NH3 has **ONE** lone pair
each lone pair can "share" an H with another moleucle; ie form a H-bond
In order for hydrogen bonds to exist, there must be a hydrogen atom that is joined or bonded with either an oxygen, flourine or nitrogen atom. Hydrogen bonds cannot exist without either an oxygen, flourine or nitrogen atom in which a hydrogen atom cannot bond with.
The reasoning is that HCN cannot form hydrogen bonds because the hydrogen is not bound to an electronegative atom.(fluorine, oxygen or nitrogen) to produce a sufficiently polarised X-H bond.
In ethanal (CH3CHO) all the hydrogens are bonded to carbon atoms...so no hydrogen bonding occurs.
CH2O, or formaldehyde, does not have hydrogen bond donors, ie. an acidic hydrogen, such as N-H, O-H, or F-H. BUT, it can accept hydrogen bonding, since the C=O bond is polar enough. So, if you put formaldehyde into something that can hydrogen bond, like water, the Hydrogens in the water can be hydrogen-bonded to the Oxygen in the formaldehyde, BUT the Hydrogens in the formaldehyde will NOT hydrogen bond to the Oxygens in the water.
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