About hydrogen bond

1.
The O atom can form 2 hydrogen bonds because it has 2 lone pairs of electrons.

Each H atom can form 1 hydrogen bond, and the 2 H atoms can form 2 hydrogen bonds.

Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 2 (= (2 + 2)/2).


2.
The N atom can form 1 hydrogen bond because it has 1 lone pair of electrons.

Each H atom can form 1 hydrogen bond, and the 3 H atoms can form 3 hydrogen bonds.

Mole ratio N : H = 1 : 3, and thus the H atoms are in excess to form hydrogen bonds. Averagely, only 1 H atom in each NH3 molecule can form hydrogen bond.
Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 1 (= (1 + 1)/2).


3.
The F atom can form 3 hydrogen bonds because it has 3 lone pairs of electrons.

The H atom can form 1 hydrogen bond .

Mole ratio H : F = 1 : 1, and the lone pairs on F atoms are in excess to form hydrogen bonds. Averagely, the F atom only form 1 hydrogen bond.

Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 1 (= (1 + 1)/2).


4.
Pure CHCl3 (A) cannot form hydrogen bonds, because the negative charges on the Cl atoms are NOT sufficent enough to form hydrogen bonds.

Pure CH3­CHO (B) cannot form hydrogen bonds, becasue the positive charges on the H atoms are NOT sufficient enough to form hydrogen bonds.

The mixture of CHCl3 (A) and CH3­CHO (B) can form hydrogen bonds. To form hydrogen bond, the H atom in A is positively charged enough and the O atom in B is negatively charged enough.
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呢句唔係好明>"Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 2 (= (2 + 2)/2)."

一個O有兩pair lone pairs,加一個H可以form一個H-bond,咁一個H2O唔係可以form4個H-bond?

thanks~