AS Electronic configuration #9701 #alevelchemistry #alevel #chemistryCAIE

*ELECTRONIC STRUCTURE SUMMARY* *1. Principal Quantum Shells (n)* Electrons are arranged in *energy levels* or principal quantum shells numbered $n = 1, 2, 3, 4$, etc. *Energy & Distance:* Shells further from the nucleus have *higher energy* and are held less tightly. *Max Electron Capacities:* Shell 1: *2* electrons. Shell 2: *8* electrons. Shell 3: *18* electrons. Shell 4: *32* electrons. *2. Sub-shells and Orbitals* *Structure:* Shells are split into *sub-shells* ($s, p, d, f$), which contain **atomic orbitals**. *Capacity:* Each atomic orbital can hold a **maximum of two electrons**. *Sub-shell Breakdown:* *s sub-shell:* 1 orbital (**2** electrons max). *p sub-shell:* 3 orbitals (**6** electrons max). *d sub-shell:* 5 orbitals (**10** electrons max). *f sub-shell:* 7 orbitals (**14** electrons max). *3. Orbital Shapes* *s Orbitals:* Spherical shape. Size increases as the principal quantum number ($n$) increases. *p Orbitals:* Dumbbell-shaped with two lobes. There are three per shell (except $n=1$): $p_x$, $p_y$, and $p_z$, arranged at right angles to each other. *4. Filling Order (Ground State)* *Rule:* Electrons fill the *lowest energy* sub-shells first (the ground state). *Order of Filling:* $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$. *The 4s Overlap:* The **$4s$ sub-shell is lower in energy than the $3d$ sub-shell**, so it is filled first. *Degeneracy:* Orbitals within the same sub-shell (e.g., the three $2p$ orbitals) have the same energy and are called **degenerate**. #chemistry #0620 #9701 #5070 #5070chemistry #p1 #caie #alevelchemistry #p3