Atoms And Molecules Class 9 Notes - Chapter 3

Atoms and molecules are responsible for forming tiny sand particles, gargantuan black holes and everything in between. The atom is the most fundamental unit of matter, making up everything that we see around us. It is extremely small, measuring in at less than 0.1 to 0.5 nanometers.

What is an Atom?

Essentially, atoms are the most basic unit of matter, comprising of a proton, neutron and an electron. Protons and neutrons together form a nucleus. The electrons revolve around the nucleus. Protons, as the name suggests, are positively charged, electrons are negatively charged and neutrons are neutral subatomic particles.

What is a Molecule?

When a group of atoms bond together, it becomes a molecule. These atoms are held together by chemical bonds which are created by sharing electrons between atoms. The size and complexity of molecules can vary – hydrogen is the simplest molecule, consisting of 1 electron that revolves around the nucleus. There are far more complex molecules that consist of hundreds of atoms.

Important Question on Atoms And Molecules Class 9

  1. What are the chemical formulae for:
    1. Calcium oxide
    2. Calcium carbonate
  2. Explain the atomic mass unit.
  3. Name the elements present in the following:
    1. Baking Powder
    2. Quick Lime

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Practise This Question

J. J. Thomson performed cathode ray tube experiment in which electricity was passed through a coil in the cathode ray tube and scintillations were observed on the screen. Cathode rays which strike on the screen were nothing but: