lowest energy shell (\(n=1\)), which contains the \(1s\) orbital (\(1s^2\)).
Next, the second shell (\(n=2\)) is filled, starting with the \(2s\) orbital (two electrons), followed by the \(2p\) orbitals (six electrons). This results in the configuration \(1s^2 2s^2 2p^6\) (ten electrons). The third shell (\(n=3\)) then fills with the \(3s\) orbital (two electrons) and the \(3p\) orbitals (six electrons), accounting for eighteen electrons so far.
The \(4s\) orbital is lower in energy than the \(3d\) orbitals, so it is filled next with two electrons. The five \(3d\) orbitals are then addressed, holding ten electrons (\(3d^{10}\)), bringing the electron count to thirty.
The final six electrons are placed into the three \(4p\) orbitals (\(4p^6\)). The complete electron configuration for Krypton is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\).
Writing the Condensed Notation
Writing the full electron configuration for heavier elements like Krypton is often long. To simplify this, the condensed or noble gas notation is used. This notation replaces the configuration of the inner core electrons with the symbol of the preceding noble gas element.
For Krypton, the preceding noble gas is Argon (Ar), which has an atomic number of 18. Argon’s configuration, \(1s^2 2s^2 2p^6 3s^2 3p^6\), represents Krypton’s core electrons. This sequence is replaced by the chemical symbol \([Ar]\).
The condensed configuration then shows the electrons in the outermost shells (valence electrons) and any partially filled inner shells that occur after the noble gas core. The resulting condensed electron configuration for Krypton is \([Ar] 4s^2 3d^{10} 4p^6\).
The Stability of a Noble Gas Configuration
The electron configuration of Krypton explains its placement in Group 18 of the periodic table, the noble gases. The outermost energy shell, or valence shell, is the \(n=4\) shell, which is completely filled. This shell contains the two electrons in the \(4s\) orbital and the six electrons in the \(4p\) orbitals, totaling eight valence electrons.
This arrangement of eight electrons in the outer shell is chemically significant and is described by the octet rule. A completely filled valence shell represents a state of maximum stability and minimum potential energy for the atom. Atoms with this configuration have no tendency to gain, lose, or share electrons.
Because its electron shells are entirely full, Krypton is chemically inert and does not readily participate in chemical reactions. This inherent stability is the defining characteristic of all noble gases.