The objects around us are made up of small particles, but how small can these particles be? Can they have any special identity? Can they play any role in determining the nature of a substance? In trying to solve these and similar questions, we come across some new concepts of science, which we will study in an interesting way in these notes. After this, you will have to face an MCQs based test based on these notes so that you can test your understanding of the subject.
Atoms and Molecules Class 9 Notes and MCQs
Atoms and Molecules
First of all, let us talk about how the idea of divisibility of a substance i.e. idea of atom was born -
Divisibility of matter and idea of Atom.
When and who first gave the idea of atom?
The idea of divisibility of matter was expressed in India much earlier, around 500 BC.
It may be surprising for you to know that the philosophers of our country India have played an initial role in solving the mysteries of this mysterious world of chemistry, that is, we were one of the first people to think in this direction -
Maharshi Kanad:
Indian philosopher Maharshi Kanad had propounded that if we keep dividing matter, we will get smaller and smaller particles and eventually there will come a limit when the obtained particles cannot be divided again, that is, The smallest particle will remain indivisible. He called this indivisible smallest particle as Parmanu .
Pakudha Katyayama
Indian philosopher Pakudha Katyayama, while explaining this opinion in detail, said that these particles are generally found in combined form, which gives us different forms of substances.
Greek philosophers Democritus and Leucippus
Around this time i.e. 500 BC. Greek philosophers Democritus and Leucippus had suggested that if we keep dividing matter, a situation will come when the particles obtained cannot be divided again. He called these indivisible particles as Atoms .
However, all these suggestions remained just philosophical thoughts. No specific experimental work was done to prove the validity of these ideas till the 18th century.
Laws of Chemical Combinatination
In the 18th century, scientist Antonie L. Lavoisier established important rules of chemical combination which provided an important foundation to chemistry.
Lavoisier and Joseph L. Proust, after much experimental work, propounded the following two laws of chemical combination.
1. Law of conservation of mass:
According to the law of conservation of mass, mass can neither be created nor destroyed in any chemical reaction.
Example of the law of conservation of mass
When a candle burns, its wax reacts with oxygen to form water (H2O) and carbon dioxide (CO2). So if we take the total weight of wax and oxygen, it will be equal to the total weight of H2O and CO2. That means the wax has not disappeared anywhere. He is as he was.
2. Law of Constant Proportion:
Lavoisier and other scientists highlighted that any compound is made up of two or more elements. In the compounds thus obtained, the ratio of these elements remains constant no matter where it is obtained from or who makes it.
In other words - "The elements in any compound are always present in a definite ratio of masses".
Example of the law of constant proportion-
Ratio of masses of hydrogen and oxygen in water
The ratio of masses of hydrogen and oxygen in water is always 1:8, no matter what the source of water is.
the ratio of masses of nitrogen and hydrogen in ammonia (NH3), is
In ammonia (NH3), nitrogen and hydrogen are always present by mass in the ratio of 14:3, no matter how the ammonia is produced or from what source it is taken.
Who is John Dalton?
John Dalton was born in 1766 in a poor weaver family in England.
At the age of twelve he began his career as a teacher. After seven years he became the principal of a school.
In 1793, John Dalton went to Manchester to teach mathematics, physics and chemistry in a college.
In 1808, he presented his atomic theory , which proved to be an important theory for the study of substances.
Dalton's atomic theory
Dalton provided the idea of divisibility of substances, which was considered philosophical till that time. He gave a rational explanation of the law of conservation of mass and the law of definite proportions in his atomic theory.
According to Dalton's atomic theory – all matter, whether element, compound or mixture, is made up of microscopic particles which are called atoms. Dalton's theory can be discussed in the following way:
All matter is made up of atoms, which participate in chemical reactions.
Atoms are indivisible smallest particles which are neither created nor destroyed in chemical reactions.
All atoms of an element have the same mass and chemical properties.
The atoms of different elements have different masses and chemical properties.
Atoms of different elements combine with each other in small whole number ratios to form compounds.
The relative number and type of atoms in any compound are fixed.
Symbols and nomenclature of elements
Naming
Initially the names of the elements were derived from the names of the places where they were first found. For example, the name Copper was derived from Cyprus.
