Lewis-dot Structures

Basic concepts Octet Rule – When making covalent bonds, atoms tend to achieve 8 valence electrons. The outer atoms (except H) tend to adhere to the octet rule. Central atoms do not always follow the octet rule. Since the outer atoms behave more predictably, we use them to predict the number of bonds between two atoms.

Basic concepts Only valence electrons are involved in forming covalent bonds. Dots represent individual electrons. Lines between two elements represent covalent bonds (2 e-). It is not mandatory to replace two dots with a line, but it is more common.

Process for writing Lewis-dot structures Write the symbol for the central atom surrounded by the symbols for the outer atoms. - The identity of the central atom is usually obvious. We will later learn how to determine the central atom when it is not obvious. Draw lines from the outer atoms to the central atom based on the number of bonds they need to make. (One bond for each electron needed to have same # of electrons as a noble gas.)

Process for writing Lewis-dot structures Determine the number of valence electrons NOT used for bonding by each atom. Represent these electrons (in pairs) as dots. Any charge should be applied to the central atom. Put ions in brackets with the charge on the outside.

SF2, SF4, SF6 all exist as stable molecules. However, the only stable molecule with oxygen (the element right above sulfur) and fluorine is OF2. WHY?

Look at orbital diagrams for O and S. Can you see why each one is willing to make two bonds? Remember - All covalent bonding involves valence electrons in valence orbitals only. What orbitals exist in the 3rd energy level that don’t exist in the 2nd energy level?