Let us determine the Lewis structures of SiH 4, \(,\) NO +, and OF 2 as examples in following this procedure: Rearrange the electrons of the outer atoms to make multiple bonds with the central atom in order to obtain octets wherever possible.Place all remaining electrons on the central atom.Distribute the remaining electrons as lone pairs on the terminal atoms (except hydrogen), completing an octet around each atom.(Generally, the least electronegative element should be placed in the center.) Connect each atom to the central atom with a single bond (one electron pair). Draw a skeleton structure of the molecule or ion, arranging the atoms around a central atom.For anions, add one electron for each negative charge. For cations, subtract one electron for each positive charge. Determine the total number of valence (outer shell) electrons.See these examples:įor more complicated molecules and molecular ions, it is helpful to follow the step-by-step procedure outlined here: Writing Lewis Structures with the Octet Ruleįor very simple molecules and molecular ions, we can write the Lewis structures by merely pairing up the unpaired electrons on the constituent atoms. Oxygen and other atoms in group 16 obtain an octet by forming two covalent bonds: To obtain an octet, these atoms form three covalent bonds, as in NH 3 (ammonia). Group 15 elements such as nitrogen have five valence electrons in the atomic Lewis symbol: one lone pair and three unpaired electrons. The transition elements and inner transition elements also do not follow the octet rule: Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule. These four electrons can be gained by forming four covalent bonds, as illustrated here for carbon in CCl 4 (carbon tetrachloride) and silicon in SiH 4 (silane). For example, each atom of a group 14 element has four electrons in its outermost shell and therefore requires four more electrons to reach an octet. The number of bonds that an atom can form can often be predicted from the number of electrons needed to reach an octet (eight valence electrons) this is especially true of the nonmetals of the second period of the periodic table (C, N, O, and F). The tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule. This allows each halogen atom to have a noble gas electron configuration. The other halogen molecules (F 2, Br 2, I 2, and At 2) form bonds like those in the chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom.
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