How can the 1s orbital (in H atom for example) interfere both constructively and destructively (to give rise to bonding and antibonding molecular orbitals) with another 1s orbital, if the 1s orbital does not have nodes that make the distinction between phases? To interfere constructively/destructively the orbitals have to be in-phase/out of phase, but how is 'phase' defined in this case, as the H atom 1s orbital seems to lack nodes, and therefore two different phases...?

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    $\begingroup$ This isn't a new question, many people stumble over this conceptually. Some of the linked duplicates have a lot of maths, and the truth is that to truly understand it, you need to understand the QM that underpins this. But in the first one, I attempted to explain this with an imperfect (but easier to understand) analogy. Maybe that might help. If you know some linear algebra, then that will help, too. $\endgroup$
    – orthocresol
    Oct 2 '21 at 15:13
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    $\begingroup$ To address the very last bit, the "in-phase" and "out of phase" is not within a 1s orbital, but rather a comparison between the relative phases of the two 1s orbitals. One may be (wholly) positive and the other (wholly) negative. (This is slightly simplified, because the only requirement is that they be equal and negative, so any pair of complex values $\pm \mathrm{e}^{\mathrm{i}\phi}$ works; the choice of $\pm 1$ is simply what happens when you have $\phi = 0$. But that's going too far into theory.) $\endgroup$
    – orthocresol
    Oct 2 '21 at 15:15