Elements and Compounds Class 8 Questions and Answers Notes Basic Science Chapter 5 Kerala Syllabus

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Class 8 Basic Science Chapter 5 Elements and Compounds Question Answer Notes

Class 8 Basic Science Chapter 5 Notes Kerala Syllabus Elements and Compounds Question Answer

Elements and Compounds Class 8 Questions and Answers Notes

Let’s Assess

Question 1.
Match the following

A Element	B Symbol	C Base
Francium	Rb	continent
Curium	Eu	satellite
Rubidium	Ti	colour
Neptunium	Cm	country
Europium	Fr	planet
Titanium	Np	scientist

Answer:

A Element	B Symbol	C Base
Francium	Fr	Country
Curium	Cm	Scientist
Rubidium	Rb	Colour
Neptunium	Np	Planet
Europium	Eu	Continent
Titanium	Ti	Satellite

Question 2.
Cl is the symbol of the element chlorine. How can we represent two chlorine atoms and one chlorine molecule using this symbol?
Answer:

• Two chlorine atoms – 2 Cl
• One chlorine molecule – Cl₂

Question 3.
Find the number of atoms in each of the following and write which of them contains the most number of atoms.
5NH₃, 2H₂O, 5NO₂, 4CO₂

Element	Number of Atoms
5NH3	5 N + (5 × 3) H = 5 N + 15 H = 20
2H2O	(2 × 2)H + 2 O = 4H + 2 O = 6
5NO2	5 N + (5 × 2) O = 5 N + 10 O = 15
4CO2	4C + (4 × 2) O = 4C + 8 O = 12

Ammonia contains the most number of atoms.

Question 4.
Complete the table.

Answer:

Element	Basis of nomenclature	Symbol
Indium	Indigo – colour	In
Rutherfordium	Rutherford – Scientist	Rf
Germanium	Germany – Country	Ge
Silver	Argentum – Latin name	Ag

Question 5.
The names of some scientists are given.
(Dobereiner, Lavoisier, Newlands, Mendeleev)
Match the statements given below with the names given in brackets
i. The chemical and physical properties of the elements are functions of their atomic masses.
ii. Elements were divided into groups containing three elements (triads) with similar properties.
iii. The elements were classified into metals and non-metals.
iv. When the elements were arranged in ascending order of atomic masses, it was found that every eighth element was a repetition of the first, in terms of its properties.
Answer:
i. Mendeleev
ii. Dobereiner
iii. Lavoisier
iv. Newlands

Basic Science Class 8 Chapter 5 Question Answer Kerala Syllabus

Question 1.
What do you know about molecules? Try to write it.
Answer:
Molecules are tiny particles made of two or more atoms stuck together by invisible bonds, forming the basic building blocks of all matter.

Question 2.
Can you formulate a definition for molecules?
Answer:
Molecules are the smallest, independent and stable particles that have all the fundamental properties of a pure substance.
New molecules are formed through chemical reactions.

Question 3.
Let’s do an experiment.
a) Burn a magnesium ribbon which is scrubbed clean. Record your observations.
Answer:
Observation: Magnesium burns brightly in air. A white powder is obtained.

b) What is the white powder obtained as a result of the reaction?
Answer:
It is magnesium oxide, produced when magnesium reacts with oxygen.
This is an example of the formation of new molecules through chemical reactions between different types of molecules.
We can dissociate molecules to make new molecules.

Question 4.
Take some sugar in a dry boiling tube. Cover the mouth of the boiling tube with a piece of cotton. Heat the boiling tube strongly for a few minutes. Write your observations.
Observation – A black substance remains in the boiling tube.

a) What is the black substance left in the boiling tube?
Answer:
Carbon

b) What is the substance seen on the sides of the boiling tube?
Answer:
Water

We can observe that when sugar is heated, carbon and water are formed. Sugar molecules can be broken down through chemical reactions.

Question 5.
Let’s do another experiment.
Take some dilute hydrochloric acid in a test tube and add a small piece of magnesium to it. Introduce a burning incense stick to the mouth of the test tube. Record your observations.
Observation – A vigorous chemical reaction takes place. A gas is released. When the incense stick is introduced to the mouth of the test tube, the gas burns with a pop sound.

a) What is the gas produced as a result of the reaction?
Answer:
Hydrogen (H₂)

b) How can we represent this reaction?
Answer:
Magnesium + Hydrochloric Acid → Magnesium Chloride + Hydrogen

Question 6.
Collect the hydrogen gas obtained in this experiment into another moisture-free test tube and introduce a burning incense stick to it. Record your observations.
Answer:
The gas burns with a popping sound. Water droplets are visible on the sides of the test tube.

