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Journey Inside the Atom: CBSE Class 9 Chemistry practice questions

Practise 50 questions on Journey Inside the Atom, free, with an explanation for each answer.

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Sample questions

Why are Bohr's shells named K, L, M, N… instead of starting from A?

Letters A to N, with the first ten faded and four highlighted
  1. K stands for 'kernel', another word for nucleus
  2. Bohr named them after the first letter of his teacher's name
  3. Barkla called the first X-ray line he found K, leaving earlier letters free; Bohr kept that notation
  4. Shells A to J are hidden inside the nucleus
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C. Barkla called the first X-ray line he found K, leaving earlier letters free; Bohr kept that notation

Physicist Charles Barkla named the first X-ray line he observed K. He skipped A to J to leave room for any series found before it, though none ever was. Bohr adopted the same letters for atomic shells. The other options are made up.

Thomson always got the same kind of cathode rays. It did not matter which metal he used for the cathode or which gas filled the tube. What did this show?

A gas discharge tube connected to a high-voltage supply and a vacuum pump
  1. Cathode rays come only from metals, not from gases
  2. Electrons are a basic part of the atoms of every element
  3. Atoms cannot be broken into smaller particles
  4. Cathode rays are streams of positively charged particles
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B. Electrons are a basic part of the atoms of every element

Because the rays were the same for every cathode material and every gas, the particles in them, the electrons, must be a basic part of the atoms of every element. Bending the rays in electric and magnetic fields showed they are negatively charged and much lighter than atoms, so atoms are not indivisible. The electron's charge, −1.602×10−19 C-1.602 \times 10^{-19}\ \text{C}, is taken as −1-1 for convenience.

A sheet of card is 0.2 mm0.2\ \text{mm} thick. One atom is about 10−10 m10^{-10}\ \text{m} across. Roughly how many atoms, placed one on top of another, would fit across the card's thickness?

A thin sheet magnified to show the atoms packed inside it
  1. 2×1092 \times 10^{9}
  2. 2×1072 \times 10^{7}
  3. 2×10−142 \times 10^{-14}
  4. 2×1062 \times 10^{6}
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D. 2×1062 \times 10^{6}

0.2 mm=0.2×10−3 m=2×10−4 m0.2\ \text{mm} = 0.2 \times 10^{-3}\ \text{m} = 2 \times 10^{-4}\ \text{m}. Number of atoms =2×10−410−10=2×106= \dfrac{2 \times 10^{-4}}{10^{-10}} = 2 \times 10^{6}, about 20 lakh. Leaving 0.2 in millimetres gives 2×1092 \times 10^{9}, and treating mm as 10−2 m10^{-2}\ \text{m} (like cm) gives 2×1072 \times 10^{7}. Multiplying instead of dividing gives 2×10−142 \times 10^{-14}.

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