Shared electron behaviour in a covalent bond 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?
 A: 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.
