Lewis Symbols and the Octet Rule
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- 25292
Why are some substances chemically bonded molecules and others are an association of ions?
- depends upon the electronic structures of the atoms
- nature of the chemical forces within the compounds
A broad classification of chemical forces:
- Ionic bonds
- Covalent bonds
- Metallic bonds
Ionic bonds - electrostatic forces that exist between ions of opposite charge
- typically involves a metal with a nonmetal
Covalent bonds - results from the sharing of electrons between two atoms
- typically involves one nonmetallic element with another
Metallic bonds
- found in solid metals (copper, iron, aluminum)
- each metal bonded to several neighboring groups
- bonding electrons free to move throughout the 3-dimensional structure
Lets look at the preferred arrangements of electrons in atoms when they form chemical compounds
Lewis Symbols and the Octet Rule
Valence electrons reside in the outer shell and are the electrons which are going to be involved in chemical interactions and bonding (valence comes from the Latin valere, "to be strong").
Electron-dot symbols (Lewis symbols):
- convenient representation of valence electrons
- allows you to keep track of valence electrons during bond formation
- consists of the chemical symbol for the element plus a dot for each valence electron
Sulfur
Electron configuration is [Ne]3s23p4, thus there are six valence electrons. Its Lewis symbol would therefore be:
Note
- The dots (representing electrons) are placed on the four sides of the atomic symbol (top, bottom, left, right)
- Each side can accommodate up to 2 electrons
- The number of valence electrons in the table below is the same as the column number of the element in the periodic table (for representative elements only)
The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium. Atoms often gain, lose, or share electrons to achieve the same number of electrons as the noble gas closest to them in the periodic table. Because all noble gasses (except He) have filled s and p valence orbitals (8 electrons), many atoms undergoing reactions also end up with 8 valence electrons. This observation has led to the Octet Rule:
Octet Rule
Atoms tend to lose, gain, or share electrons until they are surrounded by 8 valence electrons
There are many exceptions to the octet rule (He and H, for example), but it provides a useful model for understanding the basis of chemical bonding.