Structure of AtomNEET Chemistry · Class 11 · NCERT Chapter 2

Chromium (Z=24) has [Ar] 3d⁵ 4s¹ instead of the expected [Ar] 3d⁴ 4s². Copper (Z=29) has [Ar] 3d¹⁰ 4s¹ instead of [Ar] 3d⁹ 4s². In both cases, one electron shifts from 4s to 3d to achieve the extra stability of a half-filled (3d⁵) or fully-filled (3d¹⁰) d subshell. This stability comes from spherical symmetry of the electron cloud and maximum exchange energy.

A shell is a main energy level described by the principal quantum number n (K=1, L=2, M=3, ...). Each shell contains one or more subshells described by the azimuthal quantum number l (s, p, d, f). Each subshell contains one or more orbitals described by the magnetic quantum number ml. An orbital is the smallest unit: a region of space that can hold at most 2 electrons with opposite spins.

The series is determined by the lower energy level (n₁) to which the electron falls. Lyman: n₁=1 (UV). Balmer: n₁=2 (visible). Paschen: n₁=3 (IR). Brackett: n₁=4 (IR). Pfund: n₁=5 (far IR). The first line of each series corresponds to the transition from n₂=(n₁+1) to n₁.

It means you cannot simultaneously know both the exact position and exact momentum of an electron. This is not a limitation of instruments. It is a fundamental quantum property: the more precisely you measure the position, the more the momentum becomes uncertain, and vice versa. For large objects like a cricket ball, the uncertainties are negligibly small. For electrons, they are significant enough to make the concept of a definite orbit meaningless.

Radial nodes = n − l − 1. For 3p: n=3, l=1. Radial nodes = 3−1−1 = 1. Angular nodes = l = 1. Total nodes = n−1 = 2. So a 3p orbital has 1 radial node and 1 angular node (a nodal plane).

Maximum electrons = 2n² = 2(4²) = 32. The n=4 shell contains the 4s (2 electrons), 4p (6 electrons), 4d (10 electrons), and 4f (14 electrons) subshells, totalling 32 electrons.