Journey Inside the Atom Class 9 Science

20 Questions | 51 Marks

An atom of an element has 11 protons and 12 neutrons. Determine the atomic number, mass number, and number of electrons in this atom. Write its electronic configuration. 3marks

Rutherford’s gold foil experiment led to the discovery of the atomic nucleus. Explain how the observations from this experiment supported this conclusion. 3marks

Describe J. J. Thomson's model of the atom. Why was this model unable to explain the results of the gold foil experiment? 3marks

Compare and contrast the isotopes of hydrogen (Protium, Deuterium, Tritium) in terms of their subatomic particles and properties. 3marks

State the postulates of Bohr's atomic model and explain how it addresses the stability of an atom. 3marks

What is valency? Explain how the valency of an element is determined by its electronic configuration, using examples of Sodium (Na) and Oxygen (O). 3marks

Assertion (A): Atoms are electrically neutral.Reason (R): The number of protons is equal to the number of electrons in an atom.Choose the correct option:(i) Both A and R are true, and R is the correct explanation of A.(ii) Both A and R are true, but R is not the correct explanation of A.(iii) A is true, but R is false.(iv) A is false, but R is true. 1mark

Elements X and Y have different atomic numbers but the same mass number. What is the relationship between X and Y? Give an example. 2marks

Explain the concept of atomic number and mass number, and how they are represented in the standard notation of an atom. 2marks

Why is it that atoms with 8 electrons in their outermost shell (except Helium) are generally unreactive? 2marks

Complete the following table for the given elements:

Element	Atomic Number	Mass Number	Number of Protons	Number of Neutrons
Aluminium	13	27
Chlorine			17	18
Neon	10
[3 marks]
12. A scientist proposed a model where electrons orbit the nucleus. Explain why this model, if it suggested electrons could orbit at any distance, would fail to explain the stability of atoms. [2 marks]
13. What is a neutron? Who discovered it and what is its significance in determining the mass of an atom? [2 marks]
14. An atom has 18 neutrons and its atomic number is 17.
(i) What is the mass number of this atom?
(ii) Write the electronic configuration of this atom.
(iii) Identify the element. [3 marks]
15. Why did Dalton's atomic theory, though revolutionary, need to be revised over time? [1 mark]
16. Calculate the average atomic mass of an element A, which has two isotopes: A1 with mass 60 u and relative abundance 65%, and A2 with mass 62 u and relative abundance 35%. [2 marks]
17. How do the physical properties of isotopes differ from their chemical properties? Explain with an example. [2 marks]
18. The symbols of some elements are derived from their Latin names. Give an example of an element whose symbol is derived from its Latin name, and state its Latin name. [1 mark]
19. The electronic configuration of an element is 2, 8, 5.
(i) What is the atomic number of this element?
(ii) What is its valency?
(iii) Is this element likely to lose or gain electrons to achieve a stable configuration? [3 marks]
20. Case Study: A student is studying different atomic models. They are shown a diagram where positive charge is spread uniformly within a sphere, and electrons are embedded in it. They are also shown a diagram where a small, dense, positively charged nucleus is at the centre, with electrons revolving around it.
(a) Identify the atomic models represented by these two descriptions. [1 mark]
(b) Explain the experimental evidence that led to the second model and disproved the first. [2 marks]
(c) What is the major limitation of the second model? [1 mark]
(d) The third model introduced the concept of electrons moving in fixed energy levels. What is the significance of these energy levels? [1 mark]

Answers

Atomic number = Number of protons = 11. Mass number = Number of protons + Number of neutrons = 11 + 12 = 23. Number of electrons = Number of protons (since the atom is neutral) = 11. Electronic configuration: K=2, L=8, M=1.

In Rutherford’s gold foil experiment, a beam of alpha particles was directed at a thin gold foil.

