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

Q1: On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Ans: As per Thomson’s model of an atom, the number of electrons and the number of protons are equal in an atom. Electrons are positively charged and protons are negatively charged, hence the + and – charges are neutralized by each other that makes atoms neutral as a whole.

Q2: On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans: The subatomic particle proton is present in the nucleus of an atom according to Rutherford’s model of an atom.

Q3: Draw a sketch of Bohr’s model of an atom with three shells.
Ans: Bohr’s model of an atom with three shells is as follows:

Q4: What do you think would be the observation if the α− particle scattering experiment is carried out using a foil of a metal other than gold?
Ans: If the α− particle scattering experiment is carried out using a foil of a metal other than gold we will get a different observation.

Q5:. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Ans: The atomic mass of an atom is the sum of masses of protons and neutrons present in its nucleus.
Given that the mass of the helium atom is 4 u and two protons present in its nucleus.
So the number of neutrons will be
Number of neutrons = atomic mass − number of protons
⇒ Number of neutrons = 4−2
∴ Number of neutrons = 2
Therefore, the helium atom has 2 neutrons.

Q6:  Write the distribution of electrons in carbon and sodium atoms.
Ans: Atomic number of carbon is 6 and the atomic number of sodium is 11.
So distribution of electrons in carbon atom is 6 = 2,4
Distribution of electrons in the sodium atom is 11 = 2,8,1.

Q7: If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Ans: K shell contains total 2 electrons and L shell contains maximum 8 electrons. If K and L shells of an atom are full, then the total number of electrons in the atom will be 10.

Q8: If number of electrons in an atom is 8 and number of protons is also 8, then
(a) What is the atomic number of the atom?
Ans: The atomic number of an atom is equal to the number of protons or electrons present in its nucleus. So the atomic number of an atom with 8 electrons and 8 protons is 8.
(b) What is the charge on the atom?
Ans: A single electron contains one negative charge and one single proton contains one positive charge. There are equal numbers of electrons and protons in an atom so they neutralize each other. The atom will be neutral.

Q9: With the help of Table 4.1, find out the mass number of oxygen and Sulphur atoms.
Ans: The mass number of an atom is equal to the sum of protons and neutrons present in its nucleus. Mass number of oxygen = 8 + 8
Mass number of oxygen = 16
Mass number of Sulphur = 16 + 16
Mass number of Sulphur = 32

Q10: What are the limitations of J.J. Thomson’s model of the atom?
Ans: The J.J. Thomson’s atomic model failed to explain the organization of electrons in an atom.

Q11: Na⁺ has completely filled K and L shells. Explain.
Ans: Sodium (Na) has atomic number 11, so the electronic configuration of Na is 2,8,1.
It has a single electron in the outermost shell, when it gives away that electron it becomes Na+ and has electronic configuration 2,8. Also the K shell contains a total 2 electrons and the L shell contains a maximum of 8 electrons. So Na+ has completely filled K and L shells.

Q12: If z = 3, what would be the valency of the element? Also, name the element.
Ans: z = 3 represents that element has 3 electrons in its shells. The electronic configuration is 2,1. It means the outermost shell electron has 1 electron, so its valency is 1. The element is Lithium.
Composition of the nuclei of two atomic species X and Y are given as under

Q13: Give the mass number of X and Y. What is the relation between the two species?
Ans: The mass number of an atom is equal to the sum of protons and neutrons present in its nucleus.
So the mass number of X is =6+6=12
Mass number of Y is =6+8=14
Number of protons is the same in X and Y but the atomic numbers are different. X and Y are isotopes.
For the following statements, write T for True and F for False.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
Ans: False
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
Ans: True
(c) The mass of an electron is about 12000 times that of proton.
Ans: True
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Ans: False

Q14: The nucleus of an atom of Bi – 210 (atomic number = 83) emits β− particle and becomes a polonium nuclide. Write as equation for the nuclear change described.
Ans: Whenever a β−particle emits from the nucleus of an atom, the atomic number of that atom is increased by 1 but the mass number remains the same.
So when a β−particle emits from Bi – 210 (atomic number = 83) it becomes polonium nuclide. The equation for the nuclear change is as follows:


Q15: How can one conclude that electrons are fundamental particles?
Ans: The e/m  ratio of electrons remains similar irrespective of the nature of gas and electrodes inside the discharge tube. So we can conclude that electrons are fundamental particles.

