Heisenberg Uncertainty Principle Class 11 Chemistry Unit – 2 (CBSE & NCERT)
Chemistry notes for Class 11: According to this principle, it is impossible to determine simultaneously both the position and velocity or momentum of a small moving particle like an electron.
In different words, the uncertainty principle says that we cannot measure the position (x) and the momentum (p) of a particle with absolute precision. The more accurately we know one of these values, the less accurately we know the other. The uncertainty principle, also called Heisenberg uncertainty principle or indeterminacy principle, statement, articulated (1927) by the German physicist Werner Heisenberg, that the position and the velocity of an object cannot both be measured exactly, at the same time.
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The concept of this principle becomes clearer by assuming that all observations have to be made by the impact of light radiations or photons. If the object is of a reasonable bigger size, its position or velocity will not be changed by the impact of light photons; and it will be possible to find out both its position and velocity. However, when the object is very small, such as an electron, it will suffer a change in its path and velocity due to the impact of even a single photon of light.The path and velocity of an electron, after the impact of a light photon, may be different from the original path and velocity.
On this basis, Bohr’s model of the atom does not hold good; which says that electrons are particles that revolve in definite orbits or well-defined path. Hence, it will be more appropriate to say that an electron is associated with certain energy, i.e. an electron belongs to a definite energy level but not a definite orbit.
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According to the Heisenberg’s uncertainty rule, it is not possible to know exactly both the position and the momentum of an electron. Thus, the certainty of determination of one property leads to uncertainty of determination of the other. The uncertainty in the measurement of position, ∆x, and uncertainty of determination of momentum, ∆p are related by Heisenberg’s relationship as-
(∆x) X (∆p) ≥ h/4π
Where h is Planck’s constant.
If ∆x is very small, i.e., the position of the electron is known more or less exactly, ∆p would be large, i.e. uncertainty of momentum will be large or vice- versa.
On the basis of the concept of probability, it is possible to state or predict the probability or relative chance of finding an electron of a particular energy in a given region of space at a given time. The volume in space around the nucleus of the atom, in which there is the maximum probability of finding an electron, is called an orbital. The charge on the electron is diffused just like a cloud. The regions in space where the density of charge cloud is highest are called as atomic orbitals. Most of the time, the possibility of finding an electron in these orbitals is very large.
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