CBSE Class 9 Science Structure of The Atom Worksheet Set 02

STRUCTURE OF THE ATOM

GIST OF THE LESSON

Thomson’s Model of an Atom

• According to this model, an atom is made of a sphere with a positive charge where electrons are placed inside.
• Because the positive and negative charges have the same strength, the atom has no net charge overall and is electrically neutral.

Rutherford’s Gold Foil Experiment

When high-speed alpha particles were shot at a very thin sheet of gold, the following points were noticed:

• A large majority of the alpha particles went directly through the gold sheet without changing direction.
• A few alpha particles were turned slightly by the gold foil at small angles.
• Only about one particle out of every twelve thousand turned back completely.

Conclusions made by Rutherford based on his observations:

• Most of the area inside an atom is empty since nearly all alpha particles passed straight through without any deflection.
• Only a tiny number of particles were deflected, showing that the positive charge is concentrated in a very limited space.
• Since a minuscule amount of alpha particles bounced straight back, the entire mass and positive charge must be packed into a highly dense, small space at the center of the atom.

Rutherford’s Model of an Atom

• An atom contains a center with a positive charge, known as the nucleus, which holds almost the entire mass of the atom.
• The electrons move in circular orbits around this center.
• The nucleus is extremely tiny when compared to the total size of the atom itself.

Drawback of Rutherford’s model of atom

An accelerating charged particle like an electron is expected to emit energy continuously. This means a moving electron would keep losing energy and eventually collapse into the nucleus, which would make atoms highly unstable.

Postulates of Bohr’s Model of Atom

• Only certain specific orbits, called discrete orbits, are permitted for electrons to occupy inside an atom.
• While moving in these unique pathways, electrons do not release any energy. These circular paths are termed energy levels.
• These orbits or energy shells are labeled as K, L, M, N or with numbers \( n = 1, 2, 3, 4 \).

The highest count of electrons a shell can hold is calculated using the formula \( 2n^2 \), where \( n \) represents the shell number.

Important Definitions:

• Valency: An atom's capacity to combine with other atoms is known as its valency.
• Atomic number: This represents the total count of protons in an atom's nucleus.
• Mass number: This is the total sum of protons and neutrons found inside the nucleus.

Isotopes

These are atoms of one element that share the same atomic number but have different mass numbers.

Applications of isotopes:

• A specific isotope of uranium serves as fuel in nuclear reactors.
• Cobalt isotopes find application in cancer therapy.
• An iodine isotope is employed to treat goiter.

Isobars

These are atoms of different chemical elements that have matching mass numbers but distinct atomic numbers.

MULTIPLE CHOICE QUESTIONS

Question. The three subatomic particles of an atom are –
(a) Positively charged Neutrons, Negatively charged Protons and Neutral Electrons
(b) Positively charged Protons , Negatively charged Neutrons and Neutral Electrons
(c) Positively charged Protons , Negatively charged Electrons and Neutral Neutrons
(d) Positively charged Electrons , Negatively charged Neutrons and Neutral Protons
Answer: (c) Positively charged Protons , Negatively charged Electrons and Neutral Neutrons
In simple words: Protons carry a positive charge, electrons carry a negative charge, and neutrons have no charge.
Exam Tip: Be careful not to mix up the charges: protons are positive, electrons are negative, and neutrons are neutral.

Question. Number of valence electrons in \( \text{Cl}^- \) (Chloride ) ion are
(a) 16
(b) 17
(c) 18
(d) 8
Answer: (d) 8
In simple words: A neutral chlorine atom has 7 valence electrons, so adding one extra electron to form a chloride ion completes its outer shell with 8 electrons.
Exam Tip: Ions of halogen elements always achieve a stable octet of 8 valence electrons in their outermost shell.

Question. Which one of the following is a correct electronic configuration of Sodium?
(a) 2, 8
(b) 2, 1, 8
(c) 8, 2, 1
(d) 2, 8, 1
Answer: (d) 2, 8, 1
In simple words: Sodium has an atomic number of 11, which is distributed as 2 in the first shell, 8 in the second, and 1 in the third.
Exam Tip: Always fill the innermost shells to their maximum capacity (2 in K, 8 in L) before placing electrons in outer shells.

