In 1916, an American chemist Gilbert Newton Lewis worked on how atoms are connected in a molecule. As a result of his research, he discovered covalent bonds and developed a method to represent the bonds between the atoms of molecules by using simple structures called lewis electrons or lewis dot structures.
Lewis dot structures are the chemical symbols of elements. They have valence electrons that are manually arranged around the sides of the symbol of elements.
These diagrams are named lewis dot structures because valence electrons are drawn in the form of dots around the four sides of atoms. A single dot represents an unpaired electron. This electron is responsible for the chemical bond formation when it comes close to the unpaired electron of other atoms. Now, these unpaired shared electrons are called bond pairs.
However, double dots show as a pair of electrons on atoms in a molecule called lone pair. They don’t take part in the formation of bonds between the atoms of a molecule or compound.
The concept is the same for both terminologies which are lewis structure and lewis dot structure. The only difference is the mode of drawing or presenting the structures of molecules or compounds. When a structure of a molecule is drawn and paired electrons are shown in the form of dots and unpaired shared electrons are shown in the form of a solid line called a lewis structure, not a lewis dot structure.
However, in the lewis dot structures, paired and unpaired electrons are shown in the form of dots. The whole structure is a combination of dots that represents electrons. These electrons are not fixed on a specific side of an atom, they can be drawn on the random sides of elements.
Both lewis structure and lewis dot structure are valid to represent the structure of molecules but it creates confusion when we use the solid lines for unpaired shared electrons and dots for paired electrons. That’s why it is better to use one type of method, lewis structure or lewis dot structure to demonstrate the paired and unpaired electrons of molecules or compounds.
Metallic elements can lose electrons and form ions. Hydrogen is an exceptional case and when combined with non-metallic elements forms a covalent compound. However, non-metals have filled s-orbitals and partially filled p-orbitals. They have fixed numbers of electrons in their outermost shell called valence electrons.
These valence electrons are equal to the group of the element in the periodic table. For example, boron (B) has three valence electrons in its outermost shell and belongs to group 3A of the periodic table. Similarly, carbon (C) and silicon (Si) have four valence electrons and belong to group 4A. This trend is followed by all groups in the periodic table, groups 5A, 6A, 7A, and 8A.
Lewis dot structures have single and double dots representing unpaired and paired electrons. By counting the number of single dots or electrons on the elements, we can predict the diagram of the element. It is because atoms form covalent bonds to complete their doublet or octet rule. This can be seen by drawing lewis dot structures of elements.
For example, hydrogen (H) has a single electron so it can form a covalent bond by completing the doublet rule. Boron (B) has three single electrons, so it can form three covalent bonds. It does not complete the octet rule like other elements. Similarly, nitrogen (N) has one lone pair (double dots) and three single electrons (single dots), and fluorine (F) has only one single electron. So, nitrogen can make three covalent bonds while fluorine can form one covalent bond. Neon (Ne) cannot form any bond because it has already completed its octet rule and has no single electron.
In a compound, when one element shares its single electron with the other element’s single electron, they form a chemical bond. For example, in the case of hydrogen fluoride (HF), hydrogen has a single electron, combined with the single electron of the fluorine to form a chemical bond.
These are the steps to draw lewis dot structures of atoms, molecules, and compounds.
- The first step is to calculate the number of valence electrons of every atom in a molecule or compound. This can be found by counting the number of electrons in the outermost shell of the atom. For example, carbon dioxide has a total of 16 valence electrons. Carbon has 4 valence electrons and each oxygen has 6 electrons.
- A molecule or compound has more than one type of atom present in it. The atom which is the least electronegative becomes a central atom in the compound.
- Now assign electrons one by one to the atoms. For example, in the case of carbon dioxide, we have 16 valence electrons and three atoms. We can start distributing electrons from any side. One electron is given to the first oxygen atom, the second electron is given to the carbon atom, and the third electron is distributed to the second oxygen atom. Similarly, this type of electron distributing pattern continues until the 16 valence electrons are completely distributed to the molecule of carbon dioxide.
