Lewis Formulas (DP IB Chemistry): Revision Note

Lewis Formulas

• Lewis formulas are also known as Lewis structures or electron dot diagrams

• They show all valence electrons in a covalently bonded species

  • This includes bonding pairs and lone pairs

• A pair of electrons can be represented by:

  • Dots

  • Crosses

  • A combination of dots and crosses

  • A line

• For example, chlorine can be shown as:

• Note: Cl–Cl is not a Lewis formula, since it does not show all the electron pairs

Steps for drawing Lewis formulas

1. Count the total number of valence electrons:

   • This is for all atoms in the molecule

2. Draw the skeletal positions:

   • This is to show how the atoms are connected

   • The central atom, usually the least electronegative, should be in the middle

3. Add electron pairs:

   • First, add the bonding pairs between each pair of bonded atoms

   • Then, add lone pairs to complete octets (or duets for hydrogen), starting with outer atoms

4. Check your structure:

   • Confirm that all valence electrons have been used

   • Ensure all atoms (except exceptions) have full octets, or duets for hydrogen

Drawing Lewis formulas

• Applying the steps to draw full Lewis formulas for some simple molecules:

Methane, CH₄

1. Count the total number of valence electrons:

   • C = 4 electrons

   • H = 1 electron

   • CH₄ = (1 x C) + (4 x H) = (1 x 4) + (4 x 1) = 8 electrons total

2. Draw the skeletal positions:

   • Carbon is the central atom surrounded by four hydrogen atoms

3. Add electron pairs:

   • First, add the bonding pairs

   • Each C-H bond contains one pair of electrons

   • Then, add lone pairs

   • The bonding pairs account for all electrons

   • So, there are no lone pairs

4. Check your structure:

   • All valence electrons used

   • Carbon has a full octet

   • Hydrogen has duets

Ammonia, NH₃

1. Count the total number of valence electrons:

   • N = 5 electrons

   • H = 1 electron

   • NH₃ = (1 x N) + (3 x H) = (1 x 5) + (3 x 1) = 8 electrons total

2. Draw the skeletal positions:

   • Nitrogen is the central atom surrounded by three hydrogen atoms

3. Add electron pairs:

   • First, add the bonding pairs

     • Each N-H bond contains one pair of electrons

   • Then, add lone pairs

     • The bonding pairs account for 6 electrons

     • So, there is one lone pair on nitrogen

4. Check your structure:

   • All valence electrons used

   • Nitrogen has a full octet

   • Hydrogen has duets

Tetrachloromethane, CCl₄

1. Count the total number of valence electrons:

   • C = 4 electrons

   • Cl = 7 electrons

   • CCl₄ = (1 x C) + (4 x Cl) = (1 x 4) + (4 x 7) = 32 electrons total

2. Draw the skeletal positions:

   • Carbon is the central atom surrounded by four chlorine atoms

3. Add electron pairs:

   • First, add the bonding pairs

     • Each C-Cl bond contains one pair of electrons

   • Then, add lone pairs

     • The bonding pairs account for 8 electrons and complete the octet for C

     • There are 24 remaining electrons

     • This means that there is a total of 12 lone pairs of electrons

     • Each chlorine atom has 3 lone pairs

4. Check your structure:

   • All valence electrons used

   • Carbon has a full octet

   • Chlorine has octets

When the octet rule does not apply

• Most elements tend to follow the octet rule, aiming for 8 electrons in their valence shell to match a noble gas configuration

• However, several elements can form stable compounds with fewer than 8 electrons:

  • Hydrogen (H):

    • Stable with 2 electrons (1s²), like helium

  • Lithium (Li):

    • Loses one electron to become Li⁺ (1s²)

  • Beryllium (Be):

    • Often forms compounds with just 4 valence electrons

  • Boron (B) and Aluminium (Al):

    • Can form stable compounds with only 6 valence electrons

• Two examples of stable molecules with incomplete octets that do not follow the typical octet rule are:

Beryllium chloride, BeCl₂

• BeCl₂ = (1 x Be) + (2 x Cl) = (1 x 2) + (2 x 7) = 16 electrons total

• Beryllium is the central atom surrounded by two chlorine atoms

• Each Be-Cl bond contains one pair of electrons

  • Be has only 4 electrons around it (2 bonding pairs)

  • Be does not achieve an octet, but the molecule is stable

• Each chlorine atom has 3 lone pairs

  • Each chlorine has a complete octet

• BeCl₂ is stable despite beryllium not reaching 8 electrons

Boron trifluoride, BF₃

• BF₃ = (1 x B) + (3 x F) = (1 x 3) + (3 x 7) = 24 electrons total

• Boron is the central atom surrounded by three fluorine atoms

• Each B-F bond contains one pair of electrons

  • B has only 6 electrons around it (3 bonding pairs)

  • B does not achieve an octet, but the molecule is stable

• Each fluorine atom has 3 lone pairs

  • Each fluorine has a complete octet

• BF₃ is stable despite boron not reaching 8 electrons