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Early models of the atom were not fully correct — and that is fine. Each model was improved as new evidence appeared. That is exactly how science moves forward, one step at a time.
By the late 19th century, scientists found that some elements give out invisible energy and particles — a phenomenon called radioactivity . This showed atoms are made of smaller parts, so they are not indivisible after all.
- Some elements were found to emit radiation (radioactivity).
- This meant atoms give out smaller particles .
- So the atom is not indivisible — it has parts inside.
In 1897 , J. J. Thomson passed electric current through gases at very low pressure in a glass tube and saw rays travelling from the cathode to the anode — cathode rays . These rays were streams of tiny negatively charged particles, later named electrons .
- Streams of negatively charged particles — electrons .
- They move from the cathode (−) to the anode (+) .
- Their nature does not depend on the gas or cathode material — electrons are in every atom.
NCERT Question 5 — Explain and arrange the following
- Radiation — invisible energy and particles emitted by certain elements.
- Radioactivity — the phenomenon in which atoms give out radiation, showing they are made of smaller particles.
- Cathode rays — rays moving from the cathode to the anode in a discharge tube; streams of electrons.
- Electron — a negatively charged subatomic particle, much lighter than an atom; charge taken as minus 1.