- Count the total number of valence electrons around each atom. Each atom should be eight electrons and complete its octet. A few elements that do not complete their octets are boron (B) and hydrogen (H).
- When a molecule is an anion. The negative charges on the molecule are added to the counting of the total valence electrons on the molecule.
- In a cation molecule, the positive charges on the molecule are subtracted from the total count of valence electrons.
- Lone pairs are assigned from the most electronegative atom to the least electronegative atom.
- After assigning lone pairs to the atoms. For atoms that are not completing their octet configuration, double or triple bonds can be drawn to complete the octet valency of each atom. In some cases, the lone pairs can be converted to bond pairs to draw the double or triple bond.
Simple inspection method is a simple method to calculate all the valence electrons of atoms in a molecule and assign them. In this way, structures of small and simple molecules can be determined by using the lewis dot structure.
Sometimes, small or complex molecules create problems when assigning double or triple bonds. For example, the lewis dot structure of carbon monoxide (CO) cannot be determined by using a simple inspection method.
Pooled electron method is used when the lewis dot structure of molecules cannot be drawn by a simple inspection method. In pooled electron method, calculate the total number of valence electrons and distribute them in such a way that each atom has eight electrons. For example, carbon monoxide (CO) has 10 valence electrons. Carbon (C) has four electrons while oxygen (O) has six electrons. These 10 electrons are distributed as the formation of a bond takes place and each atom has completed its octet.
Pooled electron method is used for determining the lewis dot structure of polyatomic ions.
Lewis dot structure of negatively charged polyatomic ions can be drawn by adding the negatively charged value in the total valence electron of the ion. For example, sulfate ion (SO4-2) has a negatively charged value of -2. In the case of sulfate ion (SO4-2), the sulfur (S) atom contributes six valence electrons while each oxygen atom gives six valence electrons. Total valence electrons are 30 but adding the negative charge value (-2) gives a total result of 32 valence electrons. In this way, the sulfate ion has a total of 32 valence electrons that can be shown in the lewis dot structure.
To draw the lewis dot structure of positively charged polyatomic ions subtract the positive charge value from the total valence electrons. For example, ammonium ion (NH4+) has 5 valence electrons of nitrogen (N) atom and 4 valence electrons of four hydrogen (H) atoms giving a total of 9 valence electrons in ammonium ion with a positive charge (+1). When we draw the lewis dot structure of ammonium ion, we subtract the one valence electron from the total number of valence electrons. In this way, 8 valence electrons are shown to draw the lewis dot structure of ammonium ion.
Resonance is due to the delocalized movement of pi-electrons in the structure. In this way, they show different structures of the same molecule or ion.
There are some ions or compounds that show resonance in their structure. So it is a challenge to draw these structures in the form of lewis dot structure. The method is the same to calculate the valence electrons of the atoms but the only difference is the position of the electrons which are delocalized in the structure.
For example, carbonate anion (CO3-2) has three equivalent lewis dot structures. In each structure, one of the three oxygen atoms has to contribute 4 valence electrons (double bonded with carbon atom) instead of 6 valence electrons.
What is the Lewis dot structure?
Lewis dot structure represents the structure of elements in the form of dots. These dots are basically valence electrons that can be paired or unpaired.
How to know where to put the dots on a Lewis structure?
There are some steps to follow to draw the structure. First dots can be distributed to the central atoms, especially lone pairs.
What would be a good method for figuring out how to draw 3-dimensional Lewis-Dot structures?
These are two methods to draw lewis dot structure.
- Simple inspection method
- Pooled electron method
What is the difference between lone and bond pairs?
Lone pairs are paired valence electrons that do not take part in a chemical reaction. However, bond pairs are unpaired shared electrons that make the bond between the two atoms.
What is the difference between lewis structures and lewis dot structures?
When the lewis structure is represented with a solid line and dots called the lewis structure while the lewis dot structure has only dots, even bonds are shown in the form of dots.