Lewis Electron Dot Structures - Practice Questions & MCQ

Lewis Symbols of Elements

The number of electrons present in the outermost shell are known as valence electrons. For example, the electronic configuration of sodium (Na) is 2, 8, 1, thus, sodium has one valence electron. According to the long form of the periodic table, in the case of representative elements, the group number is equal to the number of valence electrons. The valence electrons in atoms are shown in terms of Lewis symbols. To write Lewis symbol for an element, we write down its symbol surrounded by a number of dots or crosses equal to the number of valence electrons. Paired and unpaired valence electrons are also indicated. The Lewis symbols for some of the important elements are shown below:

Lewis Structures

We also use Lewis symbols to indicate the formation of covalent bonds, which are shown in Lewis structures, drawings that describe the bonding in molecules and polyatomic ions. For example, when two chlorine atoms form a chlorine molecule, they share one pair of electrons:

The Lewis structure indicates that each Cl atom has three pairs of electrons that are not used in bonding also known as lone pairs of electrons and one shared pair of electrons. A dash (or line) is sometimes used to indicate a shared pair of electrons:

A single shared pair of electrons is called a single bond. Each Cl atom interacts with eight valence electrons: the six in the lone pairs and the two in the single bond.

To draw the Lewis structure for any molecule like CO, we follow the following five steps:

1. Determine the total number of valence (outer shell) electrons. The sum of the valence electrons is 4 (from C) + 6 (from O) = 10. 

2. Draw a skeleton structure of the molecule. We can easily draw a skeleton with a C–O single bond:

   C–O

3. Distribute the remaining electrons as lone pairs on the terminal atoms. In this case, there is no central atom, so we distribute the electrons around both atoms. We give eight electrons to the more electronegative atom in these situations; thus oxygen has the filled valence shell:

4. Place all remaining electrons on the central atom. Since there are no remaining electrons, this step does not apply.

5. Rearrange the electrons to make multiple bonds with the central atom in order to obtain octets wherever possible. In this case, carbon has only four electrons around it. To move to an octet for carbon, we take two of the lone pairs from oxygen and use it to form a CO triple bond.

This satisfies the Octet condition for both the atoms