Lewis Formulas (HL 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
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: