NCERT Solutions for Class 9th Science Chapter 8 .1 Rediscovering the Roots of Atomic Theory — In-text Questions
Book page 1418 Updated on2026-09-08
Q1.
What are atoms made up of?
Answer
Atoms are made of electrons, protons and neutrons, arranged in two distinct regions.
The nucleus at the centre holds the protons and neutrons (together called nucleons). It carries all the positive charge and almost all the mass. Its diameter is about 10–15 m.
The electrons move around the nucleus in shells. They occupy nearly the whole volume of the atom (diameter ≈ 10–10 m) but contribute almost no mass.
Diameter of atom ÷ diameter of nucleus = (10–10 m) ÷ (10–15 m) = 105, i.e. the atom is one lakh times wider than its nucleus
Why it happens: that ratio is why the atom is described as “mostly empty space”. If an atom were the size of a cricket ground (about 100 m across), the nucleus would be a black pepper grain a few millimetres wide at the centre.
Q2.
What would atoms look like if we could see them?
Answer
Not like the diagrams in this chapter — there are no coloured spheres and no drawn orbits.
We can actually get pictures of atoms today. A Scanning Tunnelling Microscope (STM) maps a surface atom by atom; a Transmission Electron Microscope (TEM) shows how atoms are arranged inside a very thin sample. In an STM image (Fig. 8.15) atoms appear as a regular array of rounded bumps — no colour, no sharp edges, no visible internal parts.
What such an image really shows is the electron cloud — the region where electrons are most likely to be found. The nucleus, being 105 times smaller, is far too tiny to appear at all.
Tip: the textbook's own Note says it plainly — atoms have no colour. The colours in the diagrams are only to tell electrons, protons and neutrons apart.
Q3.
What makes the atoms of one element different from the atoms of another element?
Answer
The number of protons in the nucleus — the atomic number, Z.
Z is the element's identity card. Change Z and you have a different element, no matter what else stays the same:
Hydrogen: 1 proton, Z = 1.
Helium: 2 protons, Z = 2.
Sodium: 11 protons, Z = 11.
Neutrons do not change the identity — 126C, 136C and 146C all have 6 protons, so all three are carbon.
Why it happens: a neutral atom has as many electrons as protons, so Z also fixes the electron count, and therefore the electronic configuration and the number of valence electrons. Chemistry is decided by valence electrons, so Z decides how an element behaves.