Final answer:
A rock has a fixed shape because it is solid, meaning its atoms are rigidly held together. Nitrogen gas fills its whole container because gases take both the shape and volume of their container by virtue of their particles freely moving about. The differences between solids, liquids, and gases explain these characteristics.
Step-by-step explanation:
The reason a piece of rock, which is a solid, has a fixed shape is because the atoms in a solid are always in close contact with neighboring atoms, held in place by forces. On the other hand, nitrogen gas, like all gases, fills its whole container because the particles in a gas move about freely and are separated by very large distances which means the gas must be held in a closed container to prevent it from freely expanding and escaping.
The three most common states of matter on Earth are solids, liquids, and gases. A solid, like a rock, is rigid and possesses a definite shape due to the forces holding its atoms or molecules together. A liquid flows and takes the shape of its container but has a fixed volume. A gas takes both the shape and volume of its container, as seen with nitrogen gas.
In contrast, the atoms or molecules in a liquid slide over one another and are confined to the shape of the vessel in which they are placed. However, a gas will expand without limit to fill the space into which it is placed. This explains why nitrogen gas fills its whole container.
Learn more about States of Matter