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Science Class 9 - Atoms And-Molecules Notes

Comprehensive study notes for Class 9 - Atoms And-Molecules olympiad preparation

Atoms and Molecules

In this chapter, you will learn about atoms and molecules, their structure, properties, and how they combine to form different substances. By the end of this chapter, you will understand the basic building blocks of matter and how they interact in chemical reactions.

Key Concepts

  • Atom: The smallest unit of matter that retains the properties of an element.
  • Molecule: A group of two or more atoms bonded together.
  • Element: A pure substance made of only one kind of atom.
  • Compound: A substance made of two or more different atoms bonded together.

Structure of an Atom

  • Atoms are made up of protons, neutrons, and electrons.
  • Protons and neutrons are found in the nucleus (center) of the atom.
  • Electrons move around the nucleus in shells or orbits.

Example: A hydrogen atom has 1 proton and 1 electron.

Molecules and Compounds

  • A molecule can be made of the same kind of atoms (O2) or different kinds (H2O).
  • Water (H2O) is a molecule made of 2 hydrogen atoms and 1 oxygen atom.
  • Carbon dioxide (CO2) is a molecule made of 1 carbon atom and 2 oxygen atoms.

Symbols and Formulae

  • Each element has a symbol (H for hydrogen, O for oxygen).
  • A chemical formula shows the number and type of atoms in a molecule (H2O, CO2).

Laws of Chemical Combination

  • Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction.
  • Law of Constant Proportion: A compound always contains the same elements in the same proportion by mass.

Practice Questions

  1. What is an atom?
  2. Give the formula for water and carbon dioxide.
  3. Name the particles found in the nucleus of an atom.
  4. What is the law of conservation of mass?
  5. How many atoms are in a molecule of H2O?

Challenge Yourself

  • Draw and label the structure of an atom.
  • Write the chemical formula for glucose (C6H12O6).

Did You Know?

  • Atoms are so small that millions can fit on the tip of a needle!
  • The word "atom" comes from a Greek word meaning "uncuttable".

Glossary

  • Atom: The smallest unit of matter.
  • Molecule: Two or more atoms joined together.
  • Element: A pure substance made of one kind of atom.
  • Compound: A substance made of different atoms joined together.
  • Proton: A positively charged particle in the nucleus.
  • Neutron: A particle with no charge in the nucleus.
  • Electron: A negatively charged particle moving around the nucleus.

Answers to Practice Questions

  1. An atom is the smallest unit of matter that retains the properties of an element.
  2. Water: H2O, Carbon dioxide: CO2
  3. Protons and neutrons.
  4. Mass is neither created nor destroyed in a chemical reaction.
  5. Three atoms (2 hydrogen and 1 oxygen).

Atoms and molecules are the building blocks of everything around us. Explore and discover more!

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