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

Q1: Why did Rutherford select a gold foil in his alpha-ray scattering experiment?
Ans: Rutherford selected a gold foil in his alpha-ray scattering experiment because gold has high malleability and can be hammered into a 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: An orbit is the fixed path of an 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 an anion.

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

Q6: Why do noble gases show the least reactivity?
Ans: Noble gases exhibit minimal reactivity due to their completely filled valence electron shells, which makes them inherently stable and reluctant to participate in chemical reactions. This stable electron configuration, often referred to as an "octet" (or a "duplet" for helium), means they don't readily gain, lose, or share electrons with other atoms to achieve a more stable state. 

Q7: Electrons have a negative charge, protons have a positive charge. An atom has both, but why is there no charge?
Ans: The negative and positive charges of electrons and protons, respectively, are equal in number/ magnitude. So, the atom as a whole is electrically neutral.

Q8: What is the charge and mass of an alpha particle?
Ans: Charge = + 2 units, Mass = 4 u (atomic mass units)

Q9: A 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.

Q10: 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 neutralises the negative charge on the electron.

Q11: Give two uses of isotopes in the field of medicine.
Ans:
(i) An isotope of cobalt (Co-60) is used in the treatment of cancer.
(ii) An isotope of iodine (I-131) is used in the treatment of goitre.

Q12: What is meant by the electronic configuration of elements?
Ans: The systematic distribution of electrons in various orbits of an atom is called the electronic configuration of an element.

Q13: 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. For example, Chlorine has two isotopes Cl ₃₅ and Cl _37, and its average atomic mass is 35.5.

Q14: Are noble gases inert?
Ans: Yes, they are inert. The outermost shell of the atoms of noble gases is filled. So, they do not show affinity towards chemical reactions.

Q15: How can an atom become stable by losing or gaining electrons?
Ans: For the stability of an atom, it must have either 2 or 8 electrons in the outermost orbit. So, by losing or gaining electrons, it reaches the electronic configuration of the nearest noble gas.

Q16: What is the relation between the physical and chemical properties of isobars?
Ans: Isobars have different atomic numbers, so different electronic configurations and have different chemical properties. They are atoms of different elements, hence they have different physical properties.

Q17: 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.

Q18: 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 the element is 1. The element is lithium.

Q19: How are the canal rays different from electrons in terms of charge and mass?
Ans:  Canal rays and electrons differ significantly in both charge and mass. Canal rays are positively charged, while electrons are negatively charged. Additionally, the mass of canal ray particles is considerably larger than that of electrons, roughly 2000 times greater. 

Q20: What is the limitation of J.J. Thomson’s model of an atom?
Ans: J.J. Thomson's atomic model, often called the "plum pudding" model, had several limitations. It couldn't explain the stability of the atom, particularly how the positive charge held the negative electrons. It also failed to account for the nucleus of an atom and couldn't explain the results of Rutherford's alpha-particle scattering experiment, where most particles passed through a gold foil but some were deflected at different angles. 

Q21: 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.

Q22: Why is the valency of Na is 1 and not 7?
Ans: It is easy for a Na atom to lose one electron instead of gaining seven electrons. So, valency of Na is one and not seven.

Q23: How does an atom become a cation?
Ans: When an atom acquires a positive charge by losing one or more electrons, it is called a cation.