2.9 Another look at Bonding – Lewis Diagrams (Section 8.2 pg 210-214)
One way to represent atoms and/or bonding is a Bohr diagram. Another way is to use Lewis Diagrams (a.k.a.‘electron dot diagrams’). • A Lewis diagram is similar to the Bohr diagram, but focuses exclusively on the valence e (only valence e are shown).
Important: Lewis diagrams describe atoms as they prepare to covalently bond with other atoms, therefore, it shows that valence e arrange themselves as single e wherever possible.
To draw a Lewis diagram for a single atom follow these steps (also in text p.211): • Determine the number of valence e for the element (count the number of spaces to the element from the left of the periodic table, skipping transition metals) • Arrange the valence e as dots around the atom’s symbol. 2 can go to a side. Only pair up electrons if you must. • Example oxygen has 6 valence electrons
To draw a Lewis diagram for a molecule follow these steps (also in text p.212): • First, draw the individual elements with their valence e. In general, you will position it so that the least common element (the one present in the lowest amount) will be in the center . • Next, share electrons between elements to make it so that each element has 8 (recall the octet rule – don’t forget H is an exception, as it only needs 2 e to have a ‘full’ outer shell). • Example NH3 =
In some cases, you may need to form double bonds (4 e shared between two atoms). • Please note: each bonding pair of e represents a covalent chemical bond – the other pairs of e are not involved, these lone electron pairs don’t form any bonds.
It is quite uncommon to use Lewis diagrams to represent bonding b/w ionic compounds. • To draw a Lewis diagram for an ionic compound, it’s the same process as for Bohr diagrams – don’t forget the ions have lost or gained electrons (see Fig.8 p.213). • Example Na2O