Physics · Kinetic Theory · NEET
An atom is the smallest unit of an element, like a single oxygen atom O. A molecule is two or more atoms joined together, like O2 (oxygen gas) or H2O (water). In kinetic theory of gases we usually talk about molecules, because most common gases like O2 and N2 travel around as molecules, not single atoms. Noble gases like helium (He) and argon (Ar) are special: they move as single atoms, so for them one atom is also one molecule.
Each molecule does move in a straight line, but only until it hits another molecule or the wall. A gas has a huge number of molecules (about Avogadro number, 6.02 x 10^23 per mole). They keep colliding, and every collision changes their direction and speed. Because there are so many collisions happening all the time, the overall motion looks completely random. No single molecule has a fixed path, but the average behaviour stays steady. This steady average is called dynamic equilibrium.
Almost none. In a gas the molecules themselves take up only a tiny fraction of the total volume, roughly one thousandth or less. Most of the container is empty space. This is why a gas can be squeezed (compressed) easily and why it spreads to fill any container. In a liquid or solid the molecules are packed close together, so those states are very hard to compress.
Because the molecules are far apart and move fast in random directions, and the forces between them are almost zero except during collisions. Nothing holds them in one corner. They keep moving and colliding until they are spread evenly through the entire space available. That is why gas has no fixed shape and no fixed volume of its own.
No, not most of the time. On average the distance between gas molecules is much larger than the size of a molecule itself. They only 'touch' for a very short moment during a collision, then fly apart again. Between collisions they move freely in straight lines. This large spacing is exactly why gases behave so differently from liquids and solids.
The volume occupied by the molecules contained in 4.5 kg of water at STP, if the intermolecular forces vanish away, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It means all matter, whether solid, liquid or gas, is made of very small particles called atoms and molecules. Their arrangement and motion decide the properties we see. In gases the molecules are far apart and in constant random motion.
The modern atomic theory is credited to John Dalton, who used it to explain the laws of definite and multiple proportions. NCERT also notes that ancient scholars in India (the Vaiseshika school of Kanada) and in Greece had guessed the idea of indivisible particles long before.
Because in a gas the molecules are far apart and barely interact except during brief collisions. With weak interactions, simple averages describe the gas well. In solids and liquids the molecules are close and strongly interacting, which is more complicated.
It means the molecules move in all directions with a wide range of speeds, with no preferred direction. Collisions constantly change their paths. The motion never stops as long as the temperature is above absolute zero.
Yes. It is the foundation for the whole Kinetic Theory chapter, and NEET has asked questions based on it, such as the 2022 question on the volume occupied by water molecules if intermolecular forces vanish.
