Chemistry · Structure Of Atom · NEET
n tells you the SHELL of the electron. It mainly decides two things: the SIZE of the orbital and, to a large extent, the ENERGY of the orbital. Bigger n means a bigger orbital, the electron is farther from the nucleus, and it has higher energy. For NEET, remember the NCERT line: n gives the size (and mostly the energy) of the orbital.
A shell with quantum number n has n² orbitals. Each orbital can hold at most 2 electrons (because of the two spin values, +½ and -½). So maximum electrons = 2 × (number of orbitals) = 2 × n² = 2n². Example: n=3 has 3²=9 orbitals, so 2×9 = 18 electrons maximum.
A SHELL is a group of electrons with the same n (like K, L, M). An ORBIT is Bohr's old idea of a fixed circular path — this is outdated. An ORBITAL is the modern idea: a region in space where the chance of finding the electron is high. So n names the shell; inside that shell there are n² orbitals. Do not confuse orbit (path, wrong) with orbital (probability region, correct).
Use 2n². K shell (n=1): 2×1² = 2 electrons. L shell (n=2): 2×2² = 8 electrons. M shell (n=3): 2×3² = 18 electrons. N shell (n=4): 2×4² = 32 electrons. This is a very common NEET fact, so memorise 2, 8, 18, 32.
For a hydrogen atom (and hydrogen-like ions like He⁺, Li²⁺), n controls BOTH size and energy by itself, because these have only one electron. NCERT says n determines the size and 'to a large extent' the energy. In multi-electron atoms, energy also depends on the azimuthal quantum number l, but size still mainly depends on n. If NEET asks which quantum number gives the SIZE of the orbital, the answer is n.
Number of orbitals in a shell = n². For n=1 there is 1 orbital, n=2 has 4 orbitals, n=3 has 9 orbitals, n=4 has 16 orbitals. Multiply this by 2 to get the maximum electrons (2n²). Do not mix these up: n² = orbitals, 2n² = electrons.
Match List-I (Quantum Number) with List-II (Information provided) and choose the correct option: (A) m_l (B) m_s (C) l (D) n | (I) shape of orbital (II) size of orbital (III) orientation of orbital (IV) orientation of spin of electrons.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
n is a positive whole number: n = 1, 2, 3, 4, and so on. It can never be zero, a fraction, or negative.
They are the letter names for shells. n=1 is K, n=2 is L, n=3 is M, n=4 is N. They follow the alphabet from K onward.
n² is the number of ORBITALS in a shell. The number of electrons is 2n², because each orbital holds 2 electrons.
Higher n means higher (less negative) energy and a larger orbital. The electron is held less tightly and sits farther from the nucleus.
After n comes the azimuthal quantum number l, which fixes the sub-shell (s, p, d, f) and the shape of the orbital.
