Case Study Questions for Class 9 Science Chapter 8 Journey Inside the Atom (Exploration Book) 2026-27

In 1911, Geiger and Marsden, working under Ernest Rutherford, tested Thomson’s model of the atom through what became famous as the gold foil experiment. They aimed a narrow beam of alpha particles at an extremely thin sheet of gold foil. Alpha particles are tiny, positively charged particles emitted from certain radioactive elements. According to Thomson’s model, the positive charge in the atom was spread out evenly. So they expected the alpha particles to pass straight through the gold foil or be deflected only slightly.

But to their surprise, while most particles passed through undeflected, some were sharply deflected, and a few even bounced back. This deflection from the straight path is called scattering. This single surprising result completely ruled out Thomson’s ‘plum pudding model’ of the atom and led Rutherford to propose the nuclear model of the atom.

Scientists discovered that atoms of the same element can have the same number of protons (atomic number, Z) yet have different numbers of neutrons, resulting in different mass numbers. These “twin atoms” with the same atomic number but different mass numbers are called isotopes. For example, carbon has three isotopes: Carbon-12, Carbon-13, and Carbon-14. Each of these has six protons and six electrons, but they differ in their number of neutrons.

Because these isotopes occur in different proportions in nature, the atomic mass of a natural element is calculated as the weighted average of the masses of its naturally occurring isotopes. For instance, chlorine occurs in nature in two isotopic forms, 35u and 37u, in a ratio of 3:1 (or 75% to 25%). Calculating the weighted average gives us the accurate atomic mass of 35.5u for chlorine as it exists in nature.