# Why are Bohr's formulae for radius and velocity still in use when Heisenberg's uncertainty principle predicts otherwise?

I learnt about Heisenberg's Uncertainty principle which states that it is not possible to determine the position and momentum of a small particle at the same time.

I was told that it is because in order to be able to see the atoms, we need to make use of radiations, and when we do that, the electrons get excited by those radiations and their position changes. No matter how sophisticated our instruments get, there will be some error. If we use radiation of wavelength "D" to measure the position of electron, then minimum error in position will be "D"

1. I didn't really get how would we use radiations to make the atoms visible. Can you please explain that?

2. This principle clearly states that we cannot be certain about the position and velocity of an electron in an atom, and this must also be true for hydrogen like species. But we still use the formulae given by Bohr's model of atom to find out velocity of electron in nth orbit and its position for hydrogen like species. My teacher says that these formulae give satisfactory results for hydrogen like species. How could that be? Is it because the error is minimum in the case of hydrogen like species?

I am not able to intuitively arrive to an explanation to both these above question.

• Are they still in use? I'm not sure about that. – orthocresol Jan 5 '17 at 9:23
• I was told that they give satisfactory results for hydrogen like species. – Arishta Jan 5 '17 at 9:26
• They are in use (in a way), but their meaning is quite different from that in Bohr's theory. – Ivan Neretin Jan 5 '17 at 10:47
• They're in use insofar as the Bohr model is in use. And the Bohr model does a great job in terms of explaining the emission spectra for hydrogen and motivates some kind of quantization within the atom. That's about it though. – Zhe Jan 5 '17 at 13:39
• Fair enough, the Bohr radius $a_0$ is still in use and is a pretty important length scale; however its interpretation as "the radius of the first orbit" is quite obsolete though. Why does it work for hydrogenic atoms? That's more like Bohr was trying quite hard to explain the spectrum of the hydrogen atom, and he found a way that worked for it, and it turns out the quantum mechanics of a hydrogenic atom is pretty much the same as that of the hydrogen atom. – orthocresol Jan 5 '17 at 14:16