Q16: What happens to the nucleus of an atom when it emits a γ− ray?
Ans: There is no change in mass or charge of nuclide when it emits γ− ray. The energy of the nucleus decreases equal to the energy of photons emitted.

Q17: Write the electronic configuration of following ions:
(a) Cl⁻
Ans: Electronic configuration of Cl⁻ ion is 2,8,8.
(b) Mg
Ans: Electronic configuration of Mg ion is 2,8,2.
(c) Al³⁺
Ans: Electronic configuration of Al³⁺ ion is 2,8.
(d) O
Ans: Electronic configuration of O is 2,6.

Q18: State Mendeleev’s periodic law?
Ans: Mendeleev’s Periodic law states that the properties of elements are periodic functions of their atomic mass. Properties of elements depend on the atomic mass.

Q19: Define ionization energy and electron affinity?
Ans: Ionization energy of a component is that the amount of energy that has got to be supplied to at least one mole of the element within the gaseous state to get one mole of caters within the gaseous state.
Electron affinity point is that the energy change that accompanies the formation of 1 mole of anions within the gaseous state from one mole of the atoms of the element within the gaseous state.

Q20: Why is atomic number is more important than atomic weight in predicting the chemical properties of elements?
Ans: Atomic number is that the number of protons in an atom and through a reaction the number of protons remains unchanged. Atomic number also gives number of electrons. Electrons are present in shells which participate in chemical reactions and decide chemical properties. Whereas relative atomic mass is the sum of the number of protons and number of neutrons so the number is more important in predicting the chemical properties of elements.

Q21: What are the advantages of the Periodic Table?
Ans: In periodic table elements are arranged in a tabular form. So it is easy to remember the properties of elements if the position is known. Also the compounds formed by the elements are predictable if the position of the element is known. Periodic table made it easy and systematic to study chemistry.

Q22: Which of the following electronic configuration are wrong and why?
(a) 2,8,2
(b) 2,8,8,2
(c) 2,8,9,1.
Ans: From the given electronic configuration, 2,8,9,1 is wrong because in the third shell the maximum number of electrons is 8. The correct electronic configuration is 2,8,8,2.

Q23: What are ions? What are its two types?
Ans: When one or more electrons are detached from a neutral atom, a positively charged particle is formed and called an ion. Ions may be cations and anions.

Q24: Show diagrammatically the formation of O²⁻ ion?
Ans: Atomic number of oxygen is 8 and its electronic configuration is 2,6. In the outermost shell oxygen has 6 electrons. To complete its octet and become stable it needs 2 electrons. By gaining 2 electrons it becomes O²⁻ ion.
Diagrammatic representation of formation of O²⁻ ion is as follows:

Q25: Define isotopes and isobars?
Ans: Isotopes are atoms which have identical atomic numbers but different mass numbers. Examples of isotopes are  
Isobars are atoms that have different atomic numbers but the same mass number. Examples of isobars are

Q26: For the symbol H, D and T tabulate three subatomic particles found in each of them.
Ans: H represents the hydrogen atom, D represents the deuterium atom and T represents the tritium atom. Three subatomic particles present in each of them is represented as follows:

Q27: Write the electronic configuration of any one pair of isotopes and isobars.
Ans: Electronic configuration of pairs of isotopes of carbon is  .  Isotopes have the same number of electrons and protons.
Electronic configuration of a pair of isobars of argon and calcium is

Q28: Compare the properties of electrons, protons and neutrons.
Ans: Comparison of electrons, protons and neutrons is as follows:

Q29: What are the limitations of Rutherford’s model of the atom?
Ans: Rutherford’s model of atom is failed to explain the stability of atom because as per his model electrons revolve around the nucleus and while moving through orbit should emit energy and this energy loss will shrink the orbit and finally the electron would hit the nucleus and thus atom is unstable but it is not true.

