Electron Distributions Into Shells for the First
Three Periods
Lewis Dot Diagrams of Selected Elements
Lewis Diagrams for Compound Formation
The formation of many common compounds can be visualized
with the use of Lewis symbols and Lewis diagrams. In a Lewis
symbol, the inner closed shells of electrons can be considered as
included in chemical symbol for the element, and the outer shell
or valence electrons are represented by dots. The dots are placed
in four groups of one or two electrons each, with 8 electrons
representing a closed shell or noble gas configuration. Lewis
diagrams are useful for visualizing both ionic and covalent bonds.
In the idealized ionic bond, one atom gives up an electron to the other,
forming positive and negative ions.
In the idealized covalent bond, two atoms share a pair of electrons,
closing the shell for each of them.
Lewis Diagrams for Multiple Bonds
Lewis symbols and Lewis diagrams can
be used to describe multiple bonds,
but further information must be
supplied to account for the three
dimensional geometry of the resulting
molecules.
For multiple single bonds, the procedure is similar that for a single bond.
The Lewis diagrams can also help visualize double and triple bonds.
Ionic Compounds
Crystalline solids (made of ions)
High melting and boiling points
Conduct electricity when melted
Many soluble in water but not in nonpolar liquid
Covalent Compounds
Gases, liquids, or solids (made of molecules)
Low melting and boiling points
Poor electrical conductors in all phases
Many soluble in nonpolar liquids but not in water
You can anticipate some things about bonds from the positions of the constituents in
the periodic table. Elements from opposite ends of the periodic table will generally
form ionic bonds. They will have large differences in electronegativity and will
usually form positive and negative ions. The elements with the largest
electronegativities are in the upper right of the periodic table, and the elements with
the smallest electronegativities are on the bottom left. If these extremes are
combined, such as in RbF, the dissociation energy is large. As can be seen from the
illustration below, hydrogen is the exception to that rule, forming covalent bonds.
Elements which are close together in electronegativity tend to form covalent bonds
and can exist as stable free molecules. Carbon dioxide is a common example.