Class 9 Science Chapter 4 Question Answers - Structure of the Atom

Q1: Why did Rutherford select a gold foil in his a-ray scattering experiment?
Ans: Rutherford selected a gold foil in his a-ray scattering experiment because gold has high malleability and can be hammered into thin sheet.

Q2: Name the radioisotope used for examining the circulation of blood in the body.
Ans: Na-24 is the radioisotope used for examining the circulation of blood in the body.

Q3: What is an orbit?
Ans: Orbit is the path of electron around the nucleus.

Q4: What is an anion?
Ans: When an atom gains one or more electrons, it becomes negatively charged and is known as anion.

Q5: What are nucleons?
Ans: Protons and neutrons reside in the nucleus of an atom and are thus called nucleons.

Q6: What are nucleons?
Ans: Protons and neutrons reside in the nucleus of an atom and are thus called nucleons.

Q7: Why noble gases show least reactivity?
Ans: Noble gases have their outermost octet completely filled up, so these gases show least reactivity.

Q8: Electron attributes negative charge, protons attribute positive charge. An atom has both but why there is no charge?
Ans: The negative and positive charges of electrons and protons respectively are equal in magnitude. So, the atom as a whole is electrically neutral.

Q9: What is the charge and mass of alpha particle?
Ans:
Charge = + 2 units Mass = 4 units

Q10: Helium atom has two electrons in its valence shell but its valency is not two. Why?
Ans: Helium atom has two electrons in its valence shell but its valency is not two as its outermost shell is completely filled, its combining capacity or valency is zero.

Q11: If an atom contains one electron and one proton, will it carry any charge or not?
Ans: It will not carry any charge because the positive charge on the proton neutralizes the negative charge on the electron.

Q12: Give two uses of isotopes in the field of medicines.
Ans:
(i) An isotope of cobalt is used in the treatment of cancer.
(ii) An isotope of iodine is used in the treatment of goitre.

Q13: What is meant by electronic configuration of elements?
Ans: The systematic distribution of electrons in various orbits of an atom is called electronic configuration of elements.

Q14: Why do some elements possess fractional atomic mass?
Ans: Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So, accordingly their average mass is calculated.

Q15: Are noble gases inert?
Ans: The outermost shell of the atoms of noble gases are completely filled. So, they do not show affinity towards chemical reactions.

Q16: How can an atom become stable by losing or gaining electrons?
Ans: For stability of an atom it must have either 2 or 8 electrons in the outermost orbit. So, by losing or gaining electrons it reaches to the electronic configuration of the nearest noble gas.

Q17: What is the relation between physical and chemical properties of isobars?
Ans: Isobars have different atomic numbers, so different electronic configuration and have different chemical properties. They are atoms of different elements, hence they have different physical properties.

Q18: What kind of elements have a tendency to gain electrons? What are they commonly called?
Ans: The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.

Q19: If Z = 3, what would be the valency of the element? Also, name the element.
Ans:  
Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
Valence shell has 1 electron in the outermost shell, so
valency of element is 1. The element is lithium.

Q20: How are the canal rays different from electrons in terms of charge and mass?
Ans: Electrons are negatively charged particles, mass of which is approximately 1/2000 that of canal rays.

Q21: What is the limitation of J.J. Thomson’s model of an atom?
Ans: The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.

Q22: What kind of elements have a tendency to gain electrons? What are they commonly called?
Ans: The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.

Q23: Do isotopes of an element have similar chemical properties?
Ans: Isotopes of an element have similar chemical properties because they have the same atomic number and valence electrons.

Q24: Why is the valency of Na is 1 and not 7?
Ans: It is easy for Na atom to lose one electron instead of gaining seven electrons. So, valency of Na is one and not seven.

Q25: How does an atom become a cation?
Ans: When an atom acquires positive charge by losing one or more electrons, it is called cation.