а) What do you understand from the presence of water droplets on the sides of the test tube?
Answer:
Hydrogen in the test tube reacts with oxygen to form water.
Hydrogen + Oxygen → Water

Henry Cavendish proved that water is a substance made up of more than one element. His experiment of burning hydrogen gas in the presence of oxygen helped to know about the components of water.

In 1806, Sir Humphry Davy discovered that water could be split into hydrogen and oxygen by passing electricity through it.

Question 7.
Which are the substances obtained when water is dissociated?
Answer:
Hydrogen and Oxygen

Question 8.
Can hydrogen and oxygen be further split in to components?
Answer:
No

Question 9.
Look at the table below

Molecules	Atoms in a molecule
Ammonia	One atom of nitrogen Three atoms of hydrogen
Oxygen	Two atoms of oxygen
Chlorine	Two atoms of chlorine
Methane	One atom of carbon Four atoms of hydrogen
Hydrogen	Two atoms of hydrogen
Carbon dioxide	One atom of carbon Two atoms of oxygen

a) Which molecules in the table consist of the same type of atoms?
Answer:
Oxygen, Chlorine, Hydrogen

b) Which of these molecules consist of different types of atoms?
Answer:
Ammonia, Methane, Carbon dioxide

• Elements are pure substances made up of the same type of atoms. They cannot be further divided into components through chemical reactions.
• Compounds are pure substances formed by the combination of atoms of two or more elements in a fixed ratio. These can be dissociated into constituent elements through chemical reactions.

Question 10.
Categorise the following into elements and compounds.
Oxygen, water, salt, sugar, carbon, nitrogen, carbon dioxide, ammonia, magnesium oxide

Answer:

Elements	Compounds
• Oxygen • Carbon • Nitrogen	• Water • Salt • Sugar • Carbon dioxide • Ammonia • Magnesium oxide

Question 11.
Note the list of other elements represented by the first letter of their name. Expand the list by adding more elements.

Answer:

Element	Symbol
Carbon	C
Nitrogen	N
Sulphur	S
Oxygen	O
Boron	B
Phosphorus	P
Fluorine	F
Iodine	I
Vanadium	V
Yttrium	Y
Uranium	U

Question 12.
The element Helium is represented by the symbol ‘He’. The first and second letters of the English name are used here. What could be the reason?
Answer:
H is the symbol for hydrogen. More than one element may have the same first letter.

• Some elements are represented by another, letter along with the first letter of the English name in upper case. See more examples of using two letters as symbols.

Element	Symbol
Aluminium	Al
Chlorine	Cl
Curium	Cm
Calcium	Ca

• The first letter of the English name is used upper ease along with the second letter or any other prominent letter in lower case.

Question 13.
Find more examples and expand the list.
Answer:

Element	Symbol
Germanium	Ge
Neon	Ne
Manganese	Mn
Cobalt	Co
Zinc	Zn
Beryllium	Be
Magnesium	Mg
Barium	Ba
Radium	Ra
Silicon	Si

Some elements have symbols based on their Latin names.

Question 14.
Find more examples and expand the list.

Answer:

Element	Latin name	Symbol
Sodium	Natrium	Na
Potassium	Kalium	K
Iron	Ferrum	Fe
Gold	Aurum	Au
Copper	Cuprum	Cu
Silver	Argentum	Ag
Tin	Stannum	Sn
Lead	Plumbum	Pb
Antimony	Stibium	Sb
Mercury	Hydrargyrum	Hg
Element	Latin name	Symbol

• The symbol “W” is derived from the German name ‘Wolfram’ for the metal tungsten.

Question 15.
Find put how the elements given below got their names.
• Chromium
• Iridium
• Neptunium
Answer:
• Chromium – From the world “chroma,” meaning colour.
• Iridium – From the word “Iris,” meaning Rainbow.
• Neptunium – From the name of the planet “Neptune”.

