Structure of Atom Class 11 Notes for NEET & IIT JEE

There are four quantum numbers. The principal quantum number (n) describes the shell and energy level. The azimuthal quantum number (l) describes the subshell shape. The magnetic quantum number (mₗ) describes the orientation of the orbital in space. The spin quantum number (mₛ) describes the spin of the electron (+½ or −½). Together they uniquely identify every electron in an atom.

The de Broglie equation is λ = h/mv, where λ is the wavelength associated with a moving particle, h is Planck's constant, m is mass, and v is velocity. It is used when questions involve the wave nature of electrons or other particles in motion. JEE Main frequently combines this with Bohr's orbit radius to ask for the wavelength of an electron in the nth orbit.

For NEET, the highest-priority topics are Bohr's model energy and radius formulas, spectral series of hydrogen (especially Lyman and Balmer), electronic configurations of elements including exceptions (Cr, Cu), and identification of sub-atomic particles. NEET typically asks 2–3 direct questions from this chapter, and all of them can be solved if these four areas are practised thoroughly.

Use three formulas: Radial nodes = n − l − 1; Angular nodes = l; Total nodes = n − 1. For example, a 3p orbital has n=3, l=1, so radial nodes = 3−1−1 = 1, angular nodes = 1, total nodes = 2. Node-counting questions appear in JEE Main roughly once every two years and take under 30 seconds if you know these formulas.

An orbit (Bohr's model) is a fixed, well-defined circular path around the nucleus — this concept is now considered incorrect. An orbital (quantum mechanical model) is a three-dimensional region of space where there is a 90% or higher probability of finding an electron. Orbitals have specific shapes: spherical (s), dumbbell (p), double dumbbell (d).

Chromium (Z=24) has [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s² because a half-filled d subshell (3d⁵) provides extra stability due to symmetrical electron distribution and exchange energy. Copper (Z=29) has [Ar] 3d¹⁰ 4s¹ because a completely filled 3d subshell is more stable than a partially filled one. These are among the most commonly tested facts in both NEET and JEE Main.