## Heisenberg Uncertainty Principle From Unit – 2 Class 11 Chemistry

*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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