Question 16.
Analyse the Periodic Table and write down the name and symbol of any 5 elements.
Answer:

• Lithium (Li)
• Sodium (Na)
• Beryllium (Be)
• Magnesium (Mg)
• Calcium (Ca)

Question 17.
Find out which elements were added last to the periodic table.
Answer:

• Nihonium (Nh)
• Flerovium (Fl)
• Livermorium (Lv)
• Tennessine (Ts)
• Oganesson (Og)

Question 18.
List the elements existing in solid, liquid and gaseous state with the help of periodic table.

Answer:

Solid	Liquid	Gas
Lithium	Caesium	Hydrogen
Sodium	Francium	Nitrogen
Potassium	Mercury	Oxygen
Rubidium	Gallium	Fluorine
Beryllium	Bromine	Chlorine
Magnesium		Helium
Calcium		Neon
Strontium		Argon
Barium		Krypton
Radium		Xenon
		Radon

Question 19.
Find the synthetic elements with the help of the periodic Table.

• Technetium
• Dubnium
• Bohrium
• Meitnerium
• Roentgenium
• Nihonium
• Moscovium
• Tennessine
• Promethium
• Plutonium
• Curium
• Californium
• Fermium
• Nobelium
• Rutherfordium
• Seaborgium
• Hassium
• Darmstadtium
• Copemicium
• Flerovium
• Livermorium
• Oganesson
• Neptunium
• Americium
• Berkelium
• Einsteinium
• Mendelevium
• Lawrencium

Question 20.
Is there any difference between 2N and N₂?
Answer:
2N refers to two free nitrogen atoms. N₂ refers to a nitrogen molecule formed by two nitrogen atoms bonded together.

• 2N – refers to two free nitrogen atoms.
• N₂ – refers to a nitrogen molecule formed by two atoms.
• 2N₂ – refers to two nitrogen molecules.

Question 21.
What is the total number of atoms in 2N₂?
Answer:
2N₂ refers to two nitrogen molecules (N₂ + N₂)
2 atoms in a nitrogen molecule.
Total number of atoms in 2N₂ = 2 + 2 = 4

Question 22.
Check the abbreviation of the molecules of some elements.

Answer:

Molecule	Number of atoms	Category
He	1	Monoatomic elements.
O2	2	Diatomic elements.
S8	8	Polyatomic elements.
H2	2	Diatomic elements
P4	4	Polyatomic elements
Ne	1	Monoatomic element
Cl2	2	Diatomic elements
Ar	1	Monoatomic element
O3	3	Triatomic element

Question 23.
Is it clear that, monoatomic elements are elements that have only one atom in a molecule? Write the difference between diatomic elements and polyatomic elements.
Answer:
Diatomic elements are elements that have two atoms in a molecule. Elements that have more than three atoms in a molecule are called polyatomic elements.

Chemical formula of the molecule represent one molecule of a substance.

Question 24.
Find the number of molecules and the total number of atoms in the following.

Answer:

Substance	No. of molecules	No. of atoms
H2	1	1 × 2 = 2
5O2	5	5 × 2 = 10
6N2	6	6 × 2 = 12
4Cl2	4	4 × 2 = 8
8	1	1 × 8 = 8

Question 25.
Write the symbols for hydrogen and chlorine.
Answer:
Hydrogen – H
Chlorine – Cl
A molecule of hydrogen chloride contains one hydrogen atom and one chlorine atom.
We represent a molecule of hydrogen chloride as HCl.

Question 26.
How can we represent a molecule of water in the same way?
Answer:
H₂O

Question 27.
Complete the table.

Answer:

A compound has a certain ratio between the number of atoms of its constituent elements.
Example: HCl =1 : 1
H₂O = 2 : 1
NH₃ = 1 : 3
CO₂ = 1 : 2

Question 28.
Find the number of molecules and the total number of atoms present in 3CO₂.
Answer:
Number of molecules = 3
Atoms C = 3 × 1 = 3
0 = 3 × 2 = 6
Total number of atoms = 9

Question 29.
Complete the table given below.

Answer:

Compound	No. of molecules	No. of atoms
2HCl	2	H – 2 × 1 = 2 Cl – 2 × 1 = 2 4
4SO2	4	S – 4 × 1 = 4 0 – 4 × 2 = 8 12
3C2H5OH	3	C – 3 × 2 = 6 H – 3 × 6 = 18 0 – 3 × 1 = 3 27
2C6H12O6	2	C – 2 × 6 = 12 H – 2 × 12 = 24 O – 2 × 6 = 12 48

Question 30.
What are the reactants in this reaction?
Answer:
Carbon, Oxygen

Question 31.
What are the products?
Answer:
Carbon Dioxide

Question 32.
How do you write the reactants using chemical formula?
Answer:
C, O₂

Question 33.
How about products?
Answer:
CO₂
This chemical reaction can be represented as C + O₂ → CO₂

A chemical equation is the representation of a chemical reaction which helps to understand it at a glance. It uses symbols/chemical formulae to represent the reactants and products.

