9th Science NCERT Chapter 3

– Molecules of Compounds: Atoms of different elements join together in definite proportions to form molecules of compounds. Students should learn the symbols and combining capacity of elements to understand how atoms combine to form compounds.

– Writing Chemical Formulae: Students should understand how to write the chemical formula of a compound, ensuring that the valencies or charges on the ions balance to create a neutral structure. They should also be able to write formulae for simple compounds using the valencies of ions provided in tables【4:0†source】.

– Atomicity and Molecular Mass: Students should grasp the concept of atomicity of elements and understand how to calculate molecular masses by summing the atomic masses of all atoms in a molecule. This knowledge helps in determining the relative mass of a molecule expressed in atomic mass units【4:2†source】.

– Formula Unit Mass: Understanding how to calculate the formula unit mass of substances, particularly for ionic compounds, is important. This involves adding up the atomic masses of all atoms in a formula unit of a compound, similar to calculating molecular mass【4:4†source】.