NCERT Solutions for Class 10 Science Chapter 5 (2025-2026)

This essential chapter of NCERT Solutions for Class 10 Science Chapter 5, Life Processes, helps us understand the fundamental activities that maintain life in all living organisms - from the smallest bacteria to the largest mammals. Whether it's the food we digest to gain energy, the oxygen we breathe for survival, or the way our heart pumps blood throughout our body, these life processes keep us alive every moment.This chapter is part of the comprehensive NCERT Solutions Class 1o Science series, which covers all chapters in detail.

The chapter equips students with essential skills to comprehend vital processes like nutrition, respiration, transportation, and excretion in both plants and animals. Every solution has been designed keeping CBSE board exam patterns in mind, ensuring students develop both conceptual clarity and problem-solving confidence. By mastering this chapter, students build a strong foundation for human physiology, botany, and advanced biological concepts in higher classes.

NCERT Solutions for Class 10 Science Chapter 5 - All Exercise Questions

Life Processes
Easy
Q.
The kidneys in human beings are a part of the system for
(a) nutrition.
(b) respiration.
(c) excretion.
(d) transportation.
Life Processes
Easy
Q.
The xylem in plants are responsible for
  1. transport of water.
  2. transport of food.
  3. transport of amino acids.
  4. transport of oxygen.
Life Processes
Easy
Q.
The autotrophic mode of nutrition requires
  1. carbon dioxide and water.
  2. chlorophyll.
  3. sunlight.
  4. all of the above.
Life Processes
Easy
Q.
The breakdown of pyruvate to give carbon dioxide, water and energy takes place in
  1. cytoplasm.      
  2. mitochondria.
  3. chloroplast.
  4. nucleus.
Life Processes
Easy
Q.
How are fats digested in our bodies? Where does this process take place?
Life Processes
Easy
Q.
What is the role of saliva in the digestion of the food?
Life Processes
Medium
Q.
What are the necessary conditions for autotrophic nutrition and what are its by products?
Life Processes
Easy
Q.
What are the differences between aerobic and anaerobic respiration. Name some organisms that use the anaerobic mode of respiration.
Life Processes
Easy
Q.
How are the alveoli designed to maximise the exchange of gases?
Life Processes
Easy
Q.
What would be the consequences of a deficiency of haemoglobin in our bodies?
Life Processes
Medium
Q.
Describe double circulation in human beings. Why is it necessary?
Life Processes
Medium
Q.
What are the differences between the transport of materials in xylem and phloem?
Life Processes
Medium
Q.
Compare the functioning of alveoli in lungs and nephrons in the kidneys with respect to their structure and functioning.

Class 10 Chapter 5 Science Questions & Answers – Life Processes

Q1. The kidneys in human beings are a part of the system for

(a) nutrition.
(b) respiration.
(c) excretion.
(d) transportation
Solution: (c) Excretion

Explanation: Kidney is a bean-shaped organ. It helps in the removal of metabolic wastes.

 

Q2. The xylem in plants are responsible for

  1. transport of water.
  2. transport of food.
  3. transport of amino acids.
  4. transport of oxygen.

Solution:   a) transport of water

Explanation: Xylem is the vascular tissue that facilitates the movement of water in plants. Movement of food and amino acids takes place through the vascular tissue phloem. Transport of oxygen in plants occurs through stomata.

 

Q3. The autotrophic mode of nutrition requires

  1. carbon dioxide and water.
  2. chlorophyll.
  3. sunlight.
  4. all of the above.

Solution: d) all of the above

Explanation: Autotrophic nutrition takes place by the process of photosynthesis. Carbon dioxide, water, chlorophyll and sunlight are required to carry out the process of photosynthesis.

 

Q4.The breakdown of pyruvate to give carbon dioxide, water and energy takes place in

  1. cytoplasm.      
  2. mitochondria.
  3. chloroplast.
  4. nucleus.

Solution: a) mitochondria.

Explanation: Breakdown of glucose into pyruvate takes place in cytoplasm. Then further breakdown of pyruvate into CO2, water and energy takes place in mitochondria.

 

Q5. How are fats digested in our bodies? Where does this process take place?

