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NCERT Solutions for Class 9 Science Chapter 4
Class 9 Science – Chemistry – Structure of the Atom
Extra Questions for Exam preparation
- Question 1:
Why is proton so named?
- Answer 1:
Proton (H+) is formed when hydrogen atom also called protium, loses an electron. Therefore, it is so named.
- Question 2:
Is atomic number of an atom always equal to the number of electrons?
- Answer 2:
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.
- Question 3:
Were neutrons known at the time Rutherford performed the scattering experiment?
- Answer 3:
No, these were discovered later on by Chadwick in 1931 whereas scattering experiment was performed by Rutherford in 1911.
- Question 4:
Why are isotopes of an element chemically similar?
- Answer 4:
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.
- Question 5:
Nucleus of an atom has positive charge on it. Establish.
- Answer 5:
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.
- Question 6:
Why do elements which exist as isotopes have fractional atomic masses?
- Answer 6:
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.
- Question 7:
An electron is regarded as a universal particle. Explain.
- Answer 7:
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 8:
Why do the element helium, neon and argon have zero valency?
- Answer 8:
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.
- Question 9:
Why is a proton not a universal particle like electron?
- Answer 9:
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 10:
Why are isotopes of an chemically similar?
- Answer 10:
isotopes of an element have same number of electrons and therefore, same valence shell electronic distribution. Since the chemical properties of the atoms are related to valence shell configurations, the isotopes are chemically similar.