Structure of an Atom – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain the main points of Dalton’s atomic theory. Also state one limitation based on later discoveries.
Answer:
- Dalton said atoms are indivisible and indestructible.
- He said atoms are the smallest units of matter.
- He said atoms of different elements are different in mass and properties.
- He said atoms combine in fixed ratios to form compounds.
- For example, H₂O forms in a 2:1 ratio of hydrogen to oxygen. NaCl forms in a 1:1 ratio.
- Limitation: Later, the discovery of electrons and protons showed that atoms are divisible.
Q2. How do atoms combine in fixed ratios to form compounds? Explain with suitable examples.
Answer:
- Atoms combine in simple whole-number ratios.
- This gives each compound a fixed composition.
- Water, H₂O, always has 2 hydrogen atoms and 1 oxygen atom.
- Table salt, NaCl, always has 1 sodium and 1 chlorine atom.
- This is why compounds show constant properties like melting point.
- This idea supports the law of constant composition.
Q3. Describe the discovery and basic properties of electrons and protons. How did this affect atomic theory?
Answer:
- Electrons are negatively charged particles.
- Protons are positively charged particles in the nucleus.
- The electron was discovered by J. J. Thomson.
- Protons were understood later as part of the central region of atoms.
- These discoveries showed atoms have internal structure.
- They proved Dalton’s idea of indivisible atoms was not fully correct.
Q4. State the key features of Thomson’s model of an atom. Use the watermelon/pudding analogy to explain neutrality.
Answer:
- Thomson said the atom is a positively charged sphere.
- Electrons are embedded inside this sphere.
- He compared it to a Christmas pudding with currants.
- Or like a watermelon with seeds spread inside.
- The total positive charge equals the total negative charge.
- This makes the atom electrically neutral.
Q5. What do you mean by electrical neutrality and ions? Explain using Thomson’s idea.
Answer:
- An atom is neutral when positive and negative charges are equal.
- In Thomson’s model, this balance happens inside the same sphere.
- If electrons are more than positive charge, net charge is negative.
- If positive charge is more than electrons, net charge is positive.
- Such charged atoms are called ions.
- Neutrality is key to explain stable atoms and charge balance.
High Complexity (Analysis & Scenario-Based)
Q6. Analyse how the discovery of electrons and protons challenged Dalton’s idea of indivisible atoms. Use examples from water and salt.
Answer:
- Dalton said atoms are indivisible units.
- The electron’s discovery showed smaller particles exist.
- The proton showed atoms have positive charge inside the nucleus.
- Still, water being H₂O and salt being NaCl stayed fixed in ratio.
- So combining ratios were right, but structure was more complex.
- Thus, atoms are not indivisible, but compounds still form in fixed ratios.
Q7. A student says Thomson’s model explains both neutrality and compound formation. Do you agree? Explain with reasons.
Answer:
- Yes, it explains neutrality by equal positive and negative charges.
- It also allows different atoms to combine in fixed ratios.
- So it gives a simple picture to support Dalton’s composition idea.
- But it does not explain all observations in detail.
- It cannot fully show how charges are arranged during reactions.
- So it is useful, but it is also limited and needed improvement.
Q8. An atom in Thomson’s model has three electrons embedded and a total positive charge of +2. Predict its net charge and explain your reasoning.
Answer:
- Each electron has a −1 charge.
- Three electrons give −3 total negative charge.
- The positive part is +2.
- Net charge is −3 + 2 = −1.
- So the atom is not neutral; it is a negatively charged ion.
- For neutrality, positive and negative charges must be equal.
Q9. You are asked to build a classroom model of Thomson’s atom. What materials will you use and how will you justify each part?
Answer:
- Use a foam ball to show the positive sphere of the atom.
- Use small black beads to show the electrons.
- Glue the beads inside or on the ball to show embedding.
- Say the foam ball gives the uniform positive charge.
- Say the beads give the negative charges scattered inside.
- Explain that equal positive and negative charges give neutrality.
Q10. In a science talk, you must explain the shift from Dalton’s theory to Thomson’s model. Build a clear, logical argument.
Answer:
- Start with Dalton: atoms are indivisible and smallest units.
- Mention that compounds form in fixed ratios like H₂O and NaCl.
- Then add the discovery of electrons as negative particles.
- Add protons as positive particles in the nucleus.
- Since atoms have internal parts, they are not indivisible.
- Thomson’s model included both charges and kept neutrality, so it was a logical next step.