Chemistry · Chemical Bonding · NEET
Hydrogen is bonded to a strongly electronegative atom X (F, O or N). X pulls the shared electron pair far away from hydrogen. So hydrogen gets a fractional positive charge (δ+) and X gets a fractional negative charge (δ-). This δ+ hydrogen of one molecule is then attracted by the δ- atom (with a lone pair) of another molecule. That electrostatic attraction is the hydrogen bond. NCERT says it in exactly this way in section 4.9.1.
Two things must be true. (1) The atom must be highly electronegative so the H becomes strongly δ+. (2) The atom must be small so its negative charge is concentrated, not spread out. F, O and N are both very electronegative AND small. Cl is fairly electronegative but it is a big atom, so its charge is spread out and its lone pair is diffuse. That is why HCl forms only very weak hydrogen bonds, while HF, H2O and NH3 form strong ones.
It is an electrostatic force of attraction, not a shared-electron (covalent) bond. It is much weaker than a covalent bond. In diagrams the covalent bond is a solid line and the hydrogen bond is a dotted line. The hydrogen atom acts like a bridge: it is held to one atom by a covalent bond and to the other atom by the hydrogen bond.
In a bond like H-F, both atoms share an electron pair. But fluorine is far more electronegative, so it pulls that electron pair toward itself. The electrons move away from hydrogen. Hydrogen has only one electron and no inner shell to shield it, so when the electron is pulled away, the bare proton-like nucleus is exposed. This gives hydrogen a strong δ+ charge, which is the key reason it can hydrogen bond.
Yes. The δ+ hydrogen must be attracted to a δ- atom that has a lone pair of electrons to offer. F, O and N all have lone pairs. So the hydrogen sits between two electronegative atoms: one it is covalently bonded to, and one whose lone pair attracts it. This is why molecules like water (H2O) and ammonia (NH3), which have lone pairs, form hydrogen bonds easily.
The CAUSE (electronegativity + small size → δ+ H → attraction) explains many NEET facts you must know: high boiling point of water, HF being a liquid-like associated molecule, high boiling points of carboxylic acids (they form dimers), and the ortho vs para nitrophenol steam-distillation question. If you understand the cause, you can predict which compound has stronger hydrogen bonding and therefore higher boiling point.
Carboxylic acids have higher boiling points than aldehydes, ketones and even alcohols of comparable molecular mass. This is due to their:
The number of hydrogen bonded water molecule(s) associated with CuSO4.5H2O is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The main cause is a large electronegativity difference between hydrogen and the atom it is bonded to (F, O or N). This makes hydrogen δ+ and the other atom δ-, so the δ+ H of one molecule is attracted to the δ- atom of another molecule.
A covalent bond shares electrons between atoms, which is strong. A hydrogen bond is only an electrostatic attraction between a δ+ hydrogen and a δ- atom - no electrons are shared. So it is much weaker, and shown as a dotted line instead of a solid line.
Yes. The electronegative atom must be small so its charge and lone pair are concentrated. F, O and N are small and electronegative. Larger atoms like Cl spread their charge out, so their hydrogen bonds are weak.
Hydrogen bonding is maximum in the solid state and minimum in the gaseous state, because in solids the molecules are held close and fixed, allowing many stable hydrogen bonds.