Solution:  From the stomach, partially digested food, that is acidic, enters the small intestine. Bile juice is released into the small intestine through the gall bladder. The acidity of the partially digested food is nutralised by the bile salts present in the bile juices. Bile salts also emulsify fat molecules by breaking their large globules into many small globules so that the pancreatic enzymes can act effectively on them. Pancreatic enzyme like lipase present in pancreatic juice digests fats and converts them into fatty acid and glycerol.

This entire process takes place in the small intestine.

 

 Q6. What is the role of saliva in the digestion of the food?

Solution:  Saliva, a secretion of salivary glands, helps in digestion of food in the following two ways:

  1. The saliva moistens the food and helps in its easy swallowing and movement in oesophagus.
  2. The saliva contains an enzyme called salivary amylase that initiates the breakdown of carbohydrates such as starch into sugar.

Q7. What are the necessary conditions for autotrophic nutrition and what are its by products?

Solution:  The necessary conditions for autotrophic nutrition are the availability of:

1. Carbon dioxide
2. Sunlight (source of energy)
3. Chlorophyll pigment (that captures energy)
4. Water
The products formed are carbohydrate (glucose) and oxygen.

6CO2​+6H2​Ochlorophyll→light energy​C6​H12​O6​+6O2

 

Q8. What are the differences between aerobic and anaerobic respiration. Name some organisms that use the anaerobic mode of respiration.

Solution:   Differences between aerobic respiration and anaerobic respiration are:

Aerobic respiration Anaerobic respiration
It occurs in the presence of oxygen. It occurs in the absence of oxygen.
The final products are carbon dioxide, energy and water The final products are alcohol, carbon dioxide and energy
The amount of energy released is high The amount of energy released is lower than aerobic respiration.
It takes place in cytoplasm and mitochondria of a cell. It takes place in cytoplasm only.

Some of the organisms that use anaerobic mode of respiration are bacteria and fungi like yeast.

 

Q9. How are the alveoli designed to maximise the exchange of gases?

Solution:

The alveoli are small balloon-like structures present in the lungs. The walls of alveoli consist of extensive network of blood capillaries. Each lung contains 300-350million alveoli. The alveolar surface when spread out covers the area about 80m2. This large surface area makes the gaseous exchange efficient.

 

Q10. What would be the consequences of a deficiency of haemoglobin in our bodies?

Solution:

Haemoglobin is the respiratory pigment in humans. It facilitates the transportation of oxygen in the body cells to carry out cellular respiration. Thus, deficiency of haemoglobin can directly affect the oxygen supplying capacity of the blood which can further lead to deficiency of oxygen in our body cells. The deficiency of haemoglobin also causes disease such as anaemia.

 

Q11. Describe double circulation in human beings. Why is it necessary?

Solution:

Circulation in human beings comprises two phases: pulmonary circulation and systemic circulation. During each cycle of the circulation, the blood goes through the heart twice.
Flow of blood in the heart
The human heart comprises four chambers: the right atrium, the right ventricle, the left atrium and the left ventricle.

The superior and inferior vena cava collects deoxygenated blood from the upper and lower regions of the body respectively and transfers this de-oxygenated blood to the right atrium of the heart.

The right atrium then contracts and transfers the deoxygenated blood to the right ventricle.

From the right ventricle, the deoxygenated blood is then transferred into the two pulmonary arteries, which pumps the blood to the lungs for oxygenation.

From the lungs, oxygenated blood is transferred to the left atrium of the heart through the pulmonary veins. This complete process is called pulmonary circulation.

Then the left atrium contracts and pushes the oxygenated blood into the left ventricle.

The left ventricle then contracts and forces the blood into the aorta which gives rise to many arteries that distribute the oxygenated blood to all the regions of the body.

This completes the systemic circulation.

Importance of Double blood circulation:

The separation of oxygenated and deoxygenated blood in the separate chambers of heart and blood vessels allows a more efficient supply of oxygen to the body cells.

The sufficient system of oxygen supply is very useful in animals that have high energy requirements to maintain their body temperature such as warm-blooded animals like birds and mammals.

 

Q12. What are the differences between the transport of materials in xylem and phloem?

