CBSE Class 9 Answered

Atom

• An atom is the basic unit of matter.
•   An atom is defined as the ultimate particle of an element which exhibits properties of the element and which can take part in a chemical reaction.
•   An atom may or may not have an independent existence.
•   An atom is further divisible into protons, neutrons and electrons.

Molecules

•  A molecule is in general a group of two or more atoms which are chemically bonded together.
•  A molecule can be defined as the smallest particle of an element or a compound which is capable of an independent existence and shows all the properties of that substance.
•   Atoms of the same element or different elements can join together to form molecules.
•   A molecule is the smallest particle of a substance which can normally exist on its own.
•   A molecule retains the physical and chemical properties of the substance.

Element

• An element is a pure substance composed of only one kind of atom. Examples: C, H, O, Na, Ca, N etc.
•   An element cannot be broken down into two or more simple substances by any physical or chemical means.

Compound

• A compound is a pure substance composed of two or more elements combined chemically in a fixed proportion by mass.
• The properties of compounds are different from the properties of their constituent elements. Examples: H₂O, CO₂ etc.