Class 9th Science: Chapter 3 Atoms and Molecules

Atoms and Molecules

In 430 B.C. Democritus postulated that matter is made up of very small particles called “Atomos” which means"indivisible". Later, Antoine Lavoisier, from his experimental observations, established laws of chemical combinations.

Laws of Chemical Combination:
The Law of conservation of mass:  This law states that, the total mass of reactants is equal to the total mass of products. In any chemical reaction, the total mass of the substance before and after the reaction is the same although its matter undergoes a physical change.
Law of Definite Proportions:  According to law of definite proportions, in a chemical substance, the elements are always present in definite proportions by mass.
Law of Multiple Proportions:  When two elements combine to form different compounds, then the weight of one is constant and the other has a simple ratio.
Atom:  After a series of experiments Dalton concluded that all matter must be composed of tiny particles, which cannot be further divided. He called them atoms.
Definition of an atom: The smallest particle of an element is called an atom.
Dalton’s Postulates:
  • Matter is made of atoms.
  • Atoms are indivisible and indestructible.
  • Atoms of a given element are identical in mass and in properties.
  • Compounds are formed by a combination of two or more different kinds of atoms and. A chemical reaction is a rearrangement of atoms.
  • Atoms of different elements have different masses and chemical properties.
Draw backs of Dalton's atomic theory:
  • According modern atomic theory atoms are divisible and are composed of particles. The three main sub-atomic particles are proton, neutron and electron.
  • The assumption, atoms of a given element are identical in mass and in properties not applicable for isotopes of an element.
Examples: Chlorine has 2 isotopes having mass numbers 35 and 37.
Elements and symbols: Dalton proposed a scheme of notation to represent elements
Dalton's Symbols of Elements

The symbols used today were suggested by Berzelius.
Berzelius - Symbols of Elements:  Berzelius Suggested to abbreviate the names of the elements using one or two letters.
The first letter of the symbol is always in upper case. Some elements were represented by the starting alphabet of its name. For instance,
Carbon is represented as C
Boron as B
Oxygen as O
Sulphur as S
Nitrogen is written as N and
Hydrogen as H
Symbols of some elements were the first two alphabets, where the second alphabet is in the lower case.
Examples: Aluminium is represented as Al
Chlorine as Cl.
An element is represented by either the first or the first and any other letter of its Latin name. There are some exceptions while writing the symbols for some of the elements. These names were derived from Latin. In general, the first letter and the second letter of their Latin names are used to represent the symbols of these elements.
Latin Name
Atomic mass:  
Atomic mass is the mass of an atom.
The relative atomic mass of an atom of an element is the number of times an atom of that element is heavier than 1/12th of the mass of a carbon-12 atom.
Molecule:  A molecule is the smallest particle of an element or compound that can exist independently.
H2O, O2, O3 etc.
Single Hydrogen is not a molecule. When this hydrogen bonds to other hydrogen (H2) or to other elements like oxygen (H2O), a molecule is formed.
Atomicity:  The number of atoms constituting a molecule is referred to as atomicity.
A molecule which contains only one atom is called monatomic
Example: Noble gases like Helium (He), Neon (Ne), Argon (Ar) etc.
A molecule which contains two atoms is called diatomic
Example: Hydrogen (H2), Chlorine (Cl2), Nitrogen (N2) etc.
A molecule which contains three atoms is called triatomic.
Example: O3, H2O.
A molecule which contains four atoms is called teraatomic.
Example: Phosphorus (P4).
A molecule which contains more atoms is called polyatomic.
Example: Sulphur (S8).
Compound:  Elements combine to form compounds. Thus a molecule of a compound has two or more elements.
Example: Water (H2O), Glucose (C6H12O6), Calcium oxide (CaO), Sodium chloride (NaCl) etc.
Differences between a molecule and a compound:  A molecule is formed when two or more atoms bonded chemically. A compound is formed when different elements bonded chemically. Thus a molecule of a compound has two or more elements.
All compounds are molecules but all molecules are not compounds.
Example: Molecular hydrogen (H2) is a molecule but not compound. Hydrogen molecule is made up of two atoms of hydrogen. Water is a compound. Water is made up of two atoms of oxygen and one atom of oxygen these atoms combined to form a compound known as water. Smallest particle of water is called a molecule.
Common salt is a compound of sodium and chlorine, where sodium is a positively charged particle and chlorine a negatively charged particle, and each of this is called an ion.
An ion is a charged particle.
A positively charged particle in a molecule is called cation.
Example: Na+, Ca+2.
An anion is a negatively charged particle in a molecule.
Example: F-, Cl-.
Differences between Cation & Anion:
  • Cation is a positively charged particle
  • Cation is formed from Metals
  • During electrolysis cation moves towards cathode
  • The size of cation is smaller than its parent atom
      Example: Na+, K+.
  • An anion is a negatively charged particle
  • Anion is formed by Non-metals
  • During electrolysis anion moves towards anode
  • The size of anion is larger than its parent atom.
       Example: Cl-, Br-.