Question 34.
Calcium carbonate (CaCO₃) decomposes to form calcium oxide (CaO) and carbon dioxide (CO₂). How can this chemical reaction be represented?
Answer:
CaCO₃ → CaO + CO₂

Question 35.
Write the chemical equations of some chemical reactions that you are familiar with.
Answer:

• 2H₂ + O₂ → 2H₂O
• H₂ + Cl₂ → 2HCl
• 2Mg + O₂ → 2MgO
• CaO + H₂O → Ca(OH)₂
• Mg + 2HCl → MgCl₂ + H₂

Class 8 Basic Science Chapter 5 Question Answer Extended Activities

Question 1.
Make models of the molecules using materials available in your surroundings.
• Carbon tetrachloride (CCl₄)
• Ammonia (NH₃)
• Water (H₂O)
Answer:
Here are short instructions for making the molecule models:

General Materials:

• Atoms: Different coloured clay/Play-Dough/thermocol balls (e.g., Black/Grey for C, Green for Cl, Blue
  for N, White for H, Red for O).
• Bonds: Toothpicks or short stick pieces.

1. Carbon Tetrachloride (CCl₄)

• Atoms: 1 Carbon (large, black/grey), 4 Chlorine (smaller, green).
• Instructions: Connect the central Carbon ball to four Chlorine balls using toothpicks, spreading them out to form the shape of carbon tetrachloride.

2. Ammonia (NH₃)

• Atoms: 1 Nitrogen (medium, blue), 3 Hydrogen (small, white).
• Instructions: Connect the central Nitrogen ball to three Hydrogen balls using toothpicks. Arrange the Hydrogen atoms to form the base of a pyramid with Nitrogen at the top.

3. Water (H₂O)

• Atoms: 1 Oxygen (medium, red), 2 Hydrogen (small, white).
• Instructions: Connect the central Oxygen ball to two Hydrogen balls using toothpicks. Angle the Hydrogen atoms to create a V-shaped molecule.

Question 2.
Prepare a slide presentation/chart on the history of development of Periodic Table and exhibit it in the classroom.
Answer:
The Periodic Table: A Journey of Discovery
(Slide 1 / Chart Title)
Slide 2 / Section 1: Early Groupings (1800s)

• Dobereiner’s Triads (1829): Grouped elements in threes with similar properties.
• Newlands Law of Octaves (1865): Noted repeating properties every eighth element when arranged by atomic weight.

Slide 3 / Section 2: Mendeleev’s Breakthrough (1869) ‘

• Dmitri Mendeleev: Arranged elements by increasing atomic weight.
• Key Insight: Left gaps for undiscovered elements and predicted their properties.
• Significance: His predictions were later proven correct, validating his table.

Slide 4 / Section 3: Moseley’s Refinement (1913)

• Henry Moseley: Discovered that elements are best arranged by increasing atomic number (number of protons).
• Impact: Fixed drawbacks in Mendeleev’s table, leading to the modern version.

Slide 5 / Section 4: The Modem Periodic Table

• Basis: Organised by increasing atomic number.
• Structure: Elements with similar properties fall into the same groups (columns).
• Importance: Essential tool for understanding chemistry and predicting element behaviour.

