Hydrogen molecule
The bonding between hydrogen atoms to form a molecule of hydrogen can be represented as follows:
In this representation, half-filled '1s' orbital of one hydrogen atom overlaps (shown by 'dashed' lines) with that of the other having electron with opposite spin. The 's-s' overlap leads to a sigma bond formation.
Bonding in hydrogen fluoride (HF)
Fluorine atom has seven electrons in its valence shell and needs one electron to complete its octet. Hydrogen atom also has a tendency to share one electron to acquire configuration of helium. When the two atoms approach each other, the overlapping of 1s-orbital of hydrogen and 2pz, orbital of fluorine takes place along the orbital axis to form a sigma bond. This gives a stable molecule of HF.
Electronic configuration of H and F are:
Since F is more electronegative than H, F develops a slight negative charge and H a slight positive charge. This introduces a dipole in the HF molecule.
Bonding in water (H
2O) molecule
In water molecule, the two half-filled orbitals of oxygen atom overlap with '1s' orbitals of two hydrogen atoms. This gives rise to two single bonds between one oxygen atom and two hydrogen atoms.
Electronic configuration of H and O are:
The three '2p' orbitals are oriented along the 'x', 'y' and 'z' axes and so they are right angles to each other. In water molecule only two 'p' orbitals of oxygen atom are used. Thus, the overlapping of '1s' orbital of hydrogen atoms with the orbitals of the oxygen atom will occur along 'y' and 'z' axes. Therefore, the bond angle in H2O should be 90°. However, experiments show that H2O molecule is angular and bond angle is 104.5°. This is because oxygen is more electronegative than hydrogen and so the shared pair of electron is more towards oxygen. Oxygen atom develops a slight negative charge and hydrogen positive charge. The two positively charged hydrogen atoms repel each other making the angle expand from 90° to 104.5°.
Bonding in nitrogen (N
2) molecule
Nitrogen has three half-filled orbitals. In the formation of nitrogen molecule, each of the three half-filled '2p' orbitals of one nitrogen atom overlaps with the corresponding orbitals of second nitrogen atom (one pair overlaps end to end and the other two sideways). As a result, there is a triple bond between nitrogen atoms in this molecule. Since all the spins are paired, this molecule is diamagnetic (repelled by magnet).
The electronic configuration of nitrogen atom is:N 1s2 2s2 2px1 2py1 2pz1
Bonding in ammonia (NH
3) molecule
In forming a molecule of ammonia, '1s' orbital of each of three hydrogen atoms overlap with one of the three '2p' orbitals of nitrogen. This gives rise to three N-H sigma bonds oriented along three coordinate axes.
The electronic configuration of nitrogen atom is
In this molecule each H-N-H bond angle is 107.5o instead of 90o. This is because of the more electronegativity of nitrogen atom, which causes polarity in the molecule. Hence repulsion between light charges on the hydrogen atom expand both the H-N-H bond and its angle.
