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### Course: Chemistry library>Unit 5

Lesson 3: Quantum numbers and orbitals

# Heisenberg uncertainty principle

The Heisenberg uncertainty principle states that there is a limit to how precisely certain pairs of physical properties of a particle can be known simultaneously. Explore the Heisenberg uncertainty principle by calculating uncertainty in position given the uncertainty in momentum for Bohr model of hydrogen. Created by Jay.

## Want to join the conversation?

• Does light have mas? Doesn't the dibroglie equation show that it does?
• No, the De Broglie equation shows that matter can behave like a wave. Light has no rest mass, but it does have momentum and energy. Using Einstein's full equation, E² = p²c² + m²c⁴, with a m (rest mass) = 0, we see that for light, E = pc. So, with momentum, De Broglie's equation still applies to light without the need for mass. λ = ɦ/p
• at jay said if we know the position of particle in space we cant find out momentum of that particle my question is why we cant find out the momentum of that particle if we know the position?
• That's not exactly right. Every matter in the universe act as waves and particles, matter has a dual nature wave-particle. However, the smaller the mass, the most significant is wave behavior. I, as a person, have a wave behavior, BUT you cannot see it as my rest mass is big enough to make my wave behavior insignificant to your eyes, and a planet has even more rest mass and it's even more difficult to detect it's wave behavior. So when the mass of a particle is higher, it's more uncertain to predict it's wave aspects (wavelenght, amplitude, frequency), than it's to predict it's particle aspects (position, velocity, momentum). When the rest mass is lower, tending to zero, it's easier to predict wave it's wave behavior than it's to predict it's particle behavior. That's why we have a uncertainty principle regarding MOMENTUM and POSITION when talking about ATOMIC PARTICLES.
• How can one cite a Khan video such as this? Is it academically advisable to do so?

I am doing a biographical report on Heisenberg; I wish to cite this as background information in the report, and to quote directly.
• You should always cite your sources when you quote directly.
Here's one possible format:
Author name. Video name. Publisher name. Year of publication. Media source. Date of access.
For example:
Khan Academy. "Heisenberg Uncertainty Principle". N.p., 2016. Web. 24 Jan. 2016.
"N.p." means "No publisher".
Follow the punctuation exactly. Just change the 24 Jan. 2016 to the date you accessed the site.
• How do we know the uncertainty in the velocity of the particle??
• The 10% uncertainty was just made up to illustrate a calculation.

The point is this: The Bohr model assumes that the radius and velocity of the electron can theoretically be known with no uncertainty (like in classical physics). Later, it was found that there is a fundamental limit to precision in quantum mechanics, so Bohr's model can't be the whole picture.
• At Jay says that the Bohr model is incorrect. Is this because the Uncertainty Principle showed that the position of the electron would be at a distance greater than 2 times the radius. Whereas the Bohr model had the position of the electron at r1?
• The Bohr model could not be correct because it only worked for H and even for H it did not properly explain all the observations about the emission spectrum of H. That's why physicists continued to search for a better model, and through the work of DeBroglie, Schrodinger, Heisenberg, Born and others, the modern model was developed.
• Is the mass of an electron moving in an orbit constant or not?? if yes, then why didn't heisenberg use velocity instead of momentum??
• As far as I understand, the formula is valid for any particle, not just electron. If we used velocity instead of momentum, we would have to exclude particles that have no mass, like photon.
(1 vote)
• Okay now , a node is a region where one cannot find an electron . Assuming it is a p orbital , we can find a node at the center . So does that mean that an electron cannot pass through there ; ie ,does this mean to tell us that one can find one specific electron all the time in one specific "compartment" of the px, py or pz orbital?
• No, it just means that there is zero probability that you will ever locate the electron at that spot.
Electrons in an atom can go from one place to another without passing through the place in between.
That's why we think of them as probability clouds rather than little balls. Quantum objects just don't behave like the objects you are used to.
• At , why do we multiply the equation by .1 when there is a 10% uncertainty? Don't quite see the reasoning or the logic behind doing this.
• Previously, he converted the 10% uncertainty into a decimal (.1), which is more suitable for the equation and easier to use on a calculator.
• Why did we take the 10% and not 45% for example or 90%.?
• Here, Jay assumed the uncertainty to be 10%. It can be anything.
• why exactly is it that there is no way of measuring both velocity and position of electron. cant we find some better method of measurement or instrument or something?
(1 vote)
• It isn't so much that we cannot measure both momentum (and it is momentum, not velocity) and position of an electron at the same time, but rather there is a limit placed by nature on how precisely we may simultaneously measure both.

Any mechanism by which we might measure the electron's momentum will affect the position, making the position less certain. Likewise, any mechanism by which we might measure the electron's position will affect its momentum, making the momentum less certain. This is known as the observer effect.

Thus, while we can refine instruments to some extent, it is utterly impossible to construct any instrument that would overcome this problem. This is a limit imposed by nature and it can never be overcome as far as we know with current understanding of nature.

However, it goes deeper than that. Completely apart from any measurement anyone might make, the more set the momentum becomes the less the particle even HAS a set location. Likewise, the more set the position becomes the less set the momentum of the particle actually is. So, it is not MERELY a matter of our not being able to measure with arbitrary precision, but that the particle itself does not HAVE an exact momentum nor an exact location within very specific rules. The rule is that the product of the uncertainties of the position and momentum cannot be less than h/(4π) = 5.27286×10⁻³⁵ J∙s (h is the Planck constant).