Valency: The combining capacity of an element is known as valency.
The combining capacity of the atoms to form molecules either with same or different elements is defined as valency.
Atom contains less than four electrons in its outermost shell; the valency of an atom is equal to the number of electrons present in the valence shell.
Sodium has one electron in its outermost shell, so the valency of sodium is 1.
Calcium has two electrons in its outermost shell, so the valency of calcium is 2.
Aluminum has three electrons in its outermost shell, so the valency of aluminum is 3.
If the outer shell has more than four electrons, the valency = 8 - the number of electrons in the outer shell.
Question:  Find the valency of chlorine?
Solution: The atomic number of chlorine is 17
Electronic configuration of Chlorine = 2 8 7
Valency = 8 - 7 =1
Valency of chlorine is 1
Formula of simple compounds:
Steps for writing the formula of simple compound like H2O is:
  • Write the symbols of the constituent elements.
  • Write the valencies of each element below its symbol.
  • Criss cross the valencies of the constituent elements and place as the subscripts to get the desired formula.
Example : Water
Steps for writing the formulae of simple/complex ionic compounds like Na2O and (NH4)2(SO4)2 are:
  • Identify the cation and the anion.Write the symbols of the ions along with the charges on them. Always the cation is followed by the anion.
  • Cross multiply the valency of ions.
  • For poly-atomic ions place the sub script after enclosing the ion in the brackets.In the final formula, the charges on the ions are not indicated.

Example: Ammonium sulphate

Mole Concept

Mole:  Mole is the measurement in chemistry. It is used to express the amount of a chemical substance.
One mole is defined as the amount of substance of a system which contains as many entities like, atoms, molecules and ions as there are atoms in 12 grams of carbon - 12".
Avogadro number:  The number of the particles present in one mole of any substance is equal to 6.022x1023. This is called avogadro’s number or avogadro’s constant.
Number of particles in 1 mole:
1 mole of hydrogen atoms represents 6.022 × 1023 hydrogen atoms.
1 mole of hydrogen molecules represents 6.022 × 1023 hydrogen molecules.
1 mole of water molecules represents 6.022 × 1023 water molecules.
Class 9th Science: Chapter 3 Atoms and Molecules, Atoms and Molecules, Mole Concept, NCERT CBSE Solved Quetion Answers, KEY NOTES, NCERT Revision Notes, Free NCERT Solutions Online

