Atoms and Molecules Class 9 Notes Science - Chapter 3

Chapter: 3

What Are Atoms And Molecules ?

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    Laws Of Chemical Combination

    Antoine L. Lavoisier give two important laws of chemical combination.
    Law of Conservation of Mass: It states that mass can neither be created nor destroyed in a chemical reaction. Law of Constant Proportions or The Law of Definite Proportions: This law was given by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.

    Atomic Theory

    This theory was given by British chemist John Dalton. The postulates of this theory may be stated as follows:

    • All matter is made of very tiny particles called atoms.
    • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
    • Atoms of a given element are identical in mass and chemical properties.
    • Atoms of different elements have different masses and chemical properties.
    • Atoms combine in the ratio of small whole numbers to form compounds.
    • The relative number and kinds of atoms are constant in a given compound.

    Atom

    Atoms are the smallest particles of an element which can take part in a chemical reaction.

    Atomic mass unit: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. Example: Atomic mass of H is 1, C is 12, N is 14, O is 16 , Na is 23 etc.

    Representation Of Atoms Of Different Elements

    Symbols

    An element symbol is a one- or two-letter abbreviation for a chemical element name. When a symbol consists of two letters, the first letter is always capitalised, while the second letter is lowercase.

    Symbols of Some Elements

    Element Symbol
    Aluminium Al
    Argon Ar
    Barium Ba
    Boron B
    Bromine Br
    Element Symbol
    Calcium Ca
    Carbon C
    Chlorine Cl
    Cobalt Co
    Copper Cu
    Element Symbol
    Fluorine F
    Gold Au
    Hydrogen H
    Iodine I
    Iron Fe
    Element Symbol
    Lead Pb
    Magnesium Mg
    Neon Ne
    Nitrogen N
    Oxygen O
    Element Symbol
    Potassium K
    Silicon Si
    Element Symbol
    Silver Ag
    Sodium Na
    Element Symbol
    Sulphur S
    Uranium U
    Element Symbol
    Zinc Zn

    Molecule

    It can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

    Atomicity

    The number of atoms constituting a molecule is known as its atomicity. Example:

    Element Symbol
    Argon 1
    Oxygen 2
    Phosphorous 4
    Sulphur 8
    Iron 1

    Ion

    An ion is a charged particle.

    Types of Ion

    Cation Anion
    Cannot be broken down to simpler substances. For example, copper, oxygen, iron, hydrogen, mercury, etc.
    Example: Na+, Ca+2, etc.    		 A negatively charged ion is called an anion. Example: Cl–, O–2, etc.
    Chemical Formula Valency
    The chemical formula of a compound is a symbolic representation of its composition. The combining power (or capacity) of an element is known as its valency.

    Molecular Mass

    • The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
    • It is expressed in atomic mass units (u).

    Formula Unit Mass

    The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.