Most alpha particles passed straight through the foil, indicating that atoms are mostly empty space.
A few alpha particles were deflected by small angles, suggesting the presence of a positively charged region.
A very small number of alpha particles were deflected by large angles, and some even bounced back, indicating that the positive charge and most of the mass of an atom are concentrated in a very small, dense region at the center, which was named the nucleus.
3. Thomson’s model, also known as the plum pudding model, proposed that an atom is a sphere of positive charge in which electrons are embedded, like plums in a pudding. This model failed to explain the large-angle deflections of alpha particles observed in the gold foil experiment because the positive charge was thought to be spread out uniformly, which would not cause such significant deflections.
4. Protium: 1 proton, 0 neutrons, 1 electron. Deuterium: 1 proton, 1 neutron, 1 electron. Tritium: 1 proton, 2 neutrons, 1 electron. Chemical properties are similar because they have the same number of electrons and electronic configuration. Physical properties like mass, density, boiling point, and melting point differ due to the difference in the number of neutrons.
5. Bohr’s model postulates:
Electrons revolve around the nucleus in specific, fixed circular paths called orbits or shells, each with a definite energy.
While revolving in these fixed orbits, electrons do not radiate or lose energy. These are called stationary states.
Electrons can jump from one orbit to another only by absorbing or emitting specific amounts of energy equal to the difference in energy of the two orbits.
This model explains atomic stability because electrons in their stationary states do not lose energy and thus do not spiral into the nucleus.
6. Valency is the combining capacity of an atom, which is determined by the number of electrons it loses, gains, or shares to achieve a stable electronic configuration (octet or duplet).
Sodium (Na): Electronic configuration is 2, 8, 1. It has 1 valence electron. To achieve an octet, it tends to lose 1 electron. So, its valency is 1.
Oxygen (O): Electronic configuration is 2, 6. It has 6 valence electrons. To achieve an octet, it tends to gain 2 electrons. So, its valency is 2.
7. (i) Both A and R are true, and R is the correct explanation of A.
8. Elements X and Y are isobars. They have the same mass number but different atomic numbers. Example: Calcium (Ca, atomic number 20, mass number 40) and Argon (Ar, atomic number 18, mass number 40).
9. Atomic number (Z) is the total number of protons in the nucleus of an atom. Mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus of an atom. In standard notation, an atom is represented as:

\frac{A}{Z} Symbol

Atoms with 8 electrons in their outermost shell have a stable electronic configuration, known as an octet. This configuration is energetically favourable, making the atom unreactive as it has no tendency to gain, lose, or share electrons. 11.

Element	Atomic Number	Mass Number	Number of Protons	Number of Neutrons	Number of Electrons	Electronic Configuration
Aluminium	13	27	13	14	13	2, 8, 3
Chlorine	17	35	17	18	17	2, 8, 7
Neon	10	20	10	10	10	2, 8
12. If electrons could orbit at any distance, they would constantly be accelerating (changing direction). According to classical physics, an accelerating charged particle should radiate energy and lose it. This would cause the electron to spiral inwards and eventually crash into the nucleus, making the atom unstable. Bohr's model resolved this by postulating that electrons exist in specific energy levels and do not radiate energy while in these orbits.
13. A neutron is a subatomic particle with no electric charge (neutral) and a mass approximately equal to that of a proton. It was discovered by James Chadwick. Neutrons, along with protons, constitute the nucleus of an atom and significantly contribute to its mass.
14. (i) Atomic number = 17. Number of protons = 17. Number of neutrons = 18. Mass number = Number of protons + Number of neutrons = 17 + 18 = 35.
(ii) Electronic configuration: K=2, L=8, M=7.
(iii) The element with atomic number 17 is Chlorine (Cl).
15. Dalton's atomic theory proposed that atoms are indivisible and that all atoms of a given element are identical. However, later discoveries revealed that atoms are composed of subatomic particles (electrons, protons, neutrons) and that isotopes (atoms of the same element with different masses) exist. These discoveries showed that atoms are divisible and not all atoms of an element are identical in mass.
16. Relative abundance of A1 = 65%, Mass of A1 = 60 u. Relative abundance of A2 = 35%, Mass of A2 = 62 u.
Average atomic mass = (Mass of A1 × % abundance of A1) + (Mass of A2 × % abundance of A2)
Average atomic mass = $(60 \times \frac{65}{100}) + (62 \times \frac{35}{100})$
Average atomic mass = $(39) + (21.7)$
Average atomic mass = 60.7 u
17. The chemical properties of isotopes are similar because they have the same number of electrons and the same electronic configuration, which dictates chemical behaviour. However, their physical properties like mass, density, boiling point, and melting point differ because they have different numbers of neutrons, leading to different masses. For example, deuterium ( $^{2}$ H) reacts chemically with oxygen to form heavy water ( $D_{2} O$ ) much slower than protium ( $^{1}$ H) forms ordinary water ( $H_{2} O$ ).
18. Element: Iron. Symbol: Fe. Latin name: Ferrum.
19. (i) Atomic number = 2 + 8 + 5 = 15.
(ii) Valency: The atom has 5 valence electrons. To achieve an octet, it needs to gain 3 electrons. So, its valency is 3.
(iii) This element is likely to gain electrons to achieve a stable configuration.
20.
(a) The first description represents Thomson's atomic model (plum pudding model). The second description represents Rutherford's atomic model (nuclear model).
(b) The experimental evidence that led to Rutherford's model was the gold foil experiment. Most alpha particles passed through the foil undeflected, indicating that atoms are mostly empty space. A few were deflected at large angles, and some even bounced back. This indicated that the positive charge and most of the mass of an atom are concentrated in a very small region at the center (the nucleus), disproving Thomson's model where positive charge was spread out.
(c) The major limitation of Rutherford's model was its inability to explain the stability of atoms. According to classical physics, an orbiting electron should continuously lose energy and spiral into the nucleus.
(d) The significance of energy levels in Bohr's model is that electrons can only exist in these discrete energy shells and do not lose energy while revolving in them. This explained why atoms are stable.

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