The names of some elements were derived from specific colors. For example, the name gold is derived from the Latin word Aurum, which means yellow.
The symbols of some elements are derived from their names in Latin, German or Greek languages. For example: The symbol of iron is Fe, which is derived from its Latin name ferrum. Similarly, the symbol of sodium is Na and the symbol of potassium is derived from natrium and kalium respectively.
Symbols of elements
Dalton was the first scientist who used the symbols of elements in a specific meaning, that is, this symbol represented an atom of the element.
Symbols of some elements suggested by Dalton-
What is IUPAC?
The International Union of Pure and Applied Chemistry (IUPAC) is an international scientific body that approves the names, symbols and units of elements.
The symbols of most elements are made up of one or two letters of the English names of those elements, this way of naming was first suggested by Berzelius.
The first letter of a symbol is always written in capital letter and the second letter is always written in small letter.
For example-
Hydrogen, H
Aluminum, Al not AL
Cobalt, Co not CO
What is an atom?
Atom is the smallest structural unit of an element, which is not found in free state but takes part in chemical reactions.
How big are atoms?
Atoms are very small. So small that they are not even visible with our naked eyes. Therefore atomic radius is measured in nanometers (nm).
10–9 m = 1nm
1m = 109 nm
How do atoms exist?
The atoms of most elements cannot exist independently.
Atomic mass
Atomic mass measures the mass of an atom. It is equal to the sum of the number of protons and neutrons present in the nucleus of an atom. Electrons have negligible mass, so they do not contribute significantly to the atomic mass.
Atomic mass is measured in unified atomic mass units (u). The value of one u is equal to 1/12 the mass of one atom of carbon-12.
The mass of an atom plays an important role in determining its chemical properties. For example, elements with heavier atoms are generally less reactive than elements with lighter atoms.
Initially the atomic mass was written abbreviated as amu, but nowadays according to the criteria set by IUPAC it is represented by 'u' -unified mass.
Initially, scientists took 1/16 of the mass of an oxygen atom as the unit for calculating atomic masses. There were two reasons for this-
Oxygen reacts with many elements to form compounds.
By this atomic mass unit, the atomic masses of most of the elements are obtained in integers.
In 1961, carbon-12 was accepted as the standard unit for determining atomic masses, replacing oxygen as the atomic mass unit.
Today we take 1/12th of an atomic mass of carbon-12 as the standard atomic mass unit.
What is a molecule?
Generally, a molecule is a group of two or more atoms that are connected to each other by a chemical bond, a molecule can be defined as the smallest particle of an element or compound that can exist independently and Which displays all the properties of that compound.
types of molecules
The number of atoms used in the structure of a molecule is called the atomicity of that molecule .
Monoatomic:
The molecules of an element are composed of the same type of atoms. Molecules of many elements like Argon (Ar), Helium (He) etc. are formed by only one atom of the same element.
Diatomic:
The oxygen molecule is made up of two oxygen atoms, so it is called a diatomic molecule, O2.
Triatomic:
A molecule made of three atoms is called a triatomic molecule. For example, ozone (O₃),
Tetraatomic:
A molecule made of four atoms is called a tetraatomic molecule. For example, phosphorus (P4)
Polyatomic:
A molecule made of more than two atoms is called a polyatomic molecule. For example, sulfur (S8)
Molecular mass
The molecular mass of a substance is the sum of the masses of all its constituent atoms. Thus, it is the relative mass of the molecule which is expressed by atomic mass unit (u).
Relative molecular mass of water (H2O):
Atomic mass of hydrogen = 1 u
Atomic mass of oxygen = 16 u and
2 × 1 u + 1 × 16 u = 18 u
Molecular Mass of Nitric Acid (HNO3)
Atomic mass of H + Atomic mass of Ν + 3 × Atomic mass of O
Atomic mass of hydrogen = 1 u
Atomic mass of oxygen = 16 u and
Atomic mass of nitrogen is = 14 u.
Hence, molecular mass of HNO3
= 1 u + 14 u + 3 × 16 u = 63 u .
Valency of element
The combining power (or ability) of an element is called valency of that element.
What is an ion?
Compounds containing metals and non-metals are made up of charged particles. These charged particles are called ions, they are negatively charged or positively charged.
