Physics · Atoms · NEET
No. Bohr's model works only for hydrogen-like (hydrogenic) atoms, which have exactly ONE electron: H, He+, Li2+, Be3+. Neutral helium has TWO electrons, and Bohr's model fails for it. The reason is that Bohr only included the attractive force between the nucleus and one electron. In helium each electron also feels the repulsion of the other electron, and Bohr's equations do not include this electron-electron force. So the model cannot be extended even to the simplest two-electron atom.
In a multi-electron atom, every electron interacts not only with the positive nucleus but also with all the other electrons. Bohr wrote the energy using only one force: the electrostatic attraction between the nucleus and a single electron (F = kZe^2/r^2). He left out the electron-electron repulsion, which is unavoidable when there is more than one electron. Without those extra forces, the radius, velocity and energy formulas of Bohr no longer give correct results. This is why we need quantum mechanics for atoms bigger than hydrogen.
Bohr's model correctly predicts the frequencies (positions) of hydrogen spectral lines, but not their brightness. In the real hydrogen spectrum, some lines are strong and some are weak, which means some electron transitions happen more often than others. Bohr's model treats all allowed transitions the same way and gives no rule for how likely each jump is. So it cannot say why one line is bright and another is faint. Predicting relative intensities needs quantum mechanics (transition probabilities).
No. When spectral lines are viewed with a high-resolution instrument, a single line often splits into several closely spaced lines. This is called fine structure. Lines also split when the atom is placed in a magnetic field (Zeeman effect) or an electric field (Stark effect). Bohr's model gives only one energy for each n value, so it predicts a single line and cannot produce this splitting. Note: NCERT lists three official limitations; fine structure, Zeeman and Stark effects are standard exam additions that come from the same weakness.
Rutherford's model failed on stability and spectra: a classical electron orbiting the nucleus should continuously radiate energy, spiral inward and collapse in about 10^-8 s, and it should give a continuous spectrum. Bohr fixed these two problems using quantised orbits. Bohr's model still fails on the finer points: it works only for one-electron atoms, cannot explain line intensities, and mixes classical orbits with quantum jumps. In short, Rutherford failed at the basic level (stability), Bohr failed at the advanced level (multi-electron atoms and spectral detail).
No, it is not wrong, it is incomplete. Bohr's model correctly gives the energy levels and spectral line frequencies of hydrogen and hydrogen-like ions, and it introduced the crucial idea of quantised energy. Its problem is that it is a semi-classical model: it keeps the classical picture of an electron moving in a definite circular orbit while adding a quantum condition. A complete description needs full quantum mechanics, where the electron has no fixed orbit.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
(1) It applies only to hydrogenic (one-electron) atoms and cannot be extended even to helium, because it ignores electron-electron forces. (2) It correctly predicts spectral line frequencies but cannot explain their relative intensities (why some lines are strong and others weak). (3) It is a semi-classical model that mixes a classical orbit picture with a quantum condition, so it cannot be generalised to complex atoms; a full quantum-mechanical theory is needed.
Bohr's model correctly describes hydrogen-like atoms and ions that have exactly one electron: H, He+ (Z=2), Li2+ (Z=3) and Be3+ (Z=4). For these, its radius, velocity and energy formulas (with the atomic number Z included) give correct results.
Bohr built the model around a single electron orbiting the nucleus, using only the attractive Coulomb force between the nucleus and that one electron. There is no second electron in a one-electron atom, so no repulsion term appears. The moment a second electron is added, the missing electron-electron repulsion makes Bohr's equations inaccurate, which is why the model fails for helium and heavier atoms.
No. Bohr assumed the quantisation of angular momentum (mvr = nh/2pi) as a postulate without explaining why. It treats the electron as a particle in a fixed circular orbit. The wave nature and the reason behind the quantisation condition came later from de Broglie, who showed the orbit circumference equals a whole number of electron wavelengths (2*pi*r = n*lambda). This is covered in the next concept.
Yes, it is commonly listed as a limitation in exams. Bohr gives only one energy per level, so it predicts a single spectral line even in a magnetic field. In reality a magnetic field splits lines (Zeeman effect), and an electric field splits them too (Stark effect). Bohr cannot explain this splitting. NCERT states three core limitations; Zeeman, Stark and fine structure are standard additions that follow from the same single-electron, single-energy weakness.
