Atomic Structure — Part 1: Particles, Rutherford's experiment and Bohr's model: visual lesson, ICSE Class 9 Chemistry (Science Paper 2)

Lesson

Inside the atom

Part 1 — every atom has a tiny nucleus with protons and neutrons, and electrons outside it. How did scientists find this out?

In this lesson you will

Compare protons, neutrons and electrons and find their numbers from the atomic number and mass number
Describe Rutherford's gold foil experiment, its conclusions and a limitation of the model
State the main postulates of Bohr's model and how it overcomes the limitation

Concept

The three particles

Protons and neutrons sit together in the nucleus. Electrons move round the nucleus.

Table comparing the charge, relative mass and location of the proton, neutron and electron
Particle	Charge	Relative mass	Where found
Proton	Positive, plus one	One	In the nucleus
Neutron	None	One	In the nucleus
Electron	Negative, minus one	About one eighteen hundred and fortieth	Outside the nucleus

Concept

Atomic number and mass number

The atomic number is the number of protons. The mass number is the number of protons plus neutrons. A neutral atom has as many electrons as protons.

Table defining atomic number, mass number and the number of neutrons
Quantity	Meaning	How to find it
Atomic number, Z	Protons in the nucleus	Given, or the same as the electrons in a neutral atom
Mass number, A	Protons plus neutrons	Given in the symbol
Neutrons	Particles with no charge	Mass number minus atomic number

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