Anion : Negatively charged ion
Cation: Positively charged ion
For example, sodium chloride (NaCl) contains sodium (Na+) as cation and chloride ion (Cl–) as anion.
Chemical formula
The chemical formula of a compound is a symbolic representation of its constituents.
Binary Compound:
The simplest compounds formed from two different elements are called binary compounds.
1. Formula of hydrogen chloride
2. Formula of hydrogen sulphide
3. Formula of carbon tetrachloride
4. Formula of magnesium chloride
Hence formula: MgCl2
5. Formula of Aluminum Oxide:
Hence formula: Al2O3
6. Formula of calcium oxide:
Here the valencies of both the elements are same. Hence its formula will be Ca2O2, but we simplify this formula as CaO.
compounds containing polyatomic ions
1. Formula of sodium nitrate:
Hence formula: NaNO3
2. Formula of calcium hydroxide:
Hence formula: Ca(OH)2
3.Formula of sodium carbonate:
Hence formula: Na2CO3
4. Formula of ammonium sulphate:
Hence formula: (NH4)2SO4
formula unit mass
The formula unit mass of a substance is the sum of the atomic masses of all its constituent atoms.
The formula mass is calculated in the same way as the molecular mass is calculated, the only difference is that here we use the formula unit for the substance whose constituents are ions.
Unit formula mass of sodium chloride (unit formula NaCl)
1 × 23 u + 1 × 35.5 u = 58.5 u
Formula unit mass of CaCl2
Atomic mass of calcium (Ca) = 40 u
Atomic mass of chlorine (Cl) = 35.5 u
Hence the formula unit mass of CaCl2
= 1 × 40 u + 2 × 35.5 u
= 40 u + 71 u
= 111u
Let us now test our understanding on the basis of some MCQ based test-
Mcqs Test on Atom and Molecules Atoms and Molecules Class 9 Notes and MCQs
1.Who is the Indian philosopher who first introduced the concept of "atom" (atom)?
A.Maharishi Kanad
B.Democritus
C.Aristotle
D.Socrates
2.What did Democritus and Leucippus suggest about the divisibility of matter?
A.Substance can be divided infinitely
B.Substance can be divided into small indivisible particles called atoms
C.Substance cannot be divided at all
D.Substance is made of earth, water, air and fire
3. In which century did the Indian philosopher Pakudha Katayama discuss in detail the concept of atoms and their role in the formation of various forms of matter?
A.4th century BC
B.5th century BC
C.18th century
D.19th century
4.Antoine Lavoisier is known for establishing two important principles in chemistry. What are these principles?
A.Law of gravitation and law of motion
B.Law of conservation of mass and law of stable composition
C.Theory of relativity and law of thermodynamics
D.Law of attraction and law of refraction
5. What happens in a chemical reaction according to the law of conservation of mass?
A.Mass is created
B.Mass is destroyed
C.Mass is conserved
D.Mass is converted into energy
6. Joseph L. Proust is known for which fundamental principle in chemistry?
A.Law of conservation of energy
B.Law of definite proportions
C.Law of Multiple Proportions
D.Law of gravitation
7. In the context of chemical reactions, what does the law of multiple proportions state?
A.Elements combine in whole number ratio
B.Elements are combined in fixed proportions
C.Elements can be combined in any ratio
D.Elements do not combine in chemical reactions
8.What is the ratio of hydrogen and oxygen by mass in water (H2O) according to the law of multiple proportions?
A.1:1
B.2:1
C.1:8
D.14:3
9.Who is credited with developing the atomic theory which states that all matter is made up of indivisible particles called atoms?
A.Maharishi Kanad
B.Democritus
C.Antoine Lavoisier
D.John Dalton
10.According to John Dalton's atomic theory, what is the nature of atoms in a given element?
A.The atoms of an element are identical in all aspects.
B.The properties of atoms of an element may vary.
C.Atoms of an element are indivisible.
D.Atoms of an element are always found in pairs.
11.What does John Dalton's law of definite proportions tell?
A.All elements are made up of small, indivisible particles called atoms.
B.Elements combine in certain proportions to form compounds.
C.The total mass of the products of a chemical reaction is equal to the total mass of the reactants.
D.Elements can combine in various proportions to form various compounds.