Question. The atomic number of sodium is 11 and its mass number is 23. It has
(a) 11 neutrons and 12 protons
(b) 12 protons and 11 electrons
(c) 11 electrons and 12 neutrons
(d) 12 electrons and 11 neutrons
Answer: (c) 11 electrons and 12 neutrons
In simple words: Sodium's atomic number 11 means it has 11 protons and 11 electrons, while its neutron count is mass minus protons (\(23 - 11 = 12\)).
Exam Tip: Neutron count is always the mass number minus the atomic number (\(A - Z\)).

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
(a) 12
(b) 8
(c) 10
(d) 16
Answer: (c) 10
In simple words: Since the K shell holds 2 electrons and the L shell holds 8, a full set of both shells equals 10 electrons.
Exam Tip: Add the maximum capacities of K (2) and L (8) to find the total electron count.

Question. What property of an element determines its chemical behaviour?
(a) Size of an element
(b) Valency of an element
(c) Molar mass of the element
(d) None of the options
Answer: (b) Valency of an element
In simple words: Valency, or the capacity to combine, controls how an element reacts and bonds with other elements.
Exam Tip: Chemical reactivity and bonding characteristics are fundamentally determined by valence electrons and valency.

Question. Which of the two will be chemically more reactive, Sulphur(S) with atomic number 16 or Chlorine (Cl) with atomic number 17 ?
(a) Chlorine
(b) Sulphur
(c) Both are equally reactive
(d) Both are unreactive
Answer: (a) Chlorine
In simple words: Chlorine needs only one electron to complete its outer shell, whereas Sulphur needs two, making Chlorine more eager to react.
Exam Tip: Elements that require fewer electrons to complete their octet are generally more reactive.

Question. Which of the following statements is incorrect about the structure of an atom?
i. The whole mass of an atom is concentrated in the nucleus
ii. The atom is an indivisible particle
iii. The atom as a whole is neutral
iv. All the atoms are stable in their elemental state
(a) (i) and (iii)
(b) only (ii)
(c) (ii) and (iv)
(d) None of the options
Answer: (c) (ii) and (iv)
In simple words: Atoms are actually divisible into smaller parts like protons, neutrons, and electrons, and some atoms are highly unstable on their own.
Exam Tip: Identify modern atomic theory truths to spot old or incorrect Daltonian assumptions like indivisibility.

Question. The smallest fundamental particle of an atom with unit negative charge is
(a) Neutron
(b) Alpha Particle
(c) Proton
(d) Electron
Answer: (d) Electron
In simple words: The electron is the lightest subatomic particle and carries a single negative charge.
Exam Tip: Remember that electrons are the fundamental carriers of negative charge in all atomic structures.

Question. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
(a) \( _{15}\text{X}^{31} \)
(b) \( _{16}\text{X}^{31} \)
(c) \( _{31}\text{X}^{15} \)
(d) \( _{15}\text{X}^{16} \)
Answer: (a) \( _{15}\text{X}^{31} \)
In simple words: The atomic number 15 goes at the bottom left, and the mass number (15 + 16 = 31) goes at the top left.
Exam Tip: Element representation format is \( _{Z}\text{X}^{A} \), where Z is the atomic number (bottom) and A is the mass number (top).

Question. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer: (d) (ii) and (iv)
In simple words: Mass number is the sum of protons and neutrons, while atomic number represents the individual count of either protons or electrons in a neutral atom.
Exam Tip: Remember that atomic number equals the number of protons, which also equals the number of electrons in a neutral atom.

