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Atomic Mass – Long Answer Questions (CBSE Class 10 Science)

Medium Level (Application & Explanation)

Q1. Why did scientists change the standard for atomic mass from oxygen to carbon-12? What were the advantages?

Answer:

  • Earlier, oxygen was used because it formed many compounds.
  • Atomic mass unit was taken as 1/16 of oxygen atom’s mass.
  • In 1961, the standard changed to carbon-12.
  • Now, 1 u = 1/12 of the mass of one carbon-12 atom.
  • Carbon-12 is a clear and consistent
    reference
    .
  • It makes atomic masses more uniform and comparable worldwide.

Q2. Define the atomic mass unit and express a few elements’ masses on this scale.

Answer:

  • The atomic mass unit (u) is 1/12 of the mass of one carbon-12 atom.
  • Hydrogen has an atomic mass of 1 u.
  • Oxygen has an atomic mass of 16 u.
  • Sodium has an atomic mass of 23 u.
  • These values are relative to carbon-12.
  • This makes comparing different atoms simple and clear.

Q3. How does the idea of atomic mass help explain the law of constant proportions? Use water as an example.

Answer:

  • The law says a compound always has the same fixed mass ratio.
  • Water is H2O. Hydrogen is 1 u, oxygen is 16 u.
  • In water, total hydrogen mass = 2 × 1 = 2 u.
  • Oxygen mass in water = 16 u.
  • Mass ratio of hydrogen to oxygen = 2 : 16 = 1 : 8.
  • So water always has hydrogen and oxygen in a 1:8 mass ratio.

Q4. Why do most atoms exist as molecules or ions instead of single atoms?

Answer:

  • Most atoms do not stay alone in nature.
  • They join to form molecules or ions.
  • This makes them more stable.
  • Groups of these particles form the matter we see.
  • Many elements prefer to bond and make compounds.
  • That is why we often find them in combined forms.

Q5. Why can we not see atoms with the naked eye or a regular microscope?

Answer:

  • Atoms are extremely small.
  • They are much smaller than what our eyes can detect.
  • A regular microscope also cannot resolve them.
  • We need special instruments to study atoms.
  • Scientists use indirect methods to measure their properties.
  • That is why a relative mass scale is very helpful.

High Complexity (Analysis & Scenario-Based)

Q6. Scenario: You have a fruit scale and a watermelon cut into 12 equal pieces. How can you use this to compare fruit masses? Link this to the atomic mass scale.

Answer:

  • Take one piece of the watermelon as a standard.
  • Place it on one side of the scale.
  • Put another fruit on the other side until the scale balances.
  • If the apple balances one piece, its mass equals 1 piece.
  • This is like 1 u being 1/12 of carbon-12’s mass.
  • You are comparing relative masses using a fixed standard.

Q7. A lab moved from the old oxygen standard to the carbon-12 standard. What changes and what stays the same for atomic masses?

Answer:

  • The
    reference
     changed from oxygen to carbon-12.
  • Now 1 u is defined using carbon-12.
  • Reported values may be recalibrated to the new scale.
  • The ordering of light and heavy atoms stays the same.
  • Comparisons between elements remain consistent.
  • Everyday chemistry stays unaffected in meaning and use.

Q8. Use given atomic masses to show constant proportions in magnesium oxide and carbon dioxide.

Answer:

  • Magnesium (Mg) = 24 u, Oxygen (O) = 16 u, Carbon (C) = 12 u.
  • In magnesium oxide (MgO), the ratio is 24 : 16.
  • Simplify to 3 : 2. This stays the same in pure MgO.
  • In carbon dioxide (CO2), oxygen is 2 × 16 = 32 u.
  • Ratio of carbon to oxygen is 12 : 32 = 3 : 8.
  • These fixed ratios show constant proportions in compounds.

Q9. Using the table, compare the relative heaviness of some elements. What does this mean when they form compounds?

Answer:

  • Sodium (23 u) is slightly heavier than magnesium (24 u is actually heavier; correct this): Magnesium (24 u) is heavier than sodium (23 u).
  • Oxygen (16 u) is heavier than nitrogen (14 u).
  • Chlorine (35.5 u) is heavier than sulphur (32 u).
  • When atoms join, their masses add up.
  • The ratio in a compound depends on their atomic masses and formula.
  • So compounds have fixed mass ratios, based on these values.

Q10. A student tries to measure the mass of a single atom on a kitchen scale. Explain why this fails and how scientists still compare atomic masses.

Answer:

  • A single atom has an extremely tiny mass.
  • A kitchen scale cannot detect such small values.
  • Scientists use a standard to compare atoms instead.
  • They use 1 u = 1/12 of carbon-12’s atom mass.
  • All atomic masses are written on this relative scale.
  • This method is practical, clear, and reliable.

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