I have just learned the Bohr model of the atom that electrons can be viewed as standing waves at discrete energy levels. However standing waves on, say, a string are created by 2 waves in opposite directions, but in an atom there may be just 1 electron travelling in 1 direction, so how does it create a standing wave? Thanks in advance!
I wrote a comment that might be unfair on you, as to pass exams etc., you have to jump through the hoops you are given in your introductory physics lectures, so please stick to your text and learn that.....but:
The history of physics is a long list of models, and attempts to get physical pictures that we can relate in the real world with what goes on at the smaller and smaller levels.
This is ok, until we get to the atomic level, and then it fails completely. Trying to imagine something "weird" in ordinary terms, such as standing waves is never going to work and Bohr and his colleagues knew this, that's why the Bohr model of the atom is called semi classical. It tries to take physics as it was understood at the time, and make it fit atomic and molecular structure, which it couldn't.
Today, and actually since Bohrs' model, very few physicists even try to imagine a physical picture of what an atom, or an electron, "look" like. Instead, math is used to make predictions of how the electron will behave rather than "what" an electron actually is.
After all, when we measure the mass or electric charge of an electron, we are providing a better and better description and measurement of it's physical properties, but we are not really any closer than Bohr was to pinning down what an electron is, because there is nothing like it in the world around us.