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⬆️ TOP EDUCATION ⬆️
CLASS 9th
SCIENCE CHAPTER=3
ATOMS AND MOLECULES
1. Matter: Matter is anything that occupies space and has mass. It is composed of tiny particles called atoms.
2. Atom: An atom is the smallest particle of an element that retains the chemical properties of that element.
Atoms are indivisible and cannot be created or destroyed in chemical reactions.
3. Elements: Elements are pure substances made up of only one type of atom. There are around 118 known elements, each represented by a unique chemical symbol.
4. Molecule: A molecule is a group of two or more atoms held together by chemical bonds. Molecules can be made up of atoms of the same element (e.g., oxygen gas - O2) or different elements (e.g., water - H2O).
5. Symbols of Elements: Elements are represented by symbols derived from their English or Latin names. For example, hydrogen is represented by H, oxygen by O, carbon by C, and so on.
6. Chemical Formula: A chemical formula represents the composition of a molecule or a compound. It
consists of symbols of the constituent elements along with numerical subscripts denoting the number of
atoms of each element present. For example, H2O represents a molecule of water with two hydrogen
atoms and one oxygen atom.
7. Atomic Mass: The atomic mass of an element is the average mass of all the isotopes of that element.
It is expressed in atomic mass units (amu) and is usually found below the symbol of the element on the
periodic table.
8. Mole Concept: The mole concept is a fundamental concept in chemistry that relates the mass of a
substance to the number of particles (atoms or molecules) it contains. One mole of any substance
contains Avogadro's number of particles, which is approximately 6.022 × 10^23.
9. Molar Mass: The molar mass of a substance is the mass of one mole of that substance. It is
calculated by adding up the atomic masses of all the atoms in the chemical formula.
10. Laws of Chemical Combination: The chapter also covers two important laws of chemical
combination - the Law of Conservation of Mass and the Law of Definite Proportions.
☆These notes provide a brief overview of the key concepts covered in Class 9 Chapter 3, "Atoms
and Molecules." It's important to refer to your textbook and class notes for a more comprehensive
understanding of the topic.