Elements and Compounds Class 8 Notes

Class 8 Basic Science Elements and Compounds Notes Kerala Syllabus

• When sugar is heated, carbon and water are formed. Sugar molecules can be broken down through chemical reactions.
• Molecules are the smallest particles obtained by dissociating various substances having their fundamental properties.
• Atoms are obtained when molecules are split.
• Dalton’s Atomic theory
  • All matter is made up of minute particles called atoms.
  • An atom is the smallest particle that can participate in a chemical reaction.
  • An atom cannot be divided during a chemical reaction.
  • An atom cannot be created or destroyed.
  • All atoms of an element are of the same size, property and mass.
  • Atoms of different elements differ in their size, property and mass.
• Elements are pure substances made up of the same type of atoms. They cannot be further divided into components through chemical reactions.
• Compounds are pine substances formed by the combination of atoms of two or more elements in a fixed ratio. These can be dissociated into constituent elements through chemical reactions.
• A symbol represents an atom of an element.
• The modern symbol system of representing elements by letters was introduced by a scientist named Berzelius.
• The element hydrogen is represented by the letter ‘H’. That is, the first letter of name of the element is used here as the symbol.
• Some elements are represented by another, letter along with the first letter of the English name in upper case.
• The first letter of the English name is used upper case along with the second letter or any other prominent
  letter in lower case.
• Some elements have symbols based on their Latin names.
• Elements are named on the basis of country, scientists, colour, name of planets, satellites, continents, etc. Example: Francium (Country – France)
  Einsteinum (Scientist – Albert Einstein)
• Lavoisier made the earliest attempts to classify elements. He classified the then-known elements into metals and non-metals.
• Dobereiner classified the elements of similar properties into groups of three. These groups were called triads. He stated that the atomic mass of the element in the middle of the triads was approximately the average of the atomic masses of the other two elements. Only a few elements could be classified in this way.
• Newlands found that when elements were arranged in ascending order of atomic mass, each eighth element was a repetition of the first in its properties. He compared this to the seven notes in music. This classification was known as the Law of Octaves.
• A scientist named Mendeleev arranged the 63 elements known till that day in ascending order of their atomic masses and prepared a table. It was found that elements with common properties were repeated at regular intervals.
• Mendeleev’s periodic law: The physical and chemical properties of elements are the periodic functions of their atomic mass.
• The horizontal rows in the table are called periods and the vertical columns are called groups. Elements in a group show similar chemical properties.
• Advantages of Mendeleev’s periodic table:
  • Elements were arranged in the ascending order of atomic mass.
  • Blank spaces were left for elements to be discovered and their properties were predicted.
  • Elements with similar properties were grouped together to simplify the study of elements and their compounds.
• • Limitations of Mendeleev’s periodic table
  • The order of atomic mass was not strictly followed in the case of all elements. Hydrogen could not be given an exact position.
  • Elements with very different properties had to be placed in the same group.
• Henry Moseley later discovered that the properties of elements depend on their atomic number. The periodic table developed by him is the modern periodic table used today.
• Chemical formula of the molecule represent one molecule of a substance.
• Atomicity is the number of atoms in a molecule.
• In diatomic and polyatomic elements, the atomicity is written as a subscript on the right of the symbol to obtain their chemical formula.
• The total number of molecules is written on the left side of the chemical formula.
• A chemical equation is the representation of a chemical reaction which helps to understand it at a glance. It uses symbols/chemical formulae to represent the reactants and products.

INTRODUCTION

All the objects we see around us are made up of different molecules. Atoms are the smallest particles that are obtained by dividing molecules. Elements are pure substances that contain the same type of atoms. 118 Elements have been discovered so far. Compounds are formed when atoms of elements combine with each other. In the language of chemistry, symbols are used to represent elements, and chemical formulas are used to represent compounds. The periodic table, which has been prepared by classifying the elements comprehensively, makes the study of compounds and elements easier. In this unit, we will learn about the origins of element names, their symbols, chemical formulas of molecules, and the history of element classification.

We know that all objects around us are made up of different molecules. Let us examine how each of these molecules differ with respect to their structural characteristics.

ATOM

• Molecules are the smallest particles obtained by dissociating various substances having their fundamental, properties.
• Atoms are obtained when molecules are split.
• The molecules of every substance are made up of minute atoms. All substances in the universe are made up of atoms. The word “atom” comes from the Greek word “atomos,” which means indivisible.
• Democritus proposed that all matter is made up of indivisible particles called atoms.
  In 1807, John Dalton proposed the atomic theory to explain the nature of atom. Dalton’s Atomic Theory is one of the important theories in modem chemistry. The ideas put forward by Dalton are given below.
  • All matter is made up of minute particles called atoms.
  • An atom is the smallest particle that can participate in a chemical reaction. .
  • An atom cannot be divided during a chemical reaction.
  • An atom cannot be created or destroyed.
  • All atoms of an element are of the same size, property and mass.
  • Atoms of different elements differ in their size, property and mass.

A water molecule is made up of two atoms of hydrogen and one atom of oxygen.

SYMBOLS

• Symbols are used to represent concepts in a clear and simple manner. Symbols make communication and interpretation easier.
• Similarly, signs are used to represent elements. These are known as chemical symbols. A symbol represents an atom of an element.
• The modern symbol system of representing elements by letters was introduced by a scientist named Berzelius.

A symbol represents an atom of an element.

ORIGIN OF SYMBOLS
• The element hydrogen is represented by the letter ‘H’. That is, the first letter of name of the element is used here as the symbol.

HOW ELEMENTS GOT THEIR NAMES
Elements are named on the basis of country, scientists, colour, name of planets, satellites, continents, etc.
Example: Francium (Country – France)
Einsteinum (Scientist – Albert Einstein)

CLASSIFICATION OF ELEMENTS
In a library we can see that books are arranged by categorising them into essays, poems, stories, novels, etc, Elements are also classified in this manner. So far, 118 elements have been discovered which include natural and synthetic elements. It is very difficult to study all these elements and their compounds separately. It would be easier if elements with similar properties are classified into groups.

• The Periodic Table is a table in which all the elements are categorised and arranged based on their properties.

EARLIER ATTEMPTS FOR THE CLASSIFICATION OF ELEMENTS
The modern Periodic Table that we use today is the result of the efforts of many scientists.

• Lavoisier made the earliest attempts to classify elements. He classified the then-known elements into metals and non-metals.
• Dobereiner classified the elements of similar properties into groups of three. These groups were called triads. He stated that the atomic mass of the element in the middle of the triads was approximately the average of the atomic masses of the other two elements. Only a few elements could be classified in this way.

Element	Atomic mass
Li	7
Na	23
K	39
Ca	40
Sr	87.6
Ba	137.3
Cl	35.5
Br	80
I	127

• Newlands found that when elements were arranged in ascending order of atomie mass, each eighth element was a repetition of the first in its properties. He compared this to the seven notes in music. This classification was known as the Law of Octaves. As more elements were discovered, it became clear that all elements could not be arranged in this way.

Li	Be	B	C	N	O	F
Na	Mg	Al	Si	P	S	Cl

• A scientist named Mendeleev arranged the 63 elements known till that day in ascending order of their atomic masses and prepared a table. It was found that elements with common properties were repeated at regular intervals.

The physical and chemifcal properties of elements are the periodic functions of their atomic mass.

MENDELEEV’S PERIODIC LAW
Mendeleev’s Periodic Table is shown in the figure. The horizontal rows in the table are called periods and the vertical columns are called groups. Elements in a group show similar chemical properties.

CHARACTERISTICS

• Elements were arranged in the ascending order of atomic mass. Blank spaces were left for elements to be discovered and their properties were predicted.
• Elements with similar properties were grouped together to simplify the study of elements and their compounds.
• However, the order of atomic mass was not strictly followed in the case of all elements. Hydrogen could not be given an exact position.
• Elements with very different properties had to be placed in the same group.

When Mendeleev’s Periodic Table was formed in 1869, there was limited knowledge about the structure of the atom and its elementary particles. Henry Moseley later discovered that the properties of elements depend on their atomic number. The periodic table developed by him is the modern periodic table used today.

CHEMICAL FORMULA
CHEMICAL FORMULA OF MOLECULES
We use symbols to identify elements easily. If we write N, it refers to nitrogen and one atom of nitrogen.

CHEMICAL FORMULA OF ELEMENTS

• In monoatomic elements, the symbol itself is considered as the chemical formula.
• In monoatomic molecules, the number written on the left of the symbol indicates, the number of molecules. This is also the number of atoms.

Example: 2He → 2 Helium atoms / 2 Helium molecules
3Ne → 3 Neon atoms / 3 Neon molecules
Atomicity is the number of atoms in a molecule. In diatomic Mid polyatomic elements, the atomicity is written as a subscript on the right of the symbol to obtain their chemical formula. The total number of molecules is written on the left side of the chemical formula.
We can find the number of molecules and the number of atoms from the chemical formula of the elements.
3O₂ → 3 oxygen molecules .
Total number of atoms = 3 × 2 = 6 oxygen atoms
5Cl₂ → 5 chlorine molecules
Total number of atoms = 5 × 2 = 10 chlorine atoms

CHEMICAL FORMULA OF COMPOUNDS
Hydrogen and Chlorine are the constituent elements in hydrogen chloride.

CHEMICAL EQUATIONS
Carbon reacts with oxygen to form carbon dioxide.