AS Electronic configuration #chemistry #9701 #chemistry9701 #chemistrycrashcourse

Electronic configuration describes the arrangement of electrons within the shells, sub-shells, and orbitals of an atom. This configuration is represented using a specific notation where the *principal quantum number* indicates the energy level, followed by the *sub-shell* letter (s, p, d, or f) and a superscript showing the *number of electrons* in that sub-shell. Key Principles of Electron Arrangement *Energy Levels:* Electrons occupy sub-shells in order of *increasing energy**. For instance, the **4s sub-shell is filled before the 3d sub-shell* because it sits at a lower energy level. *Orbital Occupancy:* Within a sub-shell, electrons initially occupy *separate orbitals* with the same spin to minimise repulsion. They only begin to pair up when no empty orbitals are available. *Electron Spin:* Electrons act as spinning charges. When two electrons occupy the same orbital, they must have *opposite spins* (represented by upward and downward arrows) to account for **spin-pair repulsion**. The Periodic Table and Blocks The Periodic Table is divided into *s, p, d, and f blocks* based on the valence sub-shell of the elements: *s-block:* Groups 1 and 2, where the outer electrons are in an s sub-shell. *p-block:* Groups 13 to 18, characterized by outer electrons in a p sub-shell. *d-block:* Primarily transition metals that add electrons to the d sub-shell. Exceptions and Ions *Stability Exceptions:* *Chromium and Copper* do not follow standard filling patterns. They adopt configurations ([Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ respectively) that provide half-full or full d-subshells, which are **energetically more stable**. *Ion Formation:* Positive ions (**cations**) are formed by removing electrons, while negative ions (**anions**) are formed by adding them. Crucially, in transition metals, electrons are **removed from the 4s orbital first**, before the 3d orbital. *Free Radicals:* These are species, such as a chlorine atom, that contain **one or more unpaired electrons**. They are often generated through homolytic fission, where a covalent bond splits evenly between two atoms. #chemistry #0620 #5070 #9701 #5070chemistry #p1 #caie #alevelchemistry #p3