Solution:

Differences between the transport of materials in xylem and phloem are:

Transport of materials in xylem Transport of materials in phloem
Xylem facilitates the transport of water in plants. Phloem facilitates the transport of food in plants.
Transport of water is in upward direction from roots to the other parts of the plants Transport of food occurs in both the directions i.e., upward and downward.
Simple physical forces like transpiration pull play an important role in the transport of water in xylem. Energy in the form of ATP is required in the transport of food in phloem.

 

Q13. Compare the functioning of alveoli in lungs and nephrons in the kidneys with respect to their structure and functioning.

Solution:

Alveoli Nephron
Structure They are tiny balloon-like structure present inside the lungs. Nephron is the structural and functional unit of kidney. They are present in the form of tubular structures inside the kidneys.
The walls of alveoli are one cell thick and posses an extensive network of blood capillaries that help in exchange of gases. It is composed of glomerulus, Bowman’s capsule and a long renal tubule and is surrounded by the cluster of thin walled blood capillaries.
Function The alveoli function as the sites for gaseous exchange. The exchange of O2 and CO2 takes place between the blood of the capillaries that surround the alveoli and the gases present in the alveoli. Being the basic filtration unit of kidney, nephron removes the nitrogenous wastes such as urea or uric acid from the blood and forms urine. Renal artery divides into many capillaries in glomerulus of the nephron. The blood gets filtered in the glomerulus and then the filtrate passes through the tubules of the nephron and gets collected in a collecting duct. As the filtrate flows in the tube, some substances such as glucose, amino acids and water are selectively re- absorbed and get converted into urine. Urine from many nephrons is collected into the collecting duct that passes the urine into the urete

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NCERT Solutions for Class 10 Science Chapter 5 – FAQs

Q1. What is the difference between autotrophic and heterotrophic nutrition?

Autotrophic nutrition is the mode of nutrition where organisms prepare their own food from simple inorganic substances like carbon dioxide and water using sunlight energy. Green plants, algae, and some bacteria perform photosynthesis, converting light energy into chemical energy stored in glucose. The equation is: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂. Autotrophs are called producers as they form the base of all food chains. Heterotrophic nutrition is the mode of nutrition where organisms cannot make their own food and depend on other organisms for nutrition. Animals, fungi, and most bacteria are heterotrophs. This includes saprophytic nutrition (feeding on dead organic matter like fungi), parasitic nutrition (obtaining food from living hosts like tapeworms), and holozoic nutrition (ingesting solid food like humans and animals). Heterotrophs are consumers in the food chain and depend directly or indirectly on autotrophs for energy.

Q2. How does the human respiratory system work and what is the mechanism of breathing?

The human respiratory system includes nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli in the lungs. Breathing involves two processes: inhalation and exhalation. During inhalation, the diaphragm and intercostal muscles contract, expanding the chest cavity and reducing pressure inside the lungs, causing air to rush in. During exhalation, these muscles relax, the chest cavity decreases in volume, pressure increases, and air is pushed out. In the alveoli (tiny air sacs), gaseous exchange occurs through diffusion—oxygen from inhaled air passes into blood capillaries, while carbon dioxide from blood passes into alveoli to be exhaled. Hemoglobin in red blood cells transports oxygen to body cells and brings back carbon dioxide. This continuous process ensures cells receive oxygen for cellular respiration and removes waste carbon dioxide.

Q3. What is the difference between aerobic and anaerobic respiration?

Aerobic respiration occurs in the presence of oxygen and completely breaks down glucose into carbon dioxide and water, releasing maximum energy (38 ATP molecules per glucose molecule). The equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP). It occurs in the mitochondria of cells and is the most efficient form of respiration used by most organisms including humans, animals, and plants. Anaerobic respiration occurs in the absence of oxygen and incompletely breaks down glucose, releasing less energy (2 ATP molecules per glucose). In muscle cells during intense exercise when oxygen supply is insufficient, glucose breaks down into lactic acid causing muscle cramps: C₆H₁₂O₆ → 2C₃H₆O₃ + Energy. In yeast and some bacteria, glucose breaks down into ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy. This process is used in making bread, beer, and wine. Aerobic respiration is more efficient and preferred when oxygen is available.

Q.1 Atomic number of an element is 19.
(i) Is it a metal or non-metal?
(ii) Will the size of this element be greater than that of sodium?

