Notes 4. Structure of The Atom - Non-atomic particles | Class 9 Science - Toppers Study

Chapter 4: The Structure of Atom

Atom: The smallest tiny particles of matter is called atom. 

There are three subatomic particles.

(i) Protron

(ii) Neutron

(iii) Electron

The two particles of these particles are charged.  

(i) Protron: It is positively charged particle which is situated inside the nucleus od an atom. It shows all the chemical properties of an atom. Increasing or decreasing the number of protons of an atom caused changing in chemical properties.

• Protons are represented by p⁺ .
• The Mass of a proton is about 2000 times larger than electrons. 
• The mass of a proton is taken 1 unit and it's charge is taken +1.
• It cann't be peeled off easily from nucleus as it located inside the nucleus. if we try to peel off nucleus can break. 

The discovery of Proton was made by E. Goldstein. 

(ii) Electrons: These are negatively charged (-) particles which revolve around the nucleus in well-defined orbits.

• These are represented by e^- .
• The mass of an electron is considered to be negligible and its charge is minus one.
• Electrons can be easily peeled off.  

The discovery of Electrons was made by J. J. Tomson.

(iii) Neutron: Nutrons are elecrically no charge non-atomic particles which are present in nuceus of an atom. They have no any charge. 

The discovery of nutrons : 

In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron.

• Neutrons are present in the nucleus of all atoms, except hydrogen.
• In general, a neutron is represented as ‘n’.
• The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.

Neutral Atom: Generally, all atoms are electrically neutral, because the number of positively charged particles protons are equal to negatively charged particles electrons in any atom. This is the cause to be an atom neutral and net charges on it is zero. 

Example : Oxygen has 8 positively charge particles protons and 8 negatively charged particles electrons. 

• J. J. Tomson is the first scientist who present the model of atom.

Canal Rays : Canal rays are stream of positively charged particles emitted from anode of immersion tube, when electric current is flown through gas at very low pressure.