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Some basic questions:

1) I can't figure out how two atoms form a covalent bond. We say two atoms make a covalent bond when they share an electron. On the other hand, we know that electrons circulate around the nucleus so fast! For example, one thousand times in a second! So does this "electron sharing" mean that an electron in an atom now circulates 500 times around its own nucleus and 500 times around the nucleus of the other atom?

2) Another question: they say after a covalent bond forms, the probability of finding an electron in the space in the middle of two atoms becomes larger than in other places. But if electrons still circulate around the nucleus, does that mean that now a shared electron mostly orbits the circle that passes through the bonding place? Or does electron motion change, basically?

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    $\begingroup$ Electrons don't circulate around nuclei. They occupy wave functions. The Bohr model is highly flawed and leads you to think these sorts of things. $\endgroup$
    – Jon Custer
    Commented Jun 29, 2016 at 18:11
  • $\begingroup$ @JonCuster Thanks jon, but if we reject circular motion of electrons around the nuclues and just say they occupy wave function, while we know electrons are negatively charged, and they attract by the positively charged nucleus, so how we can explain why electron dont absorbed by nucleus that cause atom collapse completely! $\endgroup$
    – Ang
    Commented Jun 30, 2016 at 7:49
  • $\begingroup$ @Ang This is a very, very, very popular question here. Almost every day someone asks it. SE has a search mechanism where you can find questions that have already been asked. As an example, check physics.stackexchange.com/questions/135222/… $\endgroup$
    – QuantumBrick
    Commented Jun 30, 2016 at 12:10

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Several questions of this nature were asked the last days. An electron does not orbit the nucleus as a particle. In Quantum Mechanics the electron is represented by a wavefunction, which gives you the probability of measuring something about the electron. This probability is spherically symmetrical in the ground state of hydrogen, for example: it means you have an equal probability of finding the electron in a region of constant radius around the nucleus. In fact, Quantum Mechanics says that you can actually find the electron everywhere, but the probability of finding it outside of this region falls very quickly to zero. For more info about that check this question.

Now, two atoms in a molecule share electrons. This means that the probability cloud, or the wavefunction of the electron, stops being localized around a single nucleus and starts spreading to the other nucleus. If one electron is shared between the atoms, then the bound will be covalent: the electron will have the same probability of being measured around both atoms (as you said, this probability is higher between them: the electron is strongly attracted by both atoms in the middle region). Now, if one of the atoms has an already filled orbital, then it may share two electrons instead of one. This is called a dative bound.

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  • $\begingroup$ I like your answer but your description of a dative bond is incorrect. Dative bonding occurs when one atom donates a filled atomic orbital to another atom that has an empty atomic orbital. Both electrons then occupy a shared molecular orbital, i.e. a bonding orbital. You also fall back on the dreaded 'orbiting' word. Nothing orbits there. Molecular orbitals are wave functions too. chemguide.co.uk/atoms/bonding/dative.html $\endgroup$
    – Gert
    Commented Jun 29, 2016 at 20:38
  • $\begingroup$ @Gert Thank you very much for this correction. I checked my atomic and molecular physics notes and you're absolutely correct. Since I passed that subject I probably did know what a dative bound was someday, but got it mixed after some years of molecular numbness. Thank you! $\endgroup$
    – QuantumBrick
    Commented Jun 30, 2016 at 2:17
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    $\begingroup$ You're welcome. Chemists (like me) do have some use in this world, after all! ;-) $\endgroup$
    – Gert
    Commented Jun 30, 2016 at 2:24
  • $\begingroup$ You do! In fact, your very good answer on the "wet hair keeps its form" shows how much the knowledge of a chemist may be useful (besides getting rich by using it in industry). $\endgroup$
    – QuantumBrick
    Commented Jun 30, 2016 at 3:19
  • $\begingroup$ thank you guys! i dont know, if i have to repeat my last comment here too or not! any way, would you plz read that too! thanks $\endgroup$
    – Ang
    Commented Jun 30, 2016 at 8:02

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