Question. Identify the \( \text{Mg}^{2+} \) ion from the figure where, n and p represent the number of neutrons and protons respectively.
(i) a
(ii) b
(iii) c
(iv) d
Answer: (iv) d
In simple words: A magnesium ion \( \text{Mg}^{2+} \) must have 12 protons, 12 neutrons, and only 10 electrons (configuration 2, 8) after losing 2 valence electrons.
Exam Tip: A 2+ charge means the ion has 2 fewer electrons than its proton number, so look for a configuration of 2, 8 for magnesium.

Question. An atom with 3 protons and 4 neutrons will have a valency of---
(a) 3
(b) 7
(c) 1
(d) 4
Answer: (c) 1
In simple words: With 3 protons, it has 3 electrons configured as (2, 1). Having 1 electron in its outer shell gives it a valency of 1.
Exam Tip: Write the electronic configuration first to determine how many electrons are in the outermost shell.

Question. There are three isotopes of carbon which are named as C-12, C-13 and C-14 out of which C-12 is the most abundant isotope. In the given structures of 3 isotopes, what will be the composition of the nucleus?
(a) C-12 : 6p + 6n, C-13 : 7p + 6n, C-14 : 8p + 6n
(b) C-12 : 6p + 6n, C-13 : 6p + 7n, C-14 : 6p + 8n
(c) C-12 : 6p + 6n, C-13 : 5p + 8n, C-14 : 7p + 7n
(d) C-12 : 6p + 6n, C-13 : 12p + 1n, C-14 : 5p + 9n
Answer: (b) C-12 : 6p + 6n, C-13 : 6p + 7n, C-14 : 6p + 8n
In simple words: Isotopes of carbon must always have 6 protons. The difference in their masses comes solely from having 6, 7, and 8 neutrons respectively.
Exam Tip: Isotopes of the same element must always have the exact same number of protons.

Question. The maximum number of electrons that can be accommodated in third shell ( n = 3) is:
(a) 18
(b) 8
(c) 2
(d) 10
Answer: (a) 18
In simple words: Using the formula \( 2n^2 \) for n = 3, we get \( 2 \times 3^2 = 2 \times 9 = 18 \) electrons.
Exam Tip: Don't confuse the maximum capacity of a shell with the octet rule of the outermost shell.

Question. The canal rays led to the discovery of which subatomic particle?
(a) Beta rays
(b) Protons
(c) Neutrons
(d) Electrons
Answer: (b) Protons
In simple words: Canal rays consist of positively charged particles, which led to the discovery of protons.
Exam Tip: Remember that cathode rays led to electrons, whereas canal (anode) rays led to protons.

Question. According to J.J Thomson, an atom is a positively charged sphere. Which sub-atomic particles is embedded in it?
(a) Electrons
(b) Canal Rays
(c) Protons
(d) Neutrons
Answer: (a) Electrons
In simple words: Thomson's plum pudding model compared an atom to a positive pudding with negative electrons scattered like plums inside it.
Exam Tip: Visualize Thomson's model as a cookie, where the dough is the positive charge and the chips are electrons.

Question. The scientist who gave the concept of fixed energy levels around the nucleus is
(a) Ernest Rutherford
(b) Neils Bohr
(c) J.J.Thomson
(d) None of these
Answer: (b) Neils Bohr
In simple words: Niels Bohr introduced the idea that electrons travel in specific circular orbits with fixed energy levels.
Exam Tip: Bohr resolved the stability issue of Rutherford's model by introducing quantized, non-radiating orbits.

Question. When alpha-particles are sent through a thin metal foil, most of them go straight through the foil because_______.
(a) alpha-particles are positively charged.
(b) alpha-particles are much smaller than electrons.
(c) alpha-particles move with low velocity.
(d) most part of the atom is empty.
Answer: (d) most part of the atom is empty.
In simple words: Since almost all alpha particles went straight through without hitting anything, it proved that atoms consist mostly of empty space.
Exam Tip: The direct passage of the majority of alpha particles is the key evidence for the hollow structure of atoms.

