📘 NCERT Solutions – Class 9 Science Chapter 4
Structure of the Atom | Full Question Answers PDF
Chapter Overview
In Chapter 4 “Structure of the Atom”, students learn about subatomic particles – electrons, protons, and neutrons – along with the models of atoms proposed by scientists like J.J. Thomson, Rutherford, and Bohr. These solutions help students understand the composition and structure of atoms clearly.
📥 Download PDF NowImportant NCERT Questions & Answers
Q1: Name the three subatomic particles of an atom.
Ans: The three subatomic particles of an atom are – Protons, Neutrons, and Electrons.
Q2: Who discovered the electron?
Ans: The electron was discovered by J.J. Thomson in 1897 during cathode ray experiments.
Q3: What is the charge and mass of a proton?
Ans: A proton has a positive charge of +1 and a relative mass of 1 unit (1 a.m.u.).
Q4: Explain Rutherford’s model of an atom.
Ans: Rutherford’s model proposed that the atom consists of a small, dense, positively charged nucleus surrounded by electrons that revolve around it, similar to planets around the sun.
Q5: What are the limitations of Rutherford’s model?
Ans: According to classical theory, electrons revolving around the nucleus should lose energy and fall into the nucleus, which does not happen — this was a major limitation.
Q6: State Bohr’s postulates.
Ans:
- Electrons revolve in fixed orbits or shells around the nucleus.
- Each orbit has a fixed energy level.
- Electrons do not radiate energy while in a particular orbit.
Q7: What is the atomic number and mass number of an element?
Ans:
- Atomic Number (Z): Number of protons in an atom.
- Mass Number (A): Sum of protons and neutrons in an atom.
🔍 Key Points to Remember
- Electron → Discovered by J.J. Thomson
- Proton → Discovered by Goldstein
- Neutron → Discovered by James Chadwick
- Rutherford’s model introduced the concept of a nucleus.
- Bohr explained how electrons stay in stable orbits.
- Atomic Number = Number of Protons.
- Mass Number = Protons + Neutrons.
Conclusion
Understanding the structure of atoms is the foundation of chemistry. These NCERT solutions are designed to simplify complex theories and help students prepare effectively for exams.
📘 Download Full NCERT Solutions PDF
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