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In X–H ----- Y, X and Y both are electronegative elements : -
A
Electron density on X will increase and on H will decrease
B
In both electron density will increase
C
In both electron density will decrease
D
On X electron density will decrease and on H increases
Detailed Solution
In a hydrogen bond X–H$\cdots$Y, the hydrogen atom is covalently bonded to the strongly electronegative atom X.
X pulls the shared electron pair towards itself, so X gets a partial negative charge (electron density increases on X) and H gets a partial positive charge (electron density on H decreases): $X^{\delta-}-H^{\delta+}$.
This partially positive H is then attracted by the lone pair of the electronegative atom Y of another molecule, forming the hydrogen bond.
So electron density on X will increase and on H will decrease.
X pulls the shared electron pair towards itself, so X gets a partial negative charge (electron density increases on X) and H gets a partial positive charge (electron density on H decreases): $X^{\delta-}-H^{\delta+}$.
This partially positive H is then attracted by the lone pair of the electronegative atom Y of another molecule, forming the hydrogen bond.
So electron density on X will increase and on H will decrease.