Atoms and molecules - CBSE NCERT Class 9 & 10 Science Notes

Atoms and molecules - 

CBSE NCERT Class 9 & 10 Science Notes

Hello friends! These are some short notes on atoms and molecules for 9th & 10th class students. 

Atoms and molecules are responsible to make everything like dust, sand, ink, bricks, buildings, mobile in your hands, humans, animals, plants. Everything is made up of atoms. Atom is basic fundamental unit of matter. When atoms bond together they form molecules. 

Atoms

We can simply say that Atom is basic building block of all matters. Atom have nucleus, which is very small and heaviest part of the atom. Atom is surrounded by large cloud of electrons, which is lightest part of the atom. 

• Definition: The smallest unit of an element that retains its chemical properties.
• Structure: Composed of protons, neutrons, and electrons.
  • Protons: Positively charged particles found in the nucleus.
  • Neutrons: Neutral particles also found in the nucleus.
  • Electrons: Negatively charged particles orbiting the nucleus.
• Atomic Number: The number of protons in an atom’s nucleus, which defines the element.
• Mass Number: The total number of protons and neutrons in the nucleus.

Atom

Molecules

We can simply say that, when two or more than two atoms form bond with each other they form a molecule. A group of atoms held together by chemical bonds. 

• Definition: The smallest unit of a compound that retains its chemical properties.
• Formation: Atoms bond together to form molecules.
  • Types of Bonds:
    • Covalent Bonds: Atoms share electrons.
    • Ionic Bonds: Atoms transfer electrons, resulting in charged ions that attract each other.
• Examples:
  • Oxygen Molecule (O₂): Two oxygen atoms bonded together.
  • Water Molecule (H₂O): Two hydrogen atoms and one oxygen atom bonded together.

Laws of Chemical Combination

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

Dalton’s Atomic Theory

John Dalton was an English chemist, physicist, and meteorologist. He developed modern atomic theory. 

1. Matter is made up of tiny particles called atoms.
2. Atoms are indivisible and indestructible.
3. Atoms of the same element are identical in mass and properties.
4. Atoms of different elements have different masses and properties.
5. Atoms combine in simple whole-number ratios to form compounds.

Some Important Terminology from chapter Atom and Molecule

• Atomic Mass: The mass of an atom, typically measured in atomic mass units (amu).
• Molecular Mass: The sum of the atomic masses of all atoms in a molecule.
• Mole Concept: A mole is a unit that measures the amount of substance, defined as containing exactly (6.022 x 10^{23}) particles (Avogadro’s number).

What is the Mole Concept?

The mole concept is a fundamental principle in chemistry used to express the amount of substance. It provides a way to quantify very large numbers of tiny particles such as atoms, molecules, or ions that are present in even small masses of material. 

• Definition of a mole: One mole is defined as the amount of a substance that contains exactly 6.02214076×10^23 elementary entities (atoms, molecules, ions, etc.). This number is known as the Avogadro constant or Avogadro number.
• Relation to Carbon-12: A mole was historically defined as being equal to the number of atoms in 12g of pure Carbon-12 (^12C) isotope. Hence, 12g of carbon-12 contains 1 mole (6.022×10^23) of carbon atoms.
• Purpose and Usage of Mole concept: The mole concept connects the macroscopic world (measurable quantities like mass and volume) to the microscopic world (number of atoms or molecules). It allows chemists to calculate reactants and products in chemical reactions conveniently, using moles as a counting unit for particles. 

Important Formulas:

• Number of Moles (n):

$$n=\frac{\text{Number of entities (N)}}{{\text{Avogadro constant (N}}_A)}$$

Where:

• $n$: number of moles

• $N$: total number of elementary entities (atoms, molecules, etc.)

• $N_A$: Avogadro constant ≈ 6.022×10^23 mol^-1.

• Mass to Mole Conversion:

$$\text{Number of moles (n)}=\frac{\text{Given mass (g)}}{\text{Molar mass (g/mol)}}$$

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  • Molar mass is the mass of one mole of a substance, expressed in grams per mole.

Example
One mole of water (H₂O):

• Contains 6.022×10^23 molecules of H₂O.
• Mass of one mole = 18g (since molar mass of H₂O = 18g/mol).
• Each molecule has 2 hydrogen atoms and 1 oxygen atom, so 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.

Why is Mole Concept Important?

It makes calculations in chemical reactions feasible by providing a direct link between the mass of substances and the number of particles they contain.
It is central to concepts like molar concentration (mol/L), stoichiometry, and quantitative analysis in chemistry.

The mole is used similarly in chemistry what "dozen" is used in daily life to count bananas. 
Mole is useful for grouping of huge numbers of fundamental entities that becomes then manageable for laboratory work.

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What is Viscosity in Chemistry?

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