Question

Only triple bonds are present in?
A. ${{N}_{2}}O$
B. $C{{O}_{2}}$
C. HCN
D. ${{N}_{2}}$

Answer 1

The total number of bonds present between the atoms of a molecule depends on the valency of the atom. The number of bonds are drawn such that the valency of the atom is satisfied.

Complete answer:
- Every atom tries to reach the electronic configuration of the noble gases to become stable like them. The configuration of noble gases is of the form $n{{s}^{2}}n{{p}^{6}}$ .This configuration is reached either by sharing of electrons or losing of electrons.
- When the configuration is reached by sharing of electrons, then the bond formed is covalent bond. It generally occurs between the atoms with little difference in their electronegativities.
- So, valency is the number of outermost electrons of the atom. It states the combining capacity of the atom. It can be found if we know the atomic number of an atom as it tells us the total number of electrons present in the atom.
Atomic no. = No. of proton = No. of electron
- The number of electrons to be filled in each shell depends on Aufbau’s principle and Hund’s rule. Valency tells us only the total number of electrons present in the outermost shell of the atom and so it is different from the atomic number.
- In the given options, the elements are nonmetals and so they form bonds by sharing electrons to satisfy the octet rule. C has 6 electrons in its valence shell, N has 7, O has 8 and H has 1 electron.
- C,N,O reach octet stability by sharing 4,3,2 electrons respectively. So the total number of bonds possible in C is 4, N is 3 and oxygen is 2. H has only 1 electron in it and so it attains the configuration of He by sharing one more electron and so makes 1 bond only.
- In ${{N}_{2}}O$, there is a triple bond between N-N but a single bond between N-O which becomes double due to resonance for all the atoms. So it cannot have only triple bonds at any stage.
- C needs 4 bonds and oxygen needs 2 and they are satisfied in $C{{O}_{2}}$ as the structure of O=C=O. So this compound only has double bonds and no triple bonds.
- In HCN, H needs 1 bond, C needs 4, N needs 3. It is satisfied when C and N are connected with a triple bond and H with a single bond with C. This way, all the valency's are satisfied. Triple bond is present between C and N but between C and H, single bond is present and so this is not the answer.
- N needs 3 bonds to form a complete octet. When it is linked to its own atom, both the atoms form triple bonds with each other sharing the electrons equally to become stable. So this compound has only triple bonds in it.

Therefore the correct option is D.

Note:
When two or more atoms share their electrons to form covalent bonds, the electrons are not always shared equally among them. The sharing tendency depends on the electronegativity of the atom. Higher the electronegativity, higher is the attraction force of that atom towards the lone pairs.