Question. The proton has a charge equal in magnitude but opposite in sign to that of:
(a) Electron
(b) deuterons
(c) Neutron
(d) canal ray
Answer: (a) Electron
In simple words: The proton's positive charge is equal in size to the electron's negative charge, allowing them to balance each other out.
Exam Tip: Though their masses are vastly different, protons and electrons carry exactly equal and opposite electrical charges.

ASSERTION - REASON TYPE QUESTIONS

Directions: Each of these questions contains two statements Assertion (A) and Reason (R). Select one of the codes (a), (b), (c) and (d) given below:
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.

Question. Assertion: Alpha particles are doubly positively charged helium nucleus.
Reason: Alpha particles have a mass of 4u
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: An alpha particle is a helium ion (\( \text{He}^{2+} \)) with two protons and two neutrons, giving it a mass of 4u and a double positive charge.
Exam Tip: An alpha particle is chemically identical to a helium nucleus and carries a +2 charge with 4 units of atomic mass.

Question. Assertion: Number of electrons present in the shell M is 32
Reason: Maximum number of electrons present in a shell is given by the formula 2n2
Answer: (d) A is false, but R is true.
In simple words: The M shell (n = 3) can hold up to 18 electrons, not 32, though the formula \( 2n^2 \) is correct.
Exam Tip: Remember the capacities of shells: K=2, L=8, M=18, N=32.

Question. Assertion: Neutrons are present in the nucleus of all atoms. expect H- atom.
Reason: Mass of the neutron is equal to that of electron
Answer: (c) A is true, but R is false.
In simple words: Ordinary hydrogen has no neutrons, but the mass of a neutron is actually about 1840 times heavier than an electron, not equal to it.
Exam Tip: Neutrons and protons have nearly equal masses, which are vastly larger than the mass of an electron.

Question. Assertion: Mass of an atom is given by the sum of the masses of protons and neutrons
Reason: Mass of the electron is equal to mass of proton
Answer: (c) A is true, but R is false.
In simple words: The mass of an atom is concentrated in the protons and neutrons of the nucleus because electrons are so light they are ignored, not because they are equal in mass.
Exam Tip: Electrons have a negligible mass compared to protons and neutrons, which are about 1840 times heavier.

Question. Assertion: He atom has an atomic mass of 4u.
Reason: It has two protons and two neutrons.
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Helium has a mass of 4u because its nucleus contains exactly two protons and two neutrons, each weighing about 1u.
Exam Tip: Atomic mass is calculated by adding the number of protons and neutrons in the nucleus.

Question. Assertion: The stability of the atom could not be explained by Rutherford model of atom.
Reason: Revolving electrons would lose energy and finally fall into the nucleus.
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Rutherford's model was unstable because classical physics says moving charges must radiate energy, which would cause electrons to spiral into the nucleus.
Exam Tip: Bohr solved this major drawback of Rutherford's model by introducing stable, non-radiating quantum orbits.

Question. Assertion: Combining capacity of an element is known valency.
Reason: Valency of the metal sodium is unity
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: Both statements are true, but the fact that sodium has a valency of 1 does not explain the general definition of what valency is.
Exam Tip: Valency is defined as combining capacity, and for metals like sodium, it equals the number of valence electrons lost.

Question. Assertion: Atomic numbers of an element is number of electrons or protons present in an atom
Reason: Atomic number denoted by the letter Z
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: Both points are correct, but representing the atomic number with the letter Z does not define what an atomic number is.
Exam Tip: Atomic number always represents the proton count in the nucleus, which equals the electron count in a neutral atom.

Question. Assertion: Atomic number of C is 8
Reason: Number of electrons present in Carbon is '6'
Answer: (d) A is false, but R is true.
In simple words: Carbon has an atomic number of 6 (meaning it has 6 protons and 6 electrons), so the claim that its atomic number is 8 is false.
Exam Tip: Carbon has an atomic number of 6, whereas oxygen is the element with an atomic number of 8.

