Class 9 Science, Chapter 8 (NCERT). 8 questions by concept. Open “Show answer” for the answer and why.
How to teach this chapter · Common misconceptions & worked visual for this chapter
What did the rare large deflections in Rutherford's experiment show?
(d) Most positive charge and mass are concentrated in a tiny nucleus.
A small fraction approached a compact positive region strongly enough to deflect.
What key feature did Bohr add to the nuclear model?
(d) Electrons occupy allowed discrete energy levels.
Bohr restricted electrons to particular energy states rather than arbitrary paths.
An atom has atomic number 11 and mass number 23. How many neutrons does it contain?
(b) 12
The atomic number Z is the number of protons, so this atom has 11 protons. The mass number A counts protons and neutrons only. Number of neutrons .
An atom has electron distribution 2,8,1. What is its usual valency?
(b) Valency 1
Losing one outer electron gives a stable 2,8 arrangement.
A neutral atom has 8 protons. How many electrons does it have?
(a) 8 electrons
An atom has no charge only when its electrons balance its protons. This atom has 8 protons, so it has 8 electrons.
Where is nearly all the mass of an atom concentrated?
(b) In the nucleus, in protons and neutrons
Protons and neutrons are far more massive than electrons.
Chlorine-35 and chlorine-37 both have atomic number 17. What type of atoms are they?
(c) Isotopes
Both atoms have 17 protons, so both are chlorine. Chlorine-35 has 35 − 17 = 18 neutrons, and chlorine-37 has 37 − 17 = 20 neutrons. Atoms of the same element with different mass numbers are isotopes.
Argon-40 has atomic number 18. Calcium-40 has atomic number 20. Both have mass number 40. What are these two atoms called?
(c) Isobars
Both atoms have mass number 40. Their atomic numbers, 18 and 20, are different, so they belong to different elements. Atoms of different elements with the same mass number are called isobars.
Wrong
A sodium atom becomes a different element when it loses an electron.
Why it feels right. The charge and behaviour change.
Right
The proton count determines the element.
Worked example
| Particle | Protons | Electrons |
|---|---|---|
| Na | 11 | 11 |
| Na⁺ | 11 | 10 |
