Structure of the Atom Class 9 Worksheet Science Chapter 4

Multiple Choice Questions

Q1: The nucleus of an atom consists of ______.
(a) Protons
(b) Electrons and neutrons
(c) Protons and neutrons
(d) Neutrons 
Ans: (c)
Sol: The nucleus of an atom consists of neutrons and protons.

Q2: What is the maximum number of electrons which can be present in K and L shells in an atom?
(a) 2 and 8
(b) 2 and 18
(c) 2 and 32
(d) 8 and 18
Ans: (a)
Sol: K and L shells of an atom can have maximum 2 and 8 electrons.

Q3: Rutherford's alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus
(b) Electron
(c) Proton
(d) Neutron
Ans: (a)

Q4: Isotopes of element have:
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers
Ans: (c)
Sol: Isotopes have same atomic number hence have similar chemical properties. They have different number of neutrons hence their mass number is different.

Q5: Number of valence electrons in Cl^– ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Ans:  (b)
Sol: Cl^– Ion has 18 electrons (17+1). Hence the number of valence electrons in Cl^– ion is 8. Electronic distribution:
Cl – 2,8,7
Cl^– – 2,8,8

Q6: An element has electronic configuration 2,8,4 it will be classified as
(a) Metal
(b) Non metal
(c) Metalloid
(d) None of these
Ans: (c)

Q7: Amongst element X (2,8,6) and (2,8,8) which is more reactive and why ?
(a) X because it is a metal
(b) Y because it is non metal
(c) X because it has 6 valence electrons
(d) Y because it is gas
Ans: (c)

Q8: The nucleus of the hydrogen atom is called as
(a) Neutron
(b) Electron
(c) Proton
(d) Nucleons
Ans: (c)

Q9: Cathode raysget deflected in a electric field towards
(a) Positive plate
(b) Negative Plate
(c) No deflection takes place
(d) First towards negative plate and then towards positive plate
Ans: (a)

Q10: The atomic number of an element ‘y’ is 20. The electronic configuration of the ion having inert gas configuration is
(a) 2,8,10
(b) 2,18
(c) 2,10,8
(d) 2,8,8
Ans: (d)

Fill in the Blank

Q1: According to Maharishi Kanad, the tiniest to tiny particle of a pure substance is called _________.
Ans: According to Maharishi Kanad, the tiniest to tiny particle of a pure substance is called Anu.

Q2: An atom is the smallest unit of an element which takes part in a _________.
Ans: An atom is the smallest unit of an element which takes part in a Chemical reaction.

Q3: Mass of an electron is 1/1837 times less than the mass of one atom of__________.
Ans: Mass of an electron is 1/1837 times less than the mass of one atom of Hydrogen.

Q4: The K-shell of any atom cannot have more than _________ electrons.
Ans: The K-shell of any atom cannot have more than Two electrons.

Q5: Isotopes are the atoms of ___________ element, having same atomic number but different mass number.
Ans: Isotopes are the atoms of Same element, having same atomic number but different mass number.

True or False

Q1: Radioactive isotope of iodine is used for making the medicine called tincture iodne.
Ans: False
Q2: J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
Ans: False
Q3: The K-shell of any atom cannot have more than _________ electrons.
Ans: True
Q4: The innermost atomic shell can hold a maximum of 18 electons.
Ans: False
Q5: Nuclear chemistry has nothing to do with the electrons present in an atom.
Ans: True

Very Short Answer Questions

Q1: Out of proton and neutron, which is heavier?
Ans: Neutron is slightly heavier (1.675 × 10^–27 kg) than proton (1.67 × 10^–27 kg).

Q2: Were neutrons known at the time Rutherford performed the scattering experiment?
Ans: No, neutrons were discovered later on by Chadwick in 1931 whereas scattering experiment was performed by Rutherford in 1911.

Q3: Why is proton so named?
Ans: Proton (H⁺) is formed when hydrogen atom also called protium, loses an electron.

Q4: What is the number of electrons in the valence shell of chlorine (Z = 17)?
Ans: The electronic distribution of Cl is 2,8,7 Valence shell of chlorine has 7 electrons.

Q5. What is the basic difference between the isotopes of an element?
Ans: Isotopes of an element differ in the number of neutrons leading to different mass numbers.

Short Answer Questions

Q2: Describe Bohr’s model of the atom.
Ans: To remove the drawbacks of Rutherford’s atomic model and to explain structure of atom in detail Neils Bohr in 1912 proposed a model of atom. The special features of Bohr’s model are given below:
(1) An electron revolves in the orbit of atom with well-defined energy.
(2) Energy of orbits increases from inner shell to the outer shells i.e. energy for orbit nearest the nucleus is lowest.
(3) If energy is supplied then electron moves from lower orbit to the higher orbit and if an electron jumps from higher orbit (energy level) to the lower orbit (energy level) then energy is radiated as electromagnetic waves.
(4) Each orbit or shell represents an energy level. Such orbits are represented as K,L,M,N,O……….. and named from centre to outwards.
(5) The shell or orbits are associated with certain amount of energy and energy of orbits/shells increases from inward to outwards.eg K<L<M<N<O…………

Q3: What are the limitations of Rutherford’s model of the atom?
Ans: Rutherford’s model of atom didn’t explain the stability of atom because as per his model revolving charged electrons while moving through orbit should emit energy and this energy loss will shrink the orbit and ultimately the electron would hit the nucleus and thus atom is unstable but it is not true.

Q4: Write the postulates of Bohr theory?
Ans: The postulates of Bohr’s theory are:
(1) Electron move around the nucleus in definite circular path called orbits.
(2) Each orbit is associated with a fixed amount of energy.
(3) The larger the radius of the orbit, the greater is the energy of the electrons in them.
(4) Electrons can move from one orbit to another by gaining or losing a fixed amount of energy.

Q5: Why metals are electropositive and non-metals are electronegative in nature?
Ans: Metals are electropositive in nature because all metals loose electrons from their outermost shell in order to become stable and hence become positively charged. Non-metals are electronegative in nature because all non-metals gain electrons in order to become stable and hence become negatively charged.