(i) The electronic configuration of the given element is 2,8,8,1. Therefore, it has 1 electron in the valence shell. Hence, it is a metal.
(ii) The element with atomic number 19 is potassium. Since it is placed below sodium in periodic table, it is larger in size as compared to sodium.

Q.2 What change will be observed in atomic size when we move from top to bottom in group 2. Give reasons also.

As we move from top to bottom in a group, new shells are being added and the distance between the nucleus and the outermost shell goes on increasing and size of the atom increases. Therefore, when we move down in group 2, the size of atom increases.

Q.3 An element (X) belongs to second period and first group of the periodic table. Find:
(i) the number of valence electrons present in its atom.
(ii) the valency of the element.
(iii) whether element (X) forms acidic or basic oxide.

(i) The number of valence electrons in element X is one (because it belongs to group 1).
(ii) As element X has one electron in outermost shell, its valency is one.
(iii) Since element (X) belongs to group one, it is a metal. It will form basic oxide.

Q.4 When a metal M reacts with a non-metal X, it forms a compound of MX type.

(a) Identify the group in which element M and X are placed in modern periodic table.
(b) Write the number of valence electrons present in element M and element X.
(c) Write an example of MX type of compound.

(a) The metal M belongs to group1 (alkali metals) and X belongs to group 17 (Halogen group).
(b) The number of valence electrons present in M is one and the mumber of valence electrons present in X is 7.
(c) NaCl or KCl

Q.5 Element P belongs to group 2 of modern periodic table and element Q belongs to group 17 of modern periodic table. Write the formula of:

(i) the compound formed by elements P and Q.
(ii) the oxide of element P.
(iii) the chloride of element P.

(i)The valency of element P is 2 (because it belongs to group 2) and that of element Q is 1 (because it belongs to group 17). The formula of compound will be PQ2.
(ii) PO
(iii) PCl2

Q.6 What was the drawback of the law of octaves?

The law of octaves was applicable only upto calcium out of the total 56 elements known.

Q.7 The positions of three elements A, B, and C in the periodic table are shown here.

Group 16 Group 17
– –
– A
– –
B C

(a) State whether A is a metal or non-metal
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?

(a) The group 17 is placed in right hand side of the periodic where non-metals are placed. So, A is a non-metal.
(b) The chemically reactivity of non-metals decreases on going down the group of the periodic table. So, C is less reactive than A.
(c) On going from left to right in a period, the atomic radii decreases. So, C is smaller than B.

Q.8 By considering their position in the periodic table, which one of the following elements would you except to have the maximum metallic character?
C , O, F, B, Be

The maximum metallic character is found in elements on the extreme left side of the periodic table. Out of the above given elements, Be will have the maximum metallic character because it is on the extreme left side in the periodic table.

Q.9 What does each period in the periodic table indicate?

Each period in the periodic table indicates the number of shells.

Q.10 An atom has electronic configuration 2, 8, 7:-
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar?
N (7), F (9), P (15), Ar (18).

(a) Atomic number of this element is 17.
(b) F will chemically be similar to this element because F has similar valence electrons.

Q.11 Name two elements you would choose to show chemical reactions similar to magnesium. Write the basis for your choice?

Calcium and Strontium would show chemical reactions similar to magnesium because they have same number of valence electrons like Mg.

Q.12 Write the characteristics of the groups.

Characteristics of the groups are: –
(i) Valence Electrons: – All the elements of a group of the periodic table have the same number of valence electrons.
(ii) Size of Atoms: – On moving down the group atomic size of the atom increases due to increase in number of shells.
(iii) Metallic Character: – On moving down the group metallic character of elements increases.
(iv) Valency: – Since the number of valence electrons in a group is the same, so all the elements in a group have the same valency.
(v) Non-metallic Character: – On moving down the group non-metallic character of elements decreases.

Q.13 Write the merits of the modern periodic table.

Merits of the modern periodic table are: –
1.It is based on the atomic number of elements which is the most fundamental property of elements.
2. It explains the reason for the periodicity in properties of elements.
3. It helps us understand why elements in a group show similar properties but elements in different groups show different properties.
4. It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements. There are no anomalies in the arrangement of elements in the modern periodic table.

Q.14 Consider the following elements:Na, Ca, Al, K, Mg, Li
(a) Which of these elements belong to the same period of the periodic table?
(b) Which of these elements belong to the same group of the periodic table?

