NCERT Solutions for Class 9 Science Chapter 4

NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom Exercise and Intext question answers in PDF form to free download. All the solutions are updated for new academic session 2020-2021 based on latest CBSE Syllabus 2020-21. Class 9 Science chapter 4 Intext questions given on Page 43 or Page 49 or Page 50 or Page 52 or Page 53 and Exercises in English Medium are available. You can download these solutions from पेज 53 के उत्तर or पेज 56 के उत्तर or पेज 57 के उत्तर or पेज 58 के उत्तर or पेज 59 के उत्तर or पेज 60 के उत्तर and अभ्यास के प्रश्न उत्तर to study online or in PDF form.

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NCERT Solutions for Class 9 Science Chapter 4 PDF

Class:9
Subject:Science – विज्ञान
Chapter 4:Structure of the Atom

9th Science Chapter 4 Answers in English & Hindi Medium

NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom Intext Questions and chapter end exercise question answers are given below to free download. All the solutions are free to download without any registrations or login. We have updated all the contents on the basis of user’s suggestions and Feedback received from last academic session.

Is atomic number of an atom always equal to the number of electrons?

No, it is the case when the atom has no charge. In case of cation (positively charged), atomic number is more than the number of electrons and in case of anion (negatively charged), it is less than the number of electrons.

Nucleus of an atom has positive charge on it. Establish.

This can be established on the basis of Rutherford scattering experiment. Since some alpha particles were repelled by the nucleus of the atom, it is expected to have the same charge as on alpha particles. Therefore, nucleus of an atom has positive charge.

Were neutrons known at the time Rutherford performed the scattering experiment?

No, these were discovered later on by Chadwick in 1931 whereas scattering experiment was performed by Rutherford in 1911.

Why are isotopes of an element chemically similar?

Isotopes of an elements have same number of electrons and therefore, same value shell electronic distribution. Since the chemical properties of the atoms are related to valence shell configurations, the isotopes are chemically similar.

Why do elements which exist as isotopes have fractional atomic masses?

The different isotopes of an element differ in their mass number as well as atomic masses. In order to represent the atomic mass of the element, we have to consider average of the atomic masses of the different isotopes and also the ratio in which these are present. In most of the cases, the average comes out to be in fraction. Therefore, these elements have fractional atomic masses.

Why is a proton not a universal particle like electron?

A proton is the positively charged residue left when hydrogen gas is enclosed in the in the discharge tube. For the other gases, the positive residues formed contain different number of protons. Therefore, proton is not a universal particle like electron.

Questions for Practice on 9th Science Chapter 4

Question 1:
An electron is regarded as a universal particle. Explain.
Answer 1:
The value of charge (e) and mass (m) of the electron always remain the same whatever may be the source of their emission. In the discharge tube, the electrons may be emitted either from the cathode or form the gas enclosed in the discharge tube. Whatever may be the metal which forms the cathode or the gas present in the discharge tube, these values remain the same. Therefore, electron is regarded as a universal particle.
Question 2:
Why do the element helium, neon and argon have zero valency?
Answer 2:
Helium has two electrons in its only energy shell (K-shell). The other two elements have eight electrons in their valence shells. Since these are the maximum number of electrons which the atoms of these elements can have therefore, they do not have any urge or desire to take part in chemical combination. These elements are known as zero valent elements. They have therefore, valency equal to zero.

Important Questions on 9th Science Chapter 4

What are the limitations of J.J. Thomson’s model of the atom?
According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the center of the atom called the nucleus, and the electrons revolve around the nucleus.
What are the limitations of Rutherford’s model of the atom?
According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the nucleus. In such a case, the atom would be highly unstable and collapse.
केनाल किरणें क्या हैं?
केनाल किरणें धनावेशित विकिरण हैं जो प्रोटॉनों से मिलकर बनी होती हैं। इसकी खोज 1886 में ई. गोल्डस्टीन ने की थी।
यदि किसी परमाणु में एक इलेक्ट्रान और एक प्रोटॉन है, तो इसमें कोई आवेश होगा या नहीं?
प्रोटॉन पर जितना धनात्मक आवेश होता है इलेक्ट्रॉन पर उतना ही ऋणात्मक आवेश होता है। अतः यदि किसी परमाणु में एक इलेक्ट्रान और एक प्रोटॉन है, तो इसमें कोई आवेश नहीं होगा। परमाणु विधुत उदासीन होगा।
Define valency by taking examples of silicon and oxygen.
The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element. If the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons. Thus, the valency of silicon is four. On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.
परमाणु उदासीन है, इस तथ्य को टॉमसन के मॉडल के आधार पर स्पष्ट कीजिए।
टॉमसन के मॉडल के आधार पर, परमाणु में ऋणात्मक और धनात्मक आवेश परिमाण में समान होते हैं। इसलिए परमाणु वैधुतीय रूप से उदासीन होते हैं।
रदरफ़ोर्ड के परमाणु मॉडल के अनुसार, परमाणु के नाभिक में कौन सा अवपरमाणुक कण विद्यमान है?
रदरफ़ोर्ड के परमाणु मॉडल के अनुसार, परमाणु के नाभिक में प्रोटॉन (धनात्मक आवेशित कण) विद्यमान है।
क्या अल्फ़ा कणों का प्रकीर्णन प्रयोग सोने के अतिरिक्त दूसरी धातु की पन्नी से संभव होगा?
अल्फ़ा कणों का प्रकीर्णन सोने की पतली पन्नी के अलावा किसी अन्य धातु की पन्नी से संभव नहीं है। क्योंकि किसी अन्य धातु की 1000 परमाणु मोटी पन्नी बनाना संभव नहीं है।
Explain Atomic number with examples.
Atomic number: The atomic number of an element is the total number of protons present in the atom of that element. For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.
What do you understand by isobars?
Isobars: They are atoms of different elements having same mass number but different atomic number.
For example calcium, atomic number 20 and argon, atomic number 18.
The number of electrons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of neutrons is the same in the atoms of this pair of elements.
अगर किसी परमाणु का K और L कोश भरा है, तो उस परमाणु में इलेक्ट्रॉनों की संख्या क्या होगी?
K कोश में इलेक्ट्रॉनों की कुल संख्या = 2
L कोश में इलेक्ट्रॉनों की कुल संख्या = 8
इसलिए, कुल इलेक्ट्रॉन = 2 + 8 = 10
अतः, उस परमाणु में इलेक्ट्रॉनों की संख्या 10 होगी।
यदि किसी परमाणु में इलेक्ट्रॉनों की संख्या 8 है और प्रोटॉनों की संख्या भी 8 है तब, (a) परमाणु की परमाणु संख्या क्या है? (b) परमाणु का क्या आवेश है?
(a) हम जानते हैं कि, परमाणु संख्या = प्रोटॉनों की संख्या
अतः, परमाणु की परमाणु संख्या = 8

(b) परमाणु का आवेश = प्रोटॉनों की संख्या – इलेक्ट्रॉनों की संख्या = 8 – 8 = 0
अतः, परमाणु पर कोई आवेश नहीं है और परमाणु उदासीन है।

If Z = 3, what would be the valency of the element? Also, name the element.
By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron). Therefore, the element with Z = 3 is lithium.

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