Chemistry · Structure Of Atom · NEET
A path (trajectory) is a line you can draw only if you know two things at the same instant: WHERE the object is (position) and HOW FAST and in which direction it moves (velocity). If you know both, you can predict where it will be next, and next, forming a line. Heisenberg's principle says that for a tiny particle like an electron you can NEVER know both position and velocity exactly at the same time. If one is known well, the other becomes very uncertain. So the two things needed to draw a path are never both available. That is why an electron cannot have a fixed path or orbit.
Significance = the important result or consequence. NCERT states it clearly: the principle RULES OUT the existence of definite paths or trajectories of electrons. In simple words, the big takeaway is 'no fixed orbit for an electron.' Because of this, we cannot say the electron moves in a neat circle; we can only speak of the PROBABILITY (chance) of finding it in a region. This one idea is what NEET usually tests.
Bohr said the electron moves in fixed circular orbits with a known radius and known speed. But a fixed orbit is a clearly defined path, and a path needs exact position AND exact velocity at the same time. Heisenberg's principle says this is impossible for an electron. So Bohr's fixed orbits cannot really exist. NCERT says Bohr's model both ignores the dual (wave) nature of the electron AND contradicts the uncertainty principle. That is why it failed for atoms with more than one electron.
It is not a measuring-instrument problem. It is a basic law of nature for tiny particles. The electron does not have a sharp position and a sharp velocity at the same time. So we describe it using a wave function and speak of probability density (chance per unit volume) of finding it. The electron is somewhere in the atom, but its exact location and speed together are not defined, so no line-path can be drawn.
The uncertainty (Δv × Δx) exists for everything, but for heavy objects it is so small it does not matter. For a milligram or heavier object the uncertainty is far too tiny to notice, so a ball has a clear path. For an electron (mass 9.11 × 10⁻³¹ kg) the uncertainty is large compared to the atom's size, so it becomes important. Short rule for NEET: the effect is significant ONLY for microscopic objects and negligible for macroscopic (big) objects.
An ORBIT (Bohr) is a fixed circular PATH — it needs exact position and velocity together, which the uncertainty principle forbids, so orbits have no real meaning. An ORBITAL is a quantum idea: it is the wave function ψ, a REGION where the probability of finding the electron is high. So the uncertainty principle is the reason we replaced the word 'orbit' (a path) with 'orbital' (a probability region).
Which one is the wrong statement?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Its main significance is that it rules out the existence of definite paths or orbits for electrons. Since we cannot know an electron's exact position and velocity at the same time, we cannot draw its path, so we describe it using probability instead.
Technically yes, but the uncertainty is so extremely small for heavy (macroscopic) objects that it has no real effect. So big objects still follow clear paths. The effect is significant only for microscopic particles like electrons.
A path needs both exact position and exact velocity at the same moment. The uncertainty principle says these two can never be known together for an electron, so no path can be drawn.
Bohr assumed fixed circular orbits with known radius and speed. That requires exact position and velocity together, which the uncertainty principle forbids. So Bohr's orbits cannot exist, which is one reason his model failed.
It led to the quantum mechanical model of the atom, where we use the wave function ψ and probability density |ψ|² to describe electrons as orbitals (probability regions) instead of orbits (fixed paths).
