Ne xe kr ar when atoms other than the noble gas atoms

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that they rarely form chemical bonds with other atoms. Ne Xe Kr Ar When atoms other than the noble gas atoms form bonds, they often have eight electrons around them in total. For example, the unpaired electron of a chlorine atom often pairs with an unpaired electron of another atom to form one covalent bond. This gives the chlorine atom an octet of eight electrons around it: two from the two‑electron covalent bond and six from its three lone pairs. This helps us explain why chlorine gas is composed of Cl 2 molecules. Cl Cl Cl Cl Note that each chlorine atom in Cl 2 has an octet of electrons. Apparently, the formation of an octet of electrons leads to stability. Cl Cl This way of depicting a molecule—using the elements’ symbols to represent atoms and using dots to represent valence electrons—is called a Lewis structure . Covalent bonds are usually represented by lines in Lewis structures, so the Lewis structure of a Cl 2 molecule can have either of two forms: Cl Cl Cl Cl or The nonbonding pairs of electrons are called lone pairs . Each atom of chlorine in a Cl 2 molecule has 1 covalent bond and 3 lone pairs. Lone pairs (nonbonding electrons) covalent bond Cl Cl Lone pairs (nonbonding electrons) Some atoms do not form octets of electrons when they bond. For example, hydrogen atoms form one bond, achieving a total of two electrons around them. The reason is similar to the reason that chlorine atoms form one covalent bond and have three lone pairs. Atoms of helium, which is one of the very stable noble gases, have two electrons. When hydrogen atoms form one covalent bond, they get two electrons around them, like helium atoms. Knowing that hydrogen atoms form one covalent bond and that chlorine atoms form one bond and have three lone pairs helps us to build the Lewis O BJECTIVE 11 O BJECTIVE 12 O BJECTIVE 12 O BJECTIVE 11
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3.3 Molecular Compounds 81 structure for a hydrogen chloride molecule, HCl: H Cl + or Cl H Cl H Like chlorine, the other elements in group 7A also have seven valence electrons, so their electron‑dot symbols are similar to that of chlorine. The unpaired dot can be placed on any of the four sides of each symbol. Br F I In order to obtain octets of electrons, these atoms tend to form compounds in which they have one bond and three lone pairs. Note how the Lewis structures of hydrogen fluoride, HF (used in the refining of uranium), hydrogen bromide, HBr (a pharmaceutical intermediate), and hydrogen iodide, HI (used to make iodine salts) resemble the structure of hydrogen chloride. Br H I H F H Hydrogen fluoride Hydrogen bromide Hydrogen iodide The nonmetallic elements in group 6A (oxygen, sulfur, and selenium) have atoms with six valence electrons: O Se S (The unpaired dots can be placed on any two of the four sides of each symbol.) These elements usually gain an octet by forming two covalent bonds and two lone pairs, as in water, H 2 O, and hydrogen sulfide, H 2 S.
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  • Fall '06
  • Mark
  • Atom, atoms, Chemical bond

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