12.According to John Dalton's atomic theory, how do atoms combine to form compounds?
A.Atoms of different elements can combine in any ratio.
B.Atoms of different elements always combine in the ratio 1:1.
C.Atoms of different elements always combine in the ratio 2:1.
D.Atoms of different elements never combine.
13.What does John Dalton's atomic theory say about the relative masses of atoms of different elements?
A.Atoms of different elements have the same mass.
B.Atoms of different elements have different masses.
C.The mass of atoms of different elements may be the same or different.
D.Atoms of different elements are indivisible.
14.In John Dalton's atomic theory, what is the role of atoms in the formation of compounds?
A.Atoms of different elements combine in any ratio.
B.Atoms of different elements combine to form molecules.
C.The mass ratio of atoms of different elements is fixed.
D.Atoms of different elements are physically mixed.
15.How are atoms usually measured because of their extremely small size?
A.Nanometer (nm)
B.Centimeter (cm)
C.Millimetre (mm)
C.meter (m)
16.What is the modern symbol of hydrogen element?
AH
B.He
C.Si
D.Hg
17.Which international scientific organization provides accepted names, symbols and atomic weights for elements?
A.UNESCO
B.IUPA
C. WHO
D.NASA
18. Who first proposed the concept of using symbols to represent elements, where each symbol represents an atom of the element?
A.Albert Einstein
B.Isaac Newton
C.John Dalton
D.Niels Bohr
19. What does “IUPAC” mean in the context of chemistry?
A.International Union of Protons and Atoms Commission
B.International Union of Pure and Applied Chemistry
C. Institute for Universal Particle Analysis and Classification
D.International Federation of Polymer and Alloys
20.The name of which chemical element is derived from the Latin word "Aurum", meaning "gold"?
A. Silver
B.copper
C. Gold
D.Iron
21.What is the chemical symbol of cobalt?
A.Co
B.Cia
C.Ca
D.Cb
22. How do chemical elements generally get their symbols according to IUPAC?
A. By their atomic number
B. By using their most common compounds
C. Using the first letter of their name in capital letters and the second letter in small letters.
D. By converting their atomic weights into symbols.
23. Who suggested the use of Atomic Mass Unit (AMU) for atomic weights of elements?
A.John Dalton
B.Albert Einstein
C. Niels Bohr
D.Marie Curie
24. The name of which element is derived from the Latin word "Cyprus"?
A.Copper
B.Carbon
C.Calcium
D.Chromium
25.What is the modern symbol of sodium?
A. To
B.Na
C.Sa
D.Sn
26. What type of ion is formed when an atom gains an electron?
A. cation
B. ion
C.Neutron
D.Proton
27. What is the sum of atomic masses of all the atoms in a molecule called?
A. Atomic radius
B.Atomic weight
C. Atomic number
D. Atomic volume
28.What is the chemical formula of sodium nitrate?
asn
B.Na2NO3
C.NaNO3
D.NaNO2
29. What is the chemical formula of calcium hydroxide?
A.CaOH
B.Ca(OH)2
C.Ca2H
D.Ca2OH
30. Calculate the molecular mass (in atomic mass units, u) of aluminum oxide (Al2O3).
A.56u
B.96u
C.102u
D.106u
Answers KEY|Atoms and Molecules Class 9 Notes and MCQ👇👇👇| Mcqs Test on Atom and Molecules
A. Maharishi Kanad
B. Matter can be divided into small indivisible particles called atoms
A. 4th century BC
B. Law of conservation of mass and law of constant composition
C. mass is conserved
B. law of definite proportions
C. Elements can be combined in any ratio
C. 1:8
D. John Dalton
A. The atoms of an element are identical in all aspects.
B. Elements combine in certain proportions to form compounds.
A. Atoms of different elements can combine in any ratio.
B. The atoms of different elements have different masses.
B. Atoms of different elements combine to form molecules.
A. Nanometer (nm)
A) H
B) IUPAC
c) John Dalton
B) International Union of Pure and Applied Chemistry
c) gold
A) Co
c) Using the first letter of their name in capital letters and the second letter in small letters.
A) John Dalton
A)copper
B) Na
B) ion
B) atomic mass
c) NaNO3
B) Ca(OH)2
D) 106 U
