Electron Configuration
Definition
Electron configuration is the arrangement of negatively charged sub-atomic electron particle in orbital shell and subshell around an atomic nucleus. Electron configuration follows specific rules based on the energy levels and sublevels of electrons.
Example: Based on Aufbau Principle electron configuration of Helium and Sodium are:
- Helium has an atomic number of 2. Its electron configuration can be represented as 1s².
- Sodium has an atomic number of 11. Its electron configuration can be 1s² 2s² 2p⁶ 3s¹.
Electron Shells
The electron shell is a grouping of electrons in an atom according to their energy level, which incircles the atomic nucleus. The electron shell of an atom can hold 2n2 electrons, where n is the energy level. For example, the first shell (closest to the nucleus) can hold 2x(1)2or 2 electrons, the second shell can accommodate 2x(2)2 or 8 electrons.
- Electrons that are in the first energy level are closest to the nucleus and will have the lowest energy on the other hand electrons further away from the nucleus will have higher energy.
Subshell
Within the shells, electrons are further grouped into subshells. There are four different types of sub-shell named as s, p, d, and f (in increasing energy order).
- The first shell has only one subshell s subshell
- The second shell has s and p subshell
- The third shell has s, p, and d subshells
- The fourth shell has s, p, d and f subshells
Atomic Orbital
Within each subshell, electrons are grouped into orbitals. The area surrounding the nucleus where electrons in their respective sub-shell are estimated to be present is known as the orbitals.
Key principles governing Electronic Configuration
The arrangement of electrons within electron shell is based on certain rules and principles. These principles/Rules are:
- Aufbau principle
- Hund’s rule
- Pauli exclusion principle
The above three principles explain the filling order of atomic orbitals and the distribution of electrons among them.
Aufbau Principle
The German word “Aufbau” means “building up.” The Aufbau principle defines a set of rules. These rules are:
Rule 1: Electrons 1st occupy the lowest energy orbitals. It means electrons of an atom occupy their position in atomic orbitals in a specific order, starting from the lowest energy level to higher energy levels.
Rule 2: Orbitals will be filled in of Increasing energy. It means the order in which orbitals are filled follows a specific sequence such as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on, is based on their energy levels.
Rule 2: Each orbital can hold only two electrons (must be of opposite spin).
Rule 3: Two or more orbitals with the same energy are each half-filled by one electron before any one orbital is completely filled by addition of the second electron.
Hund’s Rule
This rule states that:
- Electrons occupy orbitals of the same energy singly before they start pairing up.
- All of the electrons in singly occupied orbitals have the same spin.
This rule helps in determining the arrangement of electrons within subshells.
Pauli Exclusion Principle
Proposed by Wolfgang Pauli. This Principle states that, Each electron can be described by a unique set of four quantum numbers; it means no two electrons in an atom can have the same set of quantum numbers.
Valence Electrons
Valence electrons are the electrons in the outermost shell (highest occupied energy level) of the atom. The shell in which valence electron are present is termed as valence shell.
- They participate in the formation of chemical bond.
- They are held loosely
- The number of valence electron determine the properties of the atom and the way atom will bond chemically (reactivity).
- More reactive element have fewer electron in the valence cell.
- Stable atom has 8 electron in their valence cell.
Example: Boron
- Boron has 5 electrons and the electron configuration of boron is 1s² 2s² 2p¹, indicating that it has 2 electrons in the 1s orbital and 2 electrons in the 2s orbital, leaving only 1 electron in the 2p orbital, which is the outermost electron shell. Therefore, boron has 3 valence electrons. These valence electrons are crucial in defining boron’s chemical behavior.
- To achieve a stable electron configuration Boron tends to lose these 3 valence electrons.
Ionisation
Ionization is the process by which an atom or a molecule gains or loses one or more electrons, resulting in the formation of an ion with a net positive charge (cation) or negative charge (anion).
- Ion Formation: Anion is formed when an atom or molecule gains one or more electrons and becomes negatively charged. Conversely, when an atom or molecule loses one or more electrons, it becomes positively charged and termed as cation.