Q30: Define valency by taking examples of silicon and oxygen.
Ans: The valency of electrons is determined by electrons present in the outermost shell of an atom. Electrons present in the outermost shell of an atom are known as the valence electrons. Electrons gain or lose electrons to complete its octet. The valency of silicon is 14 and electronic configuration is 2,8,4. So silicon can gain or lose 4 electrons. So the valency of silicon is +4 or -4.
Atomic number of oxygen is 8 and the electronic configuration is 2,6. To complete its octet oxygen gains 2 electrons hence the valency of oxygen is 2.

Q31: If bromine atom is available in the form of, say, two isotopes  Calculate the average atomic mass of bromine atom.
Ans: The average atomic mass of bromine is

Average atomic mass of bromine atoms is 80 u.

Q32: The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes  and in the sample?
Ans: Average atomic mass of sample is given as

We get

The percentage of isotopes is

Q33: In a gold – foil experiment:
(a) Why did many α− particles pass through the gold foil undeflected?
Ans: Most of the space within the atom was empty so many α− particles passed through the gold foil undeflected.
(b) Why did few α− particles deflect through small angles.
Ans: In a gold foil at center there is a positive charge so few α− particles deflect through small angles.
(c) Why did few α− particles, after striking the gold foil, retrace their path.
Ans: In a gold foil there is a positively charged nucleus which is very tiny so few α− particles, after striking the gold foil, retrace their path.
(d) Compare the three major particles in atoms with respect to their mass and charge?
Ans: Comparison of three major particles proton, neutron and electron with respect to their mass and charge is as follows:

Inside an atom electron revolves around the nucleus in a circular path. Protons and neutrons are present inside the nucleus.

Q34: Write an experiment to show cathode rays are deflected by magnetic fields?
Ans: Experiment to show that cathode rays were deflected by magnetic fields is as follows:

• First take a discharge tube with fluorescent material inside it.
• Place a horse – shoe magnet in the center of the discharge tube.
• When cathode rays are produced and travel through the discharge tube, then cathode rays get deflected by the magnetic field of the magnets in the direction of anode. Also they are negatively charged.

The diagram of experiment is as follows:

Q35: Write the postulates of Bohr theory?
Ans: The postulate of Bohr’s theory is as follows:

• An electron revolves around the nucleus in the orbit of an atom in a definite path known as orbits or shells.
• Energy of each orbit is fixed.
• Energy increases from inner shell to the outer shells i.e. energy for orbit nearest the nucleus is lowest.
• If energy is supplied then the electron moves from lower orbit to higher orbit.

Q36:  Explain the variation of atomic radius along a period and down a group.
Ans: The atomic radius of an atom increases when we move down as an extra shell is added. The atomic radius decreases when we move from left to right as the nuclear charge of an element increases.

Q37: Why metals are electropositive and non-metals are electronegative in nature?
Ans: Metals are electropositive in nature because all metals give away electrons from their outermost shell in order to complete the octet and become stable. So metals become positively charged. Non-metals are electronegative in nature because all non-metals gain electrons in order to complete the octet and become stable. So non-metals become negatively charged.

Q38: Explain the formation of Al³⁺ ion and why is it formed?
Ans: Aluminum has an atomic number of 13. The electronic configuration of Al is 2,8,3. It has 3 electrons in the outermost shell and to become stable it needs to complete its octet. In the outermost shell, the maximum number of electrons must be 8. So it is easy to lose 3 electrons and complete the octet. By giving away the 3 outermost electrons it becomes  Al³⁺ ion.

Q39: Find the percentage composition of sucrose (C₁₂H₂₂O₁₁).
Ans: The molecular mass of sucrose (C₁₂H₂₂O₁₁) is
(C₁₂H₂₂O₁₁) = 12 × 12 + 22 + 11 × 16
(C₁₂H₂₂O₁₁) = 144+22+176
(C₁₂H₂₂O₁₁) = 342g/mol
342 g of sucrose contains 144g Carbon, 22 g Hydrogen and 176 g Oxygen.
So 100 g sucrose contains:

So the percentage composition of sucrose is
C = 42.11
H = 6.43
O = 51.46.