Chapter 3: Atoms and Molecules Class 9 Notes


Here you will find Chapter 3: Atoms and Molecules Class 9 Notes.

Atoms And Molecules Class 9 Notes

An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged. The nucleus is positively charged and contains one or more relatively heavy particles known as protons and neutrons.

 A proton is positively charged. The number of protons in the nucleus of an atom is the atomic number for the chemical element. A proton has a rest mass, denoted mp, of approximately 1.673 x 10-27 kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted mn, of approximately 1.675 x 10-27 kg. The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator.

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The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. Atoms having the same number of protons, but different numbers of neutrons, represent the same element, but are known as different isotopes of that element. The isotope for an element is specified by the sum of the number of protons and neutrons. Examples of different isotopes of an element are carbon 12(the most  common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon).

Atomic radius is measured in nanometres.

1/109 m = 1 nm

1 m = 109 nm

Protons and electrons have equal and opposite charge, and normally an atom has equal numbers of both. Thus, atoms are usually neutral. An 
ion is an atom with extra electrons or with a deficiency of electrons, resulting in its being electrically charged. An ion with extra electrons is negatively charged and is called an anion; an ion deficient in electrons is positively charged and is called a cat ion.

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Relative Sizes:

Radii(in mm)Example
10-10Atom of Hydrogen
10-9Molecule of water
10-8Molecule of hemoglobin
10-4Grain of Sand


IsotonesNuclei of atoms with the same neutron number. Example: S-36, Cl-37, Ar-38, K-39, Ca-40. These nuclei contain 20 neutrons each, but a different number of protons: sulphur 16,chlorine 17, argon 18, potassium 19 and calcium 20 protons 


Isobars are nuclides having the same mass number; i.e. sum of protons plus neutrons; Carbon- 12 and Boron-12.


Symbol of Atoms:
Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. Berzelius suggested that the symbols of elements be made from one or two letters of the name of the element.

Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name.

Examples are:
(i) chlorine, Cl, (ii) zinc, Zn etc.

Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium.
Therefore, each element has a name and a unique chemical symbol.

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Dalton was the first scientist to use the symbols for elements in a very specific sense.

Atoms and Molecules Class 9 Notes

Atomic Mass 

The most remarkable concept that Dalton’s atomic theory proposed was that of the atomic mass.
According to him, each element had a characteristic atomic mass. One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12. The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Some Elements and their Atomic Mass

ElementAtomic Mass



A molecule is in general a group of two or more atoms that are chemically bonded together that is, tightly held together by attractive forces. A molecule 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. Atoms of the same element or of different elements can join together to form molecules.
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The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the nonmetals.
For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a molecule is known as its atomicity. Molecules of metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together.

Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.
For example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions  (Na+) and negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion.

Ionic CompoundConstituting ElementsRatio By Mass
Calcium oxideCalcium and Oxygen5:02
Magnesium SulphideMagnesium and sulphur3:04
Sodium Chloridesodium and chlorine23:35.5


Chemical Formula 

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.

Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. The valency of the atom of an element can be thought of as hands or arms of that atom.
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Rules to follow while writing a chemical formula are as follows:

  • The valencies or charges on the ion must balance.
  • When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first.

For example:

calcium oxide (CaO), sodium chloride (NaCl), iron sulfide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulfur are non-metals and are written on the right, whereas calcium, sodium, iron, and copper are metals, and are written on the left.

  • In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Some Ionic Compounds and their formulas


Some Formula of Compound

1. Formula of hydrogen chloride

2. Formula of the compound would be HCl.

Formula: H2S

3. Formula of carbon tetrachloride

Formula: CCl4

4. Formula of Magnesium chloride

Formula : MgCl2

Thus, in magnesium chloride, there are two chloride ions (Cl-) for each magnesium ion (Mg2+). The positive and negative charges must balance each other and the overall structure must be neutral. Note that in the formula, the charges on the ions are not indicated.

5. Formula for aluminum oxide:

Formula : Al2O3



Take an example of the reaction of hydrogen and oxygen to form water:

2H2 + O2  →  2H2O.

The above reaction indicates that

(i) two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water, or

(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water molecules. We can infer from the above equation that the quantity of a substance can be characterized by its mass or the number of molecules. But, a chemical reaction equation indicates directly the number of atoms or molecules taking part in the reaction.
Therefore, it is more convenient to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses. So, a new unit “mole” was introduced. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.

Relationship between mole, Avogadro number and mass

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Laws of Chemical Combination


Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction


Compounds were composed of two or more elements and each such compound had the same elements in the same proportions, irrespective of where the compound came from or who prepared it.In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.

This led to the law of constant proportions which is also known as the law of definite proportions. This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.

Dalton’s atomic theory provided an explanation for the law of conservation of mass and the law of definite proportions.

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

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