Atomic mass:  The atomic mass of an element is the mass of one atom of that element in atomic mass units or (u).
Atomic mass unit (amu):  1/12th of the mass of an atom of carbon-12 is called atomic mass unit. It is a unit of mass used to express atomic masses and molecular masses.
Molar mass:  The molar mass of an element is equal to the numerical value of the atomic mass. However, in case of molar mass, the units change from ‘u’ to ‘g’. The molar mass of an atom is also known as gram atomic mass.
For example, the atomic mass of carbon =12 atomic mass units. So, the gram atomic mass of carbon = 12 grams.
Molecular mass of the molecule:  The sum of the atomic masses of all the atoms in a molecule of a substance is called the molecular mass of the molecule.
Molecular mass - calculation: Generally we use relative atomic masses of atoms for calculating the molecular mass of 1 mole of any molecular or ionic substances.
Example: Molecular mass of H2SO4
Atomic mass of Hydrogen = 1
Atomic mass of sulphur = 32
Atomic mass of oxygen = 16
Molecular mass of H2SO4 = 2(Atomic mass of Hydrogen) + 1 (Atomic mass of sulphur) + 4 (Atomic mass of oxygen)
= 2×1 + 32 + 4× 16 = 98 u.
Calculation of molecular mass of hydrogen chloride:
Atomic mass of hydrogen + Atomic mass of chlorine = 1 + 35.5 = 36.5 u.
Formula unit mass:  The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound. The term ‘formula unit’ is used for those substances which are made up of ions.
Formula unit mass of NaCl: 1 X Atomic mass of Na + 1 X Atomic mass of Cl
1x23 +1 x 35.5 = 58.5 atomic mass units.
Formula unit mass of ZnO:
= 1 X Atomic mass of Zn + 1 X Atomic mass O
= 1 X 65 + 1 X 16 = 81 u.
Avogadro's number, number of particles and moles - conversion formulae:
   Number of moles (n) = Given number of particles (N) / Avogadro number (N0)

Example: Find the number of moles present in 24.088X1023 particles of carbon dioxide
1 mole of carbon dioxide contains 6.022x1023
Solution: The number of moles (n ) = Given number of particles (N) / Avogadro number (N0).
                                                  = 24.088X1023 / 6.022x1023
                                                  = 4 moles.

  Number of atoms (n) = Given mass (m) / Molar mass (M) x Avogadro number (N0)

Example: Calculate the number of atoms in 48g of Mg
Solution: Number of atoms (n) = Given mass (m) / Molar mass (M) x Avogadro number (N0).
                                           = 48/24 X 6.022x1023
                                           = 12.04 X1023 atoms.

  Number of molecules (n) = Given mass (m) / Molar mass (M) x Avogadro number (N0)

Example: Calculate the number of molecules in 3.6 g of water
Solution: The molecular weight of H2O = 18
                                           18g of water - 6.022x1023 molecules
                                           3.6g of water - 6.022x1023 X 3.6/18
                                           = 1.206 X 1022 molecules.

Number of particles (n) = Number of moles of particles (n) x Avogadro number (N0)

Example:  Calculate the number of atoms in 0.5 moles of carbon
Solution: Number of atoms = 0.5 X 6.022x1023
                                       = 3.0115 atoms

Extra problems:

1) Calculate the number of moles in 12g of oxygen gas
Ans: 32g of oxygen gas = 1 mole
       12g of oxygen gas = ?
        = 12 X 1 / 32 =  0.375 mole.

2) Calculate the number of moles present in 14g of carbon monoxide .
Ans: 28g of CO = 1 mole
14g of CO = ?
= 14 X 1 / 28 = 1/2 = 0.5 mole.

3) Find the mass of 5 moles of aluminium atoms?
Ans: Atomic mass of aluminium atom = 27g
5 moles of aluminium atoms = 5 X 27g = 135 g.

4) Calculate the molar mass of sulphur.
Ans: Sulphur is a poly atomic molecule. It is made up of 8 sulphur atoms.
Atomic mass sulphur atom = 32 g.
Molar mass of sulphur = 8 X 32 = 256 g.

5) Calculate the mass of 0.2 mole of water molecules.
Ans: 1 mole of water molecules = 18g
0.2 mole of water molecules = ?
= 0.2 X 18 / 1 = 3.6 g.

6) Which has greater number of atoms, 100g of sodium or 100g of iron?
Atomic mass of Na = 23 u, Fe = 56 u.
Ans: 1 mole of substance contains the number of particles =  Avagadro's constant.
1 mole of Na = 23g
23g of Na contains - 6.023 X 1023 atoms
100g of Na Contains - ?
= 100 X  6.023 X 1023/23
= 4.3 X  6.023 X 1023 atoms.
1 mole of Fe =  56g
56g of Fe contains - 6.023 X 1023 atoms
100g of Fe contains - ?
= 100 X 6.023 X 1023/ 56
= 1.78 X  6.023 X 1023 atoms
Therefore, 100g of Na has greater number of atoms than 100 g of Fe.