Question. Assertion : \( _7^{14}\text{N} \) , No. of proton is equal to 7
Reason: Number of neutron present in N- atom is 8
Answer: (c) A is true, but R is false.
In simple words: Nitrogen-14 has 7 protons, but its neutron count is \( 14 - 7 = 7 \), not 8.
Exam Tip: Subtract the atomic number from the mass number (\( 14 - 7 \)) to find the correct number of neutrons.

CASE BASED QUESTIONS

Question 1. Read the following and answer the questions from (i) to (v) given below
According to Dalton's atomic theory, atom was indivisible and indestructible. The discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of the aspect of Dalton's theory. To know the arrangement of electrons and protons within an atom, many scientists proposed various atomic models.
JJ Thomson was the first scientist to propose a model for the structure of an atom. Then Ernest Rutherford came who designed an experiment to know how the electrons are arranged within an atom. To overcome the objections raised against Rutherford's model, Niels Bohr put forward some postulates about the model of an atom.

(i) Who discovered proton?
(a) E. Goldstein
(b) J Thomson
(c) Christmas pudding
(d) Neils Bohr
Answer: (a) E. Goldstein
In simple words: Goldstein discovered positively charged anode rays, which led to the identification of protons.
Exam Tip: Always associate E. Goldstein with the discovery of the proton and J.J. Thomson with the electron.

(ii) Thomson's model of an atom was similar to ............
(a) Apple
(b) Guava
(c) Christmas pudding
(d) Strawberry
Answer: (c) Christmas pudding
In simple words: Thomson compared the atom to a Christmas pudding where electrons are embedded like dry fruits in a positive sphere.
Exam Tip: Thomson's model is also commonly referred to as the watermelon or plum pudding model.

(iii) Which metal foil was used by Rutherford in his experiment?
(a) Aluminium
(b) Gold
(c) Silver
(d) Zinc
Answer: (b) Gold
In simple words: Rutherford chose gold because it can be hammered into an extremely thin foil just a few atoms thick.
Exam Tip: Gold's high malleability is the reason it was chosen for this famous scattering experiment.

(iv) The electrons of second energy level will be present in which shell?
(a) K-shell
(b) L-shell
(c) M-shell
(d) N-shell
Answer: (b) L-shell
In simple words: The energy shells are labeled K for the first, L for the second, M for the third, and N for the fourth.
Exam Tip: Always remember the alphabetical sequence starting from K for the shells: K=1, L=2, M=3, N=4.

(v) JJ thomson taken a discharge tube in his experiment which is made up of?
(a) Rubber
(b) Metal foil
(c) Plastic
(d) Glass
Answer: (d) Glass
In simple words: The discharge tube used in the experiment was made of strong glass to withstand the vacuum inside.
Exam Tip: Glass discharge tubes are standard in gas discharge experiments because they are transparent and non-conducting.

Question 2. Read the following and answer the questions from (i) to (v) given below
Protons are present in the nucleus of an atom. It is equal to the number of protons in an atom, which determines its atomic number. It is denoted by 'Z'. All atoms of an element have the same atomic number, Z. Therefore, the atomic number is defined as the total number of protons present in the nucleus of an atom. The mass of an atom is practically due to the protons and neutrons alone. These are present in the nucleus of an atom. Hence, protons and neutrons are called nucleons. The mass number is equal to the sum of total number of protons and neutrons present in the nucleus of an atom.
Information about 6 elements is given below:

(i) What is the electronic configuration of oxygen (O)?
(a) 4,4
(b) 2,4,2
(c) 2,8,1
(d) 2,6
Answer: (d) 2,6
In simple words: Oxygen has 8 electrons, so 2 electrons go into the first shell (K) and the remaining 6 go into the outer shell (L).
Exam Tip: Always fill the K shell with 2 electrons before moving the rest to the L shell.

(ii) Which compound is liquid at room temperature?
(a) \( \text{LiO}_2 \)
(b) \( \text{CCl}_4 \)
(c) BeO
(d) \( \text{AlCl}_3 \)
Answer: (b) \( \text{CCl}_4 \)
In simple words: Carbon tetrachloride is a liquid at room temperature because it consists of covalent molecules with weak forces between them.
Exam Tip: Covalent compounds like \( \text{CCl}_4 \) often have lower melting/boiling points and exist as liquids, unlike ionic salts.

