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Elements that are naturally unstable combine with other elements to gain stability. Inert gases or noble gases are stable. Molecules can be diatomic, triatomic, or polyatomic and can be made of the same type of atoms or different types of atoms. Molecules that are made of two atoms are binary compounds, while those made of three atoms are tertiary compounds. Compounds have a fixed composition irrespective of the source from which they are prepared, and this follows the law of constant proportion. Molecules have atoms bonded around them and occupy space. They can have different bondings, like covalent and ionic.
Ionic and covalent molecules are very different from one another. An ionic bond is formed by the transfer of electrons, forms ions and is crystalline and non-directional. A covalent bond is usually formed by sharing electrons, without ions, amorphous solids, and is directional.
Molecules forming a covalent bond with the same atoms, like homoatomic molecules, are usually arranged linearly; for example, H2, I2, Br2, Cl2, F2 etc. are binary molecules. Molecules forming a covalent bond with the same atoms with more than two atoms are polyatomic molecules; for example, O3, P4, S8. Molecules forming a covalent bond with different atoms usually are arranged in various forms.
To explain molecular geometry and bond angles, the most important theory we need to discuss is the VSEPR theory.
Nitrogen dioxide (NO2)
Nitrogen and oxygen readily combine to form various oxides as nitrogen shows variable oxidation states. In the oxides of nitrogen, nitrogen always shows a positive charge, and oxygen possesses a negative charge. This is because oxygen is more electronegative, nitrogen is less electronegative than oxygen, and the electronegative element with a negative charge is more stable. Nitrogen shows a +4 oxidation state, and each oxygen shows a -2 oxidation state. The overall charge on the molecule is 1(+4) + 2(-2) = 0. The nitrogen atom can bond by coordinating a covalent bond, and the charge distribution leads to a negative charge on oxygen.
Nitrogen and not the oxygen atom is the central atom in nitrogen dioxide. Around the nitrogen atom are the two oxygen atoms surrounded by a double bond and a coordinate covalent bond. One oxygen atom bonds with a double bond to the central nitrogen atom, and the other oxygen bonds by a single bond with nitrogen. Nitrogen in this molecule has an incomplete octet and has odd electrons. To get an octet configuration, it combines with an oxygen atom. Oxygen atoms have an octet configuration in the molecule.
NO2 is a neutral molecule with odd electrons and can combine with other NO2 molecules and exist as a dimer N2O4. We can also write NO2 as NO2- ion as it contains a coordinate covalent bond and is called a nitrate ion. Nitrogen has one odd electron and is an example of an odd electron species.
Geometry
In the structure of nitrate ions, around the nitrogen atom, there are three bond pairs and one single electron or odd electron. The molecular geometry or the shape can be identified by calculating the number of lone pairs and bond pairs around the central atom. Around nitrogen, there are two sigma bonds and one single electron according to the above Lewis structure. According to VSEPR postulates, the sum of the lone pair and bond pair is the hybridisation of the molecule. In nitrite ion, the lone pair on nitrogen is zero. So, the number is 2(bond pairs) + 1(single electron) = 3. If the number for hybridisation is three it means the central atom undergoes sp2 hybridisation. So, for sp2 hybridisation, the expected geometry is trigonal planar structure and the bond angle is 120 degrees between the atoms. But nitrite ion deviates from the expected in the geometry as well as the bond angle.
The actual geometry observed in nitrite ion is the bent shape or V shape, and the bond angle is observed to be 134 degrees instead of 120 degrees. Nitrogen has an extra electron or odd electron in the molecule. In molecules containing lone pair and bond pair, there exist lone pair-bond pair repulsions, which lead to greater deviation in the structure. In the case of nitrate ions, due to the presence of odd electrons, the molecule experiences greater repulsive forces. If there is one lone pair, repulsion can be equal on both sides, but since an extra electron or odd electron is present, repulsions are not even between two oxygen atoms, which makes them move wider. This widening of bond pair atoms increases the bond angle in the molecule; moreover, the shape or the structure changes to the bent shape or V shape, similar to a water molecule.
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