The electronic configurations of the following elements are:
Na: – 2, 8, 1
Ca: – 2, 8, 8, 2
Al: – 2, 8, 3
K: – 2, 8, 8, 1
Mg: – 2, 8, 2
Li: – 2, 1
(a) Na, Al and Mg elements belong to third period and Ca and K elements belong to the fourth period of the periodic table because Na, Al and Mg elements have same number of shell (three shells) and Ca and K elements have same number of shells(four shells).
(b)The elements which have same number of valence electrons belong to the same group. So, Li, Na and K having 1 valence electron belong same group and Mg, Ca having 2 valence electrons belong same group.

Q.15 Write the limitations of Mendeleev’s classification of elements.

1. The position of isotopes could not be explained.
2. Wrong order of atomic masses of some elements could not be explained.
3. Correct position could not be assigned to hydrogen in the periodic table.

Q.16 For each the following triads, name the element with the characteristics specified below:

Elements Smallest atomic radius Chemically least reactive
F, Cl, Br ………….. ……………
Li, Na, K …………… …………….

Ans-

Elements Smallest atomic radius Chemically least reactive
F, Cl, Br F Br
Li, Na, K Li Li

Q.17 Write the merits of Mendeleev’s classification of elements.

Merits of Mendeleev’s classification of elements:
1. Mendeleev’s classification was first successful classification of elements which included all the elements.
2. It predicted the properties of elements which were not known at that time.
3. When noble gases were discovered, they could be placed in a new group without disturbing the existing order.

Q.18 Write the demerits of Newlands’ law of octaves.

Demerits of Newlands’ law of octaves: –
(i) This law was applicable upto calcium only.
(ii) Newlands’ assumed that only 56 elements existed in nature and no more elements would be discovered in future.
(iii) In order to fit elements into his table, Newland put even two elements together in one slot and that too in the column of unlike elements having very different properties.

Q.19 What are the advantages of the periodic table?

1. The periodic table has made the study of chemistry systematic and easy.
2. It is easier to remember the properties of an element if its position in the periodic table is known.
3. The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

Q.20 An element X belongs to group 2 and another element Y belongs to group 15 of the periodic table:-

(a) What is the number of valence electrons in X?
(b) What is the valency of X?
(c) What is the number of valence electrons in Y?
(d) What is the valency of Y?

Ans-

(a) Valence electrons of X = 2
(b) Valency of X = 2
(c) Valence electrons of Y = 5
(d) Valency of Y = 3

Q.21 What happens to

(i) Metallic character of the elements as we move from left to right in a period.
(ii) Atomic size when we move from top to bottom in a group.

Ans-

(i) When we move left to right in a period metallic character decreases.
(ii) Atomic size increases when we move from top to bottom in a group.

Q.22 Explain why?

(i) All the elements of a group have similar chemical properties.
(ii) All the elements of a period have different chemical properties.

Ans- Chemical properties of the elements depend upon the valence electrons, it means the elements which have the same number of valence electrons in their outermost shells, will have similar chemical properties.
(i) So all the elements of the group have similar chemical properties owing to the same number of valence electrons.
(ii) But the elements of a period have different valencies, hence they show different chemical properties.

Q.23 Element X forms a chloride XCl2, which is a solid with a high melting point. X would most likely be in the same group of the periodic table as:
(a) Na (b) Mg (c) Al (d) Si

Ans- Since element X forms a chloride XCl2, so the valency of X is 2. Now, out of Na, Mg, Al, and Si, the element of valency 2 is Mg. So, X would be in the same group as that of Mg because all the elements of the same group have same valency.

Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative. Why?

Ans- Electronic configuration of Nitrogen is 2, 5.

Electronic configuration of Phosphorus is 2, 8, 5.
Nitrogen is more electronegative than phosphorus due to its small size.

Q.25 How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Ans- The properties of the elements depend upon the valence electrons. On arranging the elements in the increasing order of their atomic numbers, the elements with the same number of valence electrons and hence, showing similar properties occur at regular intervals. Hence, the chemical properties of the elements show repetition.

Q.26Out of F and Cl, which is bigger in size and why?