7) How many atoms of oxygen are present in 300 grams of CaCO3?
Ans: Molecular mass of calcium carbonate = 40 + 12 + 3 X 16 = 40 + 12 + 48 = 100g
1 mole of Calcium carbonate contains – 3 mole of oxygen atoms.
100 g of calcium carbonate contains - 3 X 6.023 X 1023 atoms
300g of calcium carbonate contains - ?
= 300 X 3 X 6.023 X 1023/100
= 54.207 X 1023 oxygen atoms.

8) The mass of one atom of an element 'A' is 2.65 X10-23 g.  Calculate its atomic mass and name the element.
Ans: The mass of one atom of an element A = 2.65 X10-23 g
We know that 1 mole of an atom = 6.023 x 1023 atoms
1 mole of an atom = Its atomic weight
Mass of one atom of an element = 2.65X10-23 g
Mass of 6.023 x 1023 atoms of an element A = 2.65X10-23 X6.023 x 1023
= 15.69 g
The element is oxygen.

9) Calculate the number moles of magnesium  in 0.478g of magnesium ?
Ans: Molecular weight of magnesium =24g
24g of magnesium = 1 mole
0.478g of magnesium = ?
= 0.478 X 1/24 = 0.019 mole.

10) In which of the following cases the number of hydrogen atoms is more ? Two moles of HCl or one mole of NH3.
Ans: Calculation of number of  hydrogen atoms present in 2 moles of HCl
1 mole of  HCl contains 6.023X1023 molecules of HCl
2 moles of HCl contains 2 X 6.023X1023 =  12.046X1023 molecules of HCl
1 molecule of HCl contains 1 hydrogen atom
12.046X1023 molecules of HCl contains12.046X1023 hydrogen atoms
Calculation of number of hydrogen atoms present in 1 mole of NH3
1 mole NH3 Contains 6.023X1023 molecules of NH3
1 molecule of NH3 contains 3 atoms of hydrogen
6.023X1023 molecules of NH3 contains 3X 6.023X1023 atoms of hydrogen
= 18.069 X1023 atoms of hydrogen
Number of hydrogen atoms present in 2 moles of HCl = 12.046X1023
Number of hydrogen atoms present in 1 mole of NH3 = 18.069 X1023
Therefore, numbers of hydrogen atoms are more in 1 mole of NH3.

11) Find the number of oxygen atoms in 88g of CO2?

Ans: Molar mass of CO2 = 44 g/mol
Number of moles of CO2 present in 88g of CO2 = 88g/44g mol-1
= 2 moles
1 mole of CO2 contains 2 moles of Oxygen atoms.
2 moles of CO2 contains 4 moles of Oxygen atoms
1 mole of oxygen contains 6.023 X 1023 atoms of oxygen
4 moles of oxygen contains = 4 X 6.023 X 1023
= 24.092 X 1023
= 2.4092 X 1024 atoms of oxygen
Therefore, 88g of CO2 Contains 2.4092 X 1024 atoms of oxygen.

12) Calculate the number of water molecules contained in a drop of water weighing 0.06g ?
Ans: 18g of water = 1 mole
0.06 g = ?
= 0.06/18 = 3.33 X 10-3 mole of water
1 mole of water contains - 6.022X1023 molecules of water
3.33 X 10-3 mole of water contains -----?
= 6.022X1023 X 3.33 X 10-3
= 2 X 1020 molecule of water
∴ 0.06g of water contains 2 X 1020 molecules of water.

13) Find the number of aluminium ions present in 0.051g of aluminium oxide(Al2O3).
(Atomic masses: Al= 27u;O= 16u)
Ans: Molecular weight of Al2O3 = 102 g
102 g of Al2O3 contains 6.022X1023 molecules
0.051g of Al2O3 contain X number of molecules
X = 0.051 X 6.022X1023 /102 = 0.003 X 1020
X = 3 X 1020
1 molecule of Al2O3 contains – 2 Al+3ions
3 X 1020 molecules contains – 2 X 3 X 1020
= 6 X 1020 Al+3 ions
∴ 0.051 g of Al2O3 contains 6 X 1020 Al+3 ions.
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