Chemistry Chap 1

Topics: Lewis structure, Atom, Chemical bond Pages: 5 (1670 words) Published: February 24, 2013
Lewis structure
From Wikipedia, the free encyclopedia

The Lewis structure of acarbon atom, showing its fourvalence electrons

The Lewis structure of water
Lewis structures (also known as Lewis dot diagrams, electron dot diagrams, and electron dot structures) are diagrams that show the bondingbetween atoms of a molecule and the lone pairs of electrons that may exist in the molecule.[1][2][3] A Lewis structure can be drawn for any covalently bondedmolecule, as well as coordination compounds. The Lewis structure was named after Gilbert Newton Lewis, who introduced it in his 1916 article The Atom and the Molecule.[4] They are similar to electron dot diagrams in that the valence electrons in lone pairs are represented as dots, but they also contain lines to represent shared pairs in a chemical bond (single, double, triple, etc.). Lewis structures show each atom and its position in the structure of the molecule using its chemical symbol. Lines are drawn between atoms that are bonded to one another (pairs of dots can be used instead of lines). Excess electrons that form lone pairs are represented as pairs of dots, and are placed next to the atoms. Although many of the elements react by gaining, losing or sharing electrons until they have achieved a valence shell electron configuration with a full octet of (8) electrons, there are many noteworthy exceptions to the 'octet rule'. Hydrogen (H) conforms instead to a duet rule wherein it fills its first (and outermost) shell with just two electrons or empties it completely. Some compounds, such as boron trifluoride, have incomplete orbitals, while others, such assulfur hexafluoride, have a valence shell with more than eight electrons. Contents  [hide]  * 1 Construction * 1.1 Counting electrons * 2 Formal charge * 3 Resonance * 4 Example * 5 Alternative formats * 6 See also * 7 References| -------------------------------------------------

[edit]Counting electrons
Main article: Electron counting
The total number of electrons represented in a Lewis structure is equal to the sum of the numbers of valence electrons on each individual atom. Non-valence electrons are not represented in Lewis structures. Once the total number of available electrons has been determined, electrons must be placed into the structure. They should be placed initially as lone pairs: one pair of dots for each pair of electrons available. Lone pairs should initially be placed on outer atoms (other than hydrogen) until each outer atom has eight electrons in bonding pairs and lone pairs; extra lone pairs may then be placed on the central atom. When in doubt, lone pairs should be placed on more electronegative atoms first. Once all lone pairs are placed, atoms, especially the central atoms, may not have an octet of electrons. In this case, the atoms must form a double bond; a lone pair of electrons is moved to form a second bond between the two atoms. As the bonding pair is shared between the two atoms, the atom that originally had the lone pair still has an octet; the other atom now has two more electrons in its valence shell. Aside from organic compounds, only a minority of compounds have an octet of electrons. Incomplete octets are common for compounds of groups 2 and 13 such as beryllium,boron, and aluminium. Compounds with more than eight electrons in the Lewis representation of the valence shell of an atom are called hypervalent, and are common for elements of groups 15 to 18, such as phosphorus, sulfur, iodine, and xenon. Lewis structures for polyatomic ions may be drawn by the same method. When counting electrons, negative ions should have extra electrons placed in their Lewis structures; positive ions should have fewer electrons than an uncharged molecule. When the Lewis structure of an ion is written, the entire structure is placed in brackets, and the charge is written as a superscript on the upper right,...
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