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: What is an orbit?
Ans: An orbit is the fixed path followed by an electron around the nucleus according to the Bohr model of the atom. It represents a definite energy level in which an electron can remain without radiating energy.
Q3: What are nucleons?
Ans: Protons and neutrons that are present in the nucleus of an atom are collectively called nucleons.
Q4: Why do noble gases show the least reactivity?
Ans: Noble gases have completely filled outer electron shells (a full duplet in helium and an octet in other noble gases). This stable electronic configuration makes them unlikely to gain, lose or share electrons, so they show very little chemical reactivity.
Q5: Electrons have a negative charge, protons have a positive charge. An atom has both, but why is there no charge?
Ans: An atom is electrically neutral because it contains equal numbers of protons (positive) and electrons (negative). The positive and negative charges cancel each other, giving no net charge.
Q6: What is the charge and mass of an alpha particle?
Ans: Charge = +2 units; Mass = 4 u. An alpha particle is the nucleus of a helium atom, containing two protons and two neutrons.
Q7: A Helium atom has two electrons in its valence shell, but its valency is not two. Why?
Ans: Helium’s outermost shell is completely filled with two electrons, which is a stable duplet. Because its valence shell is full, helium does not need to gain or lose electrons to become stable, so its valency is zero, not two.
Q8: If an atom contains one electron and one proton, will it carry any charge or not?
Ans: It will not carry any net charge because the single positive charge of the proton is exactly balanced by the single negative charge of the electron; the atom is electrically neutral.
Q9: Give two uses of isotopes in the field of medicine.
Ans:
(i) An isotope of cobalt (Co-60) is used in radiotherapy to treat certain types of cancer.
(ii) An isotope of iodine (I-131) is used in the diagnosis and treatment of thyroid disorders such as goitre.
Q10: What is meant by the electronic configuration of elements?
Ans: The electronic configuration of an element is the systematic arrangement of its electrons in different shells or orbits around the nucleus, showing how many electrons occupy each energy level.
Q11: 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.
Q12: Are noble gases inert?
Ans: Yes. Noble gases are largely inert because their outermost electron shells are full, so they have little tendency to participate in chemical reactions.
Q13: 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.
Q14: 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)
The valence shell has 1 electron, so the valency of the element is 1. The element is lithium.
Q15: How are the canal rays different from electrons in terms of charge and mass?
Ans: Canal rays consist of positively charged particles, whereas electrons are negatively charged. The mass of a typical canal-ray particle (for example a proton) is much larger than that of an electron – roughly 2000 times greater.
Q16: 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.
Q17: Do isotopes of an element have similar chemical properties?
Ans: Yes, Isotopes of an element have the same atomic number and the same arrangement of valence electrons, so they exhibit very similar chemical behaviour.
Q18: Why is the valency of Na is 1 and not 7?
Ans: A sodium atom can attain a stable electronic configuration more easily by losing one electron than by gaining seven. Losing one electron gives it the noble-gas configuration, so its valency is 1 and not 7.