(iii) Which elements are good conductors of electricity?
(a) Li, Be, Al
(b) Be, Al, Cl
(c) C, O, Al
(d) O, Al, Cl
Answer: (a) Li, Be, Al
In simple words: Lithium, Beryllium, and Aluminium are metals, and metals have free electrons that allow them to conduct electricity well.
Exam Tip: Metallic elements are always excellent conductors of electricity due to their mobile valence electrons.

(iv) How many neutrons are there in beryllium?
(a) 0
(b) 2
(c) 5
(d) 4
Answer: (c) 5
In simple words: Beryllium has a mass of 9 and 4 protons, so the number of neutrons is \( 9 - 4 = 5 \).
Exam Tip: Subtract the atomic number from the atomic mass to get the exact neutron count.

(v) What are the charges present on proton and neutron?
(a) +1,0
(b) -1,+1
(c) 0,-1
(d) 0,0
Answer: (a) +1,0
In simple words: Protons have a single positive charge (+1), while neutrons carry no electrical charge at all (0).
Exam Tip: Remember that protons are positive, electrons are negative, and neutrons have zero charge.

Question 3. Read the following and answer the questions from (i) to (v) given below.
The maximum number of the electrons which are permitted to be assigned to an energy shell of an atom is called the electron capacity of that shell. The distribution of electrons in different orbits or shell is governed by a scheme known as Bohr-Bury scheme. According to this scheme:

• the maximum number of electrons that can be present in any shell is given by the formula \( 2n^2 \) where, n is the number of energy level.
• The maximum number of electrons that can be accommodated in the outermost shell is 8.

Electrons are filled in the shells in a stepwise manner in increasing order of energy of the energy shell.

(i) What is the maximum electron capacity of N- shell?
(a) 24
(b) 8
(c) 18
(d) 32
Answer: (d) 32
In simple words: The N shell represents the fourth shell (n = 4), so its capacity is \( 2 \times 4^2 = 2 \times 16 = 32 \) electrons.
Exam Tip: Use the \( 2n^2 \) rule where n = 4 for the N shell to find the maximum electron capacity.

(ii) Which of the following have same valency? A(2,8,2), B(2,8,4), C(2,6), D(2,8)
(a) A and B
(b) B and C
(c) A and C
(d) C and D
Answer: (c) A and C
In simple words: Element A has 2 valence electrons (valency 2), and Element C has 6 valence electrons, meaning it needs 2 more to complete its octet (valency 2).
Exam Tip: Remember that valency is determined by either the number of valence electrons (if 1-4) or 8 minus the number of valence electrons (if 5-8).

(iii) Arrange the K,L,M,N shell in increasing order of their energy.
(a) K>L>M>N
(b) K
(c) N
(d) L>M>K>N
Answer: (b) K
In simple words: Shells closer to the nucleus have lower energy, so the order is K (lowest), then L, M, and N (highest).
Exam Tip: Energy of shells increases sequentially with their distance from the nucleus.

(iv) Which element has the following configuration? K: 2, L: 8, M: 5
(a) Phosphorus
(b) Nitrogen
(c) Oxygen
(d) Carbon
Answer: (a) Phosphorus
In simple words: Adding the electrons \( 2 + 8 + 5 = 15 \). The element with atomic number 15 is phosphorus.
Exam Tip: Sum the electrons in the configuration to find the atomic number of the element.

(v) What is the total number of electrons in an atom if it's first four shells are completely filled?
(a) 60
(b) 32
(c) 26
(d) 48
Answer: (a) 60
In simple words: The sum of the maximum capacities of the K, L, M, and N shells is \( 2 + 8 + 18 + 32 = 60 \) electrons.
Exam Tip: Sum the maximum capacities of the first four shells: \( 2n^2 \) for n = 1, 2, 3, 4 to get 60.