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Molecules formed between different elements can be covalent or ionic. Molecules that contain different atoms in them are called compounds. Elements present in these compounds share electrons in various ways and form bonds between them. The sharing of electrons can be of two types between the atoms:
- The first type is where the bonding atoms contribute one electron each and share the two electrons among the atoms, known as covalent bonding
- The sharing of electrons in which one of the bonding atoms contributes two electrons, and the other atom contributes no electrons. The two electrons are shared between the two atoms. This type of sharing is called coordinate covalent bonding
Covalent bonding can be seen between two atoms with equal electronegativity or with less difference in electronegativity or two non-metals. The coordinate covalent bond is a bond similar to a covalent bond, but one of the atoms in the molecule should have an extra pair of electrons like lone pair. The electrons that do not participate in the bonding but are present in the valence shell of the central atom is called a lone pair. The number of electrons that can be shared between two atoms can be 2,4 or 6, where 2 is minimum, and 6 is maximum.
Ammonia Molecule (NH3)
NH3 is the molecular formula for ammonia. Nitrogen is the central atom in Ammonia and H are the bonded atoms around nitrogen. Ammonia is a base that has a pungent smell. Ammonia can exist as liquid as well as gas. Ammonia and compounds of ammonia can show their basic nature turning red litmus blue.
Geometry
Ammonia molecule geometry is explained by valence bond theory (VBT) and the valence shell electron pair repulsion (VSEPR) theory. The valence bond theory explains the bonding in the molecule with the overlapping of orbitals in the valence shell.
In NH3, N has 1s22s22p3 electronic configuration.
It uses 2s and 2p orbitals in the valence shell and undergoes sp3 hybridisation to give four hybrid orbitals and overlap head-on and lead to sigma bonds. Out of the four sigma bonds, three are with an unpaired electron, and one is with lone pair electrons.
According to VBT, for the sp3 hybridisation, geometry is tetrahedral, and the bond angle is 109.28 degrees.
The actual geometry for ammonia is trigonal pyramidal, and its bond angle is 107 degrees. This cannot be explained by the VBT, but can be explained by the VSEPR theory.
VSEPR Theory
Valence shell electron pair repulsion theory states that the shape and geometry of a molecule are decided by the repulsions existing between the bond pair electrons and lone pair electrons of the central atom.
Ammonia molecule:
N – atomic number 7 – 1s22s22p3
N in NH3 has three bonds around it and a lone pair.
According to the VSEPR theory, the sum of the number of lone pairs and bond pairs of the central atom gives the hybridisation of the molecule.
In ammonia, the sum of the lone pair and the bond pair is 1+3 = 4
If the number of hybridisation is four, then the hybridisation of the central atom is sp3. The expected bond angle is 109.28 degrees, and the geometry is tetrahedral.
In contrast, ammonia shows trigonal pyramidal geometry and <109 bond angle, and the reason behind this deviation is explained by the VSEPR theory.
Reason
In the case of ammonia, the molecules are around N, and there are three bonds between N and H (N-H). N has four sp3 hybrid orbitals in which three orbitals are with unpaired electrons, and one orbital is with lone pair electrons. So, there are three bond pairs and one lone pair, which are placed close to one another. There exists bond pair-bond pair repulsions as well as lone pair-bond pair repulsions. Compared to bond pair-bond pair repulsions, lone pair-bond pairs are more strong and push the bond pairs far from the lone pair, resulting in a decrease in bond angle and also a deviation in the geometry from the expected.
Briefing the Content
Covalent molecules combine with other atoms by sharing electrons and forming compounds. In covalent molecules, the atoms bond by sharing electrons, and those bonds are called bond pairs of bond pair electrons. Molecules contain different atoms in which there is one central atom and the remaining bonding atoms. The central atom is the atom in the molecule with a high valency or more number of bonds attached to it. The bonded atoms have less valency than the central atom. A central atom contains more valence electrons in its outermost shell and shares electrons with other atoms or can contain non-bonded electrons called lone pairs. All the other atoms bond and arrange themselves in space around the central atom. This arrangement of bonded atoms around the central atom gives geometry and shape to the molecules.
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