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The duet rule or duplet rule of the first shell applies to H, He and Li—the noble gas helium has two electrons in its outer shell, which is very stable. (Since there is no 1 p subshell, 1 s is followed immediately by 2 s , and thus shell 1 can only have at most 2 valence electrons).
conclusion: Methane follows the octet-rule for carbon, and the duet rule for hydrogen, and hence is expected to be a stable molecule (as we see from daily life) H 2 S, for the central S; neutral counting: S contributes 6 electrons, each hydrogen radical contributes one each: 6 + 2 × 1 = 8 valence electrons
In the diagram of methane shown here, the carbon atom has a valence of four and is, therefore, surrounded by eight electrons (the octet rule), four from the carbon itself and four from the hydrogens bonded to it. Each hydrogen has a valence of one and is surrounded by two electrons (a duet rule) – its own one electron plus one from the carbon.
Period 2 elements (carbon, nitrogen, oxygen, fluorine and neon) obey the octet rule in that they need eight electrons to complete their valence shell (lithium and beryllium obey duet rule, boron is electron deficient.), where at most eight electrons can be accommodated: two in the 2s orbital and six in the 2p subshell.
Much has been done to explain the symmetric hydrogen bond quantum-mechanically, as it seems to violate the duet rule for the first shell: The proton is effectively surrounded by four electrons. Because of this problem, some consider it to be an ionic bond.
From the right hand rule, this is directed in the −x direction, opposite to the velocity v of the metal sheet. This force accelerates the electron giving it a component of velocity opposite to the sheet. Collisions of these electrons with the atoms of the sheet exert a drag force on the sheet. Eddy current brake.
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Codenames: Duet is a cooperative version of the game where two players try to find all their agents out of codename cards. Codenames: Duet was released in October 2017 as a two-player cooperative version of the original game. The game packaging includes 200 new word cards which can also be used for the original game.