Lewis Dot Structure: An Overview

Dec 15, 2021 | Chemistry

The Basics of Lewis Dot Structure

A Lewis dot structure visually represents an atom’s electrons where the dots represent valence electrons in an atom’s outermost orbital. This structure is used to predict the chemical behavior of atoms and molecules. But how the lewis dot structure is drawn?

Let’s understand with an example of boron lewis dot structure.

Boron Lewis Dot Structure

Boron Lewis Dot Structure

Boron Lewis Dot Structure

Boron belongs to group 13 of the periodic table, and its electronic configuration is 2,3. This means it has three electrons in its valence shell, two in the 2s orbital and one in the 2p orbital.

How to Make Lewis Dot Structures?

A Lewis dot structure is a graphic representation of the electron distribution around atoms. This also predicts the type and number of bonds formed around an atom. It is also possible to predict the geometry of a molecule by using a lewis dot structure.

Here are some steps on how to draw lewis dot diagram of a moleculeto follow while drawing a lewis dot structure of any molecule:

Step 1: Calculate the Total Number of Valence Electrons

Sum up the valence electrons in the molecule from all the atoms.

Step 2: Determine how many Electrons are required to make the Atoms “Happy.”

When an atom’s outer electron shell is full, it is said to be “happy.” Elements in the periodic table up to period four require eight electrons to fill their outer electron shell. This property is commonly referred to as the “octet rule.”

Step 3: Evaluate the Molecule’s Bond Number

Covalent bonds form when an electron from every atom forms an electron pair. Step 2 indicates the number of required electrons, while Step 1 indicates the number of electrons you have. Subtracting the count in Step 1 from the count in Step 2 yields the number of electrons required to complete the octets. Each bond needs two electrons to form, so the number of bonds equals half the number of electrons required, or ((Step 2-Step 1)/2)

Step 4: Select a Central Atom

The molecule’s central atom is often the least electronegative or highest valence atom. To determine electronegativity, use periodic table trends or a table that records electronegativity values. Electronegativity decreases as one moves down the periodic table and increases as one moves from left to right across a period. Hydrogen and halogen atoms typically appear on the molecule’s periphery and are hardly ever the central atom.

Step 5: Sketch a Skeleton

A solid line indicating a bond between the two atoms connects the atoms to the central atom. A maximum of four other atoms can be connected to the central atom.

Step 6: Arrange Electrons outside Atoms

Fill the octets surrounding every outer atom. The skeletal frame from Step 5 is inaccurate if there are insufficient electrons to fill the octets. Alter your arrangement. This might need some trial and error at the beginning. It will become easier to predict skeletal structures as you gain experience.

Step 7: Distribute the Leftover Electrons surrounding the Central Atom.

With the remaining electrons, fill the octet for the central atom. Make double bonds to lone pairs on outside atoms if any bonds remain from Step 3. Two solid lines between two atoms represent a double bond. If the central atom has more than eight electrons and is not one of the octet rule exceptions, the number of valence electrons in Step 1 could have been incorrectly counted. 

This completes the molecule’s Lewis dot structure.

The Octet Rule

The octet principle is a rule of thumb in chemical chemistry that predicts how atoms will bond together. The law states that atoms will tend to bond so that they each achieve a full outer shell of electrons (an octet). This can be achieved by sharing electrons to form covalent bonds or by transferring electrons to form ionic bonds. 

The octet rule is a powerful predictor of molecular structure, but it is not always perfectly accurate. Some molecules, such as carbenes, borane, and free radicals like the methyl radical (CH3), which has an unpaired electron in a non-bonding orbital on the carbon atom and no electron of opposite spin the same orbital, violate the octet rule. 

Central bonding atoms like Lithium (Li), Beryllium (Be), and Boron (B) have less than four valence electrons and therefore contradict the octet rule. 

Lewis Dot Structure for Boron Trifluoride

To Draw the Lewis Dot Structure of Boron trifluoride BF3, compute four variables: 

  • The total number of valence electrons.
  • Required number of electrons to complete octet.
  • Bonding electrons, i.e., number of pairs.
  • Lone pairs (non-bonding pairs: when electrons do not participate in the formation of bonding between atoms)

Boron (B) and fluorine (F) have atomic numbers of 5 and 9, respectively. Hence, the number of valence electrons is determined by their electronic configurations. Boron has three valence electrons, while three fluorine atoms have (3*7= 21). As a result, the total number of valance electrons involved in forming BF3 is 24.

For each atom to share the required number of electrons, F must have 8 electrons in its outermost shell, and B must have six valence electrons: 6 + 8 (3) = 30. 

Calculate the difference between total valence and required electrons: 30 – 24 = 6. 

Six bonding pairs of electrons equals three covalent bonds between F and B.

Calculate the difference between the number of valence electrons and the number of bonding pair electrons to get lone pairs: 24 – 6 = 18, i.e., nine lone pairs. 

To complete the octets of electrons, choose the central atom before placing electrons around them.

According to the arrangement of elements in the table, B is the least electron-negative of F. Therefore, B forms three single bonds (6 electrons) with F’s atoms. Assign the 18 remaining valence electrons around so that each F has 6 electrons surrounding it.

According to the octet rule, each fluorine must contain 8 electrons to complete the octet, and the boron must include 6 electrons for their stability. Fluorine has 6 electrons, which is a rare case in boron. Fluorine also completes its final shell by forming a bond with the electrons of the boron.

Conclusion

The Lewis Dot Structure allows visibility of all electrons and determines if an atom follows the octet rule. Molecules can have both bonding and non-bonding electrons. 

Knowing the Lewis structure of an atom allows an understanding of how it will bond and how many bonds it will form. This knowledge can eventually enable us to understand the shapes of molecules and their chemical properties.