15 extra multiple-choice questions for Atoms and Molecules (10th Standard Science, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Which of the following pairs of elements represents isobars according to modern atomic theory?
- A. Chlorine-35 and Chlorine-37
- B. Argon-40 and Calcium-40Correct
- C. Oxygen-16 and Oxygen-18
- D. Protium and Deuterium
Explanation. Isobars are atoms of different elements that possess the same mass number but different atomic numbers, such as Argon-40 and Calcium-40.
Q2
Which of the following compounds exhibits an atom ratio of carbon, hydrogen, and oxygen that cannot be simplified to a simple whole number ratio of 1:2:1?
- A. Glucose
- B. Formaldehyde
- C. SucroseCorrect
- D. Fructose
Explanation. In sucrose, C12H22O11, the ratio of carbon, hydrogen, and oxygen is 12:22:11, which does not simplify to the simple 1:2:1 ratio seen in glucose.
Q3
According to modern atomic theory, which equation explains that the mass of an atom can be converted into energy?
- A. E = h\nu
- B. \(E = mc^2\)Correct
- C. p = mv
- D. PV = nRT
Explanation. Einstein's equation\(, E = mc^2\), states that mass can be converted into energy, which is a salient concept in modern atomic theory.
Q4
In an atom, which subatomic particles have negligible mass compared to the others?
- A. Protons
- B. Neutrons
- C. ElectronsCorrect
- D. Both protons and neutrons
Explanation. Protons and neutrons reside in the nucleus and have considerable mass, whereas electrons have negligible mass and do not contribute significantly to atomic mass.
Q5
Naturally occurring Boron consists of B-10 (20% abundance) and B-11 (80% abundance). What is the calculated average atomic mass of Boron?
- A. 10.0 amu
- B. 10.2 amu
- C. 10.8 amuCorrect
- D. 11.0 amu
Explanation. The average atomic mass is calculated by multiplying isotope masses by their abundances: (10 * 0.20) + (11 * 0.80) = 10.8 amu.
Q6
Which of the following statements correctly represents the relationship between molecules and compounds?
- A. All molecules are compounds, and all compounds are molecules.
- B. All compounds are molecules, but all molecules are not compounds.Correct
- C. Molecules are made of similar atoms only, whereas compounds are made of different atoms.
- D. Compounds do not contain chemical bonds, but molecules do.
Explanation. All compounds are molecules because they consist of bonded atoms. However, homonuclear molecules like oxygen gas are not compounds because they contain only one element.
Q7
Based on the classification of molecules, which of the following is a heteroatomic triatomic molecule?
- A. Ozone
- B. Carbon dioxideCorrect
- C. Ammonia
- D. Phosphorus
