Lewis Formulas (SL IB Chemistry)

• Lewis formulas are simplified electron shell diagrams and show pairs of electrons around atoms.
• A pair of electrons can be represented by dots, crosses, a combination of dots and crosses or by a line. For example, chlorine can be shown as:

Different Lewis Formulas for chlorine molecules

 

• Note: Cl–Cl is not a Lewis formula, since it does not show all the electron pairs.
• The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of 8 electrons

Steps for drawing Lewis Formulas

1. Count the total number of valence electrons
2. Draw the skeletal structure to show how many atoms are linked to each other.
3. Use a pair of crosses or dot/cross to put an electron pair in each bond between the atoms.
4. Add more electron pairs to complete the octets around the atoms ( except H which has 2 electrons)
5. If there are not enough electrons to complete the octets, form double/triple bonds.
6. Check the total number of electrons in the finished structure is equal to the total number of valence electrons

Further examples of Lewis formulas

• Follow the steps for drawing Lewis structures for these common molecules

Molecule	Total number of valence electrons	Lewis formula
CH4	C + 4H 4 + (4 x 1) = 8
NH3	N + 3H   5 + (3 x 1)=8
H2O	2H + O (2x 1) + 6 =8
CO2	C + 2O 4 + (2 x 6) = 16
HCN	H+C+N 1+ 4 + 5 = 10

Incomplete Octets

• For elements below atomic number 20 the octet rule states that the atoms try to achieve 8 electrons in their valence shells, so they have the same electron configuration as a noble gas
• However, there are some elements that are exceptions to the octet rule, such a H, Li, Be, B and Al
  • H can achieve a stable arrangement by gaining an electron to become 1s², the same structure as the noble gas helium
  • Li does the same, but losing an electron and going from 1s²2s¹ to 1s² to become a Li⁺ ion
  • Be from group 2, has two valence electrons and forms stable compounds with just four electrons in the valence shell
  • B and Al in group 13 have 3 valence electrons and can form stable compounds with only 6 valence electrons

• There are two examples of Lewis structures with incomplete octets you should know, BeCl₂ and BF₃:

Molecule	Total number of valence electrons	Lewis formula
BeCl2	Be + 2Cl =  2 + ( 2 x 7) = 16
BF3	B + 3F=  3 + (3 x 7) = 24

• Test your understanding of Lewis diagrams in the following example: