Class 9 Atomic Structure and Chemical Bonding Mock Test (ICSE)

Topics in Atomic Structure and Chemical Bonding

• Dalton's theory and its limitations
• Thomson, Rutherford and Bohr models
• Electronic configuration
• Isotopes and isobars
• Electrovalent bonding
• Covalent bonding

Atomic Structure and Chemical Bonding practice questions with answers

1. Q1Multiple choice·1 mark
   Which of the following compounds is formed by the complete transfer of electrons and has a high melting point?

   1. (a)$NaCl$
   2. (b)$H_2O$
   3. (c)$CCl_4$
   4. (d)$NH_3$
   Show answer and step marks

   1. Correct option: (a) $NaCl$+1

   Answer: (a)
2. Q2Multiple choice·1 mark
   Which of the following postulates of Dalton's atomic theory was contradicted by the discovery of isotopes?

   1. (a)Atoms can neither be created nor destroyed.
   2. (b)All atoms of a given element are identical in mass and properties.
   3. (c)Compounds are formed by combination of atoms in a fixed ratio.
   4. (d)Atoms are indivisible.
   Show answer and step marks

   1. Correct option: (b) All atoms of a given element are identical in mass and properties.+1

   Answer: (b)
3. Q3Multiple choice·1 mark
   Rutherford's $\alpha$-particle scattering experiment led directly to the discovery of the:

   1. (a)Atomic nucleus
   2. (b)Electron
   3. (c)Proton
   4. (d)Neutron
   Show answer and step marks

   1. Correct option: (a) Atomic nucleus+1

   Answer: (a)
4. Q4Multiple choice·1 mark
   The number of covalent bonds present in a molecule of nitrogen ($\text{N}_2$) gas is:

   1. (a)Two
   2. (b)Four
   3. (c)One
   4. (d)Three
   Show answer and step marks

   1. Correct option: (d) Three+1

   Answer: (d)
5. Q5Long answer·4 marks
   (a) Define the term isotopes. Name two common isotopes of hydrogen. (b) State why isotopes of an element have identical chemical properties. (c) Define isobars. Give one pair of isobars and state why they possess different chemical properties.
   Show answer and step marks

   1. (a) Isotopes are atoms of the same element having the same atomic number but different mass numbers.+0.5
   2. (a) First isotope of hydrogen named: protium ($_1^1\text{H}$), deuterium ($_1^2\text{H}$) or tritium ($_1^3\text{H}$).+0.5
   3. (a) Second isotope of hydrogen named, different from the first: protium, deuterium or tritium.+0.5
   4. (b) Isotopes have the same atomic number, hence the same number of electrons and the same electronic configuration (valence electrons), which decide chemical properties.+1
   5. (c) Isobars are atoms of different elements having different atomic numbers but the same mass number.+0.5
   6. (c) Pair of isobars, any one of: $_{18}^{40}\text{Ar}$ and $_{20}^{40}\text{Ca}$; $_{19}^{40}\text{K}$ and $_{20}^{40}\text{Ca}$; $_{6}^{14}\text{C}$ and $_{7}^{14}\text{N}$; or any other valid pair of isobars.+0.5
   7. (c) Isobars have different atomic numbers, hence different numbers of electrons and different electronic configurations (valence electrons), so their chemical properties differ.+0.5

   Answer: (a) Isotopes are atoms of the same element with the same atomic number but different mass numbers; any two of protium ($_1^1\text{H}$), deuterium ($_1^2\text{H}$), tritium ($_1^3\text{H}$). (b) They have the same number of electrons and the same electronic configuration, which decides chemical properties. (c) Isobars are atoms of different elements with different atomic numbers but the same mass number, e.g. $_{18}^{40}\text{Ar}$ and $_{20}^{40}\text{Ca}$; their different atomic numbers give different electronic configurations, so their chemical properties differ.
6. Q6Long answer·4 marks
   An element exists in two isotopic forms with mass numbers $35$ and $37$. Both have an atomic number of $17$. (a) Define isotopes and explain why these two atoms have identical chemical properties. (b) Calculate the number of protons, electrons, and neutrons in the isotope with mass number $37$.
   Show answer and step marks

   1. (a) Isotopes are atoms of the same element having the same atomic number but different mass numbers.+1
   2. (a) Both atoms have $17$ electrons and the same electronic configuration ($2, 8, 7$), hence the same valence electrons, so their chemical properties are identical.+1
   3. (b) Number of protons = atomic number = $17$.+0.5
   4. (b) Number of electrons = number of protons = $17$.+0.5
   5. (b) Number of neutrons = $37 - 17 = 20$.+1

   Answer: (a) Isotopes are atoms of the same element with the same atomic number but different mass numbers; both atoms have the same number of electrons and electronic configuration ($2, 8, 7$), so their chemical properties are identical. (b) Protons: $17$, Electrons: $17$, Neutrons: $20$.

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