• Lewis structures?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 Structures for chlorine molecules

• Note: Cl–Cl is not a?Lewis structure, 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 Structures

1. 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

Answer:

Steps in drawing the Lewis Structure for CCl₄

Further examples of Lewis structures

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

• 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 1s²2s^1?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:

Incomplete Octets Examples

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

Answer:

The correct option is?D.

• • You must count the lone pairs on N and O as well as the bonding pairs. There are 5 ‘hidden’ pairs of bonding electrons in the OH, CH₂?and NH_2?groups. Hydrogen does not follow the octet rule.