Modifications of dalton's atomic theory

Modifications of Dalton's atomic theory

Dalton’s atomic theory was modified as most postulates were practically invalid. Thus the discovery of modern atomic theory. The following include point of modern atomic theory:-

Postulates of the modern atomic theory:

i. Matter is made up of small indivisible particles called atoms was not valid due to the
existence of three particles in atom. (Proton, electron and neutron) Instead Matter is made up of a small divisible particle
called atoms.

ii. Atom is neither created nor destroyed was not valid due to the existence of radioactivity
thus Atom can be created or destroyed by either nuclear fission or fusion.

iii. Atoms of the same elements being similar especially in mass was not valid due to the
existence of isotopes. Atoms of the same elements have either same or different mass.

iv. Atoms of elements are different especially in mass was not valid. Atoms of different
elements have either same or different mass.

v. Atoms of different elements when combine they do so in small ratio of whole number was
not valid because different elements combine by using variable ratio of whole
number.

THOMPSON'S EXPERIMENT
(The discovery of electrons).

Thompson  did an experiment to investigate whether air conducts electricity.
The following circuit was used during the experiment.

Illustrations:

Working:

Figure 2.

explanations:
• The emission tube have electrode at each of the two ends connected to the external circuit. The
emission tube is connected to the vacuum pump in order to maintain the low pressure in the
emission tube.
The circuit is switched on which results into the following observation:-
• The bulb emitted light which indicate that the gas conduct electricity.
• There is glowing of emission tube or emission of light.
• There is fluorescence of emission tube.
• The stream of rays running from cathode to the anode. Through investigation of properties of cathode rays by using magnetic field, electric field and gold electroscope results into
discover of electron. The cathode rays were the electrons.

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