In an ice crystal, each water molecule is hydrogen bonded to ____________ neighbouring molecules.

Solution

Chemistry ma’am · 4 steps

Count what one water molecule gives and what it takes: two hydrogens out, two lone pairs in.

1. Donating hydrogens. Each H₂O has two O–H hydrogens. Each one points at the oxygen of a neighbouring molecule, so the molecule donates 2 hydrogen bonds.
2. Accepting hydrogens. The oxygen also carries two lone pairs. Each lone pair holds the hydrogen of another neighbour, so the molecule accepts 2 more hydrogen bonds.
3. Tetrahedral network. These 2 + 2 = 4 bonds point to the corners of a tetrahedron around each oxygen. That gives the open, cage-like structure of ice, which is why ice is less dense than liquid water.
4. So. In ice, each water molecule is hydrogen bonded to 4 neighbouring molecules.

The common slip: 2. Counting only the bonds made through the two hydrogens of the molecule gives 2. The two lone pairs on oxygen accept two more hydrogen bonds.

The idea to keep. For hydrogen bonds, count both sides: the H atoms a molecule donates and the lone pairs it accepts with. Water has 2 of each, so ice has 4 per molecule in a tetrahedral cage.

Answer from the NTA final answer key. Solution by PrepWiser.