& Lewis Symbols
Matter is made up of one or more different types of elements. No element except for noble gases exists as an independent atom under normal conditions . However, two or more atoms may combine together to form a molecule which exists as one species. Obviously, there must be a force that holds these constituent atoms together in molecules. This force is called a chemical bond.
A chemical bond is an attractive force which holds various constituents (atom, ion etc.) together in different chemical species.
Why do atoms combine
Atoms combine together to lower down the energy of the system to attain stability. In order to be stable, they try to attain the nearest inert gas configuration.
Noble gases, also known as inert gases, are the least reactive of all elements. The reason for their least reactivity can be understood with the help of the following table :
|Name||Atomic number||Outer electronic Configuration||Electrons per shell|
Noble gases have 8 electrons in their outermost shell (except for helium which has only two electrons). In other words, their outermost s and p orbitals are completely filled (the only s orbital in case of He) making noble gases most stable, and consequently, the least reactive of all known elements. This property of noble gases leads to the following conclusion :
The atoms of different elements combine with each other in order to complete their respective octets (i.e. 8 electrons in their outermost shell) or duplets (in case of H, Li, and Be since their outermost shell can contain maximum 2 electrons) to attain stable inert gas configuration.
As said above, atoms combine to complete their octets. It can occur in two ways :
- By complete transfer of valence electrons
- By sharing of electrons
Octet rule is discussed in detail in Octet rule.
The outermost shell electrons are responsible for bond formation. Inner shell electrons are well protected and generally do not take part in the combination process.
Valence electrons of elements with atomic number 1 to 20 are given in the following table:
|Name||Atomic number||Valence electrons|
Because of the importance of valence electrons in bond formation, the dots (.) are sometimes put around the symbol of the element to represent valence electrons. These symbols along with the dots are called Lewis symbols. Lone pairs are usually put together.
Question : Write Lewis symbols of the these atoms: Lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O).
Answer : Lewis dot symbols of above atoms are given in the table below:
|Name||Atomic number||Electrons per Shell||Valence Electrons||Lewis Symbol|
Significance of Lewis Symbols
The dots on Lewis symbol help us calculate the valency of the element.
The combining power of an element is known as its valency. Valency helps us find out how many electrons an atom needs to gain, lose, or share to form a bond with other atom. For example, sodium has only one electron in its outermost shell; therefore, it can lose one electron. So, the valency of sodium is one. Chlorine, on the other hand, has seven valence electrons. It's easier for chlorine to gain an electron to complete its octet rather than lose seven electrons. Hence, its valency can be calculated as 8 − 7 = 1.
In general, the valency of an element is either equal to the number of dots in the Lewis symbol (if the dots are less than or equal to 4 ) or 8 minus the number of dots (if the dots are greater than 4).
Some atoms with their valencies are given below:
Valency : 1
Valency : 2
Valency : 4
Valency : 8 - 6 = 2
Case of Carbon : Carbon has four valence electrons. It's very difficult for carbon to either gain or lose four electrons. So, carbon shares electrons to achieve its octet.