Introduction

·          Scientists needed a new theory which can explain how electrons and protons are arranged in an atom after (the failure of Daltons theory).

·         Thomson gave his model but that failed too.

·         Fast moving alpha particles were bombarded on gold foil.

·         Gold foil : because he needed thin layer.

·         Alpha particles: Helium ions with +2 charge.

(i)  Most of the fast moving ?-particles passed straight through the gold foil.

(ii) Some of the ?-particles were deflected by the foil by small angles.

(iii) One out of every 12000 particles reflected back.

·         (i) There is a positively charged center of an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

·         (ii) The electrons revolve around the nucleus in well-defined orbits. 

·      (iii) The size of the nucleus is very small as compared to the size of the atom.

Pre-Requisites:

Daltons atomic theory, molecules

Activity:

Atoms are the smallest units of any substance.

Real Life Question:

What does an atom look like?

Key Words / FlashCards

Learning aids / Gadgets

Real life uses :

Atoms make up all substances.

Places to visit :

Laboratory.

Practical examples around us

What you learn in Theory:

·         Scientists needed a new theory which can explain how electrons and protons are arranged in an atom after (the failure of Daltons theory).

·         JJ Thomson first proposed the model for the structure of an atom.

·         His model proposed that :

·         1) An atom consists of a positively charged sphere and the electrons are embedded in it.

·       2) The negative and positive charges are equal in magnitude. So the atom as a whole is electrically neutral.

What you learn in Practice:

Atoms are the smallest unit of any substance.