Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev

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Class 9 : Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev

The document Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev is a part of the Class 9 Course Science Class 9.
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Atoms and molecules
Introduction
The structure of matter has been a subject of speculation from very early times. According to Greek philosopher Democritus, if we go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be divided further. He called these particles 'atoms' meaning indivisible.

Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRevFig. Particles of an atom

Conclusion:
All matter is made up of small particles called atoms. Different kinds of atoms and molecules have different properties due to which different kinds of matter also show different properties.

Dalton's Atomic Theory:
On the basis of laws of chemical combination, John Dalton proposed the atomic theory in 1808. The main points of Dalton’s atomic theory are:

The main postulates of Dalton's atomic theory are:

  1.  All matter is made up of very tiny particles called atoms.
  2.  Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  3.  Atoms of a given element are identical in mass size and chemical properties.
  4. Atoms of different elements have different mass size and chemical properties.
  5.  Atoms combine in the ratio of small whole numbers to form compounds.
  6. The relative number and kinds of atoms are constant in a given compound. Dalton's atomic theory was based on the laws of chemical combination.
  7.  Atoms of the same element can combine in more than one ratio to form more than one compound. For example, hydrogen and oxygen combine to give water and hydrogen peroxide. In water, two atoms of hydrogen combine with one atom of oxygen while in hydrogen peroxide; two atoms of hydrogen combine with two atoms of oxygen.Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRevFig. Dalton's atomic theory

For example: The postulates of Dalton's atomic theory that "atoms can neither be created nor destroyed", was the result of law of conservation of mass and the postulates of Dalton's atomic theory that "the elements consist of atom having fixed mass", and that the number and kind of atom in a given compound is fixed came from the law of constant proportions. 

Drawbacks of Dalton's Atomic Theory:
Some of the drawbacks of Dalton's atomic theory of matter are given below:

  • According to Dalton's atomic theory, atoms were thought to be indivisible. But it is now known that atoms can be further divided into still smaller particle called electrons, protons and neutrons.
  • Dalton's atomic theory said that all the atoms of an element have exactly the same mass. But it is now known that atoms of the same element can have slightly different masses, as in case of isotopes.
  • Dalton's atomic theory said that atoms of different elements have different masses. But it is now known that even atoms of different elements can have the same mass as in case of isobars. 

 Atoms
All the matter is made up of atoms. An atom is the smallest particles of an element that can take part in a chemical reaction. Atoms of most of the elements are very reactive and do not exist in the free state (as a single atom). They exist in combination with the atoms of the same elements or another element. Atoms are very small in size. The size of an atom is indicated by its radius which is called atomic radius (radius of an atom). Atomic radius is measured in nanometer (nm). 
(1 meter = 109 nanometers or 1 nm = 10-9m). Hydrogen atoms are the smallest atom of all, having an atomic radius of 0.037 nm. Atoms are so small that we cannot see them under the most powerful optical microscope.

Symbol of elements: 
 The symbol may be defined as the abbreviation used for the name of an element. The symbol of an element is generally either the first letter or the first two letters or the first and the third letters of the name of the element. For example, the symbols of the following elements are the first letter of the name of that element.    Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRevFig. Symbols of elements proposed by Dalton

The first letter of the name of that element. 

  • Some symbols derived from the first two letters of the names of the element.Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev
  • Some symbol derived from the first and the third letter of the names of the elements.Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev
  • There are certain symbols which seen to here no relationship to their names. The symbol of these elements is derived from their Latin names.

Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev

Atomic Mass:
Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Atomic mass: Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev

How do Atoms occur?
The atoms of only a few elements called noble gases (such as helium, neon, argon and krypton etc.) which are chemically unreactive and exist in the free state (as a single atom). Atoms of the elements are chemically very reactive and do not exist in the free state (as a single atom).

Molecules:
A molecule is the smallest particle of an element or compound that has an independent existence. A molecule contains one or more than one atoms.
The molecules of elements contain atoms of only one kind.
The number of atoms in a molecule of an element is known as atomicity of the element. 

For example -The atomicity of the noble gases is 1, that of hydrogen, nitrogen, oxygen etc. is 2 each and of ozone is 3. Thus, noble gases, hydrogen and ozone are respectively monoatomic, diatomic and triatomic molecules.

Molecules of elements:
The molecules of an element contain two similar atoms chemically bonded together, for example, ozone gas has 3 oxygen atoms combined together, so ozone exists in the form of O3. A recently discovered form of carbon, called Buckminsterfullerene has molecular formula C60.

Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRevFig. Molecule of elements

Molecules of compounds:
The molecules of a compound contain two or more different types of atoms chemically bonded together. For example the molecule of Sulphur dioxide (SO2) contain one atom of Sulphur chemically bonded with two atoms of oxygen.

Molecular mass and formula mass:
The molecular mass of a substance (an element or a compound) may be defined as the average relative mass of a molecule of the substance as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Molecular Mass =  Basic units and Concepts - Atoms and Molecules Class 9 Notes | EduRev

The molecular mass of a compound can be obtained by adding atomic masses of all the atoms present in the molecule of the compound. For example, molecular mass of CO2 is -

    12 × 1 + 16 × 2 = 44 u

Gram Molecular Mass:
Gram molecular mass of a substance is defined as that much quantity of the substance whose mass expressed in grams is numerically equal to its molecular mass.
For example, The molecular mass of CO2 is 44 u, its gram molecular mass is 44 g. Gram molecule mass of a substance is also known as the gram-molecular mass of the substance.
Formula Mass:
Formula mass of an ionic compound is obtained by adding atomic masses of all the atoms in a formula unit of the compound.

For example Formula mass of potassium chloride (KCl)
= Atomic mass of potassium + atomic mass of chlorine
= 39 + 35.5 = 74.5.

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