Ans- Fluorine and chlorine belong to same group. When we move down the group the size of atom increases due to increase in number of shells. Therefore, the size of chlorine atom will be larger as compared to the atom of chlorine.

Q.27 Write the electronic configuration of Ca in terms of K, L, M, N shells.

(Atomic number of Ca is 20)

Ans- The electronic configuration of Ca is

K L M N
2 8 8 2

Q.28 Define groups.

Ans- The vertical columns in the long form of the periodic table are called groups.

Q.29 In the Modern Periodic Table, which are the metals among the first ten elements?

Ans- Li and Be are the metals amongst first ten elements of the Modern Periodic Table.

Q.30 Which period is the longest period in the modern periodic table?

Ans- 6th period is the longest period and it has 32 elements.

Q.31 In the following diagram for the first three periods (showing only group 1,2, 13 to 18)of the periodic table, five elements have been represented by the letters a, b, c, d and e (which are not their chemical symbols):

(i) Select the letter which represents a halogen.
(ii) Select the letter which represents a noble gas.
(iii) What type of bond is formed between a and b?
(iv) What type of bond is formed between c and d?
(v) Which element will form a divalent anion?

Ans-

(i) The halogens are placed in group 17 of the periodic table. So, letter ‘b’ represents a halogen.
(ii) The noble gases are placed in group 18 of the periodic table. So, letter ‘e’ represents a noble gas.
(iii) ‘a’ and ‘b’ both are non-metals. So, the bond formed in between ‘a’ and ‘b’ will be covalent.
(iv) ‘c’ is metal and ‘d’ is non-metal. So, the bond formed in between ‘c’ and ‘d’ will be electrovalent.
(v) ‘d’ element will form a divalent anion because it has 6 valence electrons and it wants to 2 more electrons to completes its octet.

Q.32 What is the basis of classification of elements?

Ans- Elements are classified on the basis of similarities in their properties.

Q.33 How many periods are present in Modern periodic table?

Ans- Seven periods are present in the modern periodic table.

Q.34 Besides gallium, which other elements have been since discovered to fill the gaps left by Mendeleev in his periodic table? (Any two)

Ans- Eka-boron (Scandium) and Eka-silicon (Germanium)

Q.35 What is the fundamental difference in the electronic configuration of the group I and group II elements?

Ans- Group I elements have 1 electron in their outermost shell whereas group II elements have 2 electrons in their outermost shell.

Q.36 How many groups are in the long form of the periodic table?

Ans- There are 18 vertical columns called groups in the long form of periodic table.

Q.37 State the modern periodic law.

Ans- According to the modern periodic law the properties of elements are periodic function of their atomic numbers.

Q.38 What was the criteria used by Mendeleev in creating his periodic table?

Ans- Mendeleev used atomic mass of the elements as the criteria to create his periodic table.

Q.39 What was the main limitation of Dobereiner’s triad?

Ans- Dobereiner’s classification failed to arrange all the known elements in the form of triads of elements having similar chemical properties.

Q.40 Why do you think the noble gases are placed in a separate group?

Ans- Noble gases are placed in a separate group because all of them have completely filled shells. Therefore, these gases are inert in nature and do not generally participate in any chemical reaction.

Q.41 An element ‘X’ belongs to second period and second group of the periodic table.
Find:
(i) the number of valence electrons.
(ii) the valency of the element.
(iii) whether element X forms acidic or basic oxide.

Ans-

(i) Group number of element = Number of valence electrons
Therefore, number of valence electrons will be 2.
(ii) The valency of elements will be 2.
(iii) Nature of its oxide will be basic because it is a metal.

Q.42 Write the names of
(a) two elements that have seven electrons in their outermost shells
(b) two elements that have two electrons in their outermost shells
(c) two elements which have completely filled outermost shells

Ans-

(a) Chlorine (Cl) and Bromine (Br)
(b) Magnesium (Mg) and Calcium (Ca)
(c) Neon (Ne) and Argon (Ar)

Q.43 How many elements were known when Mendeleev classified the elements?

Ans- At the time of Mendeleev, 63 elements were known.

Q.44 Why are the elements in the same group of the periodic table show close resemblance in chemical behaviour?

Ans- The elements in the same group of the periodic table show close resemblance in their chemical behaviour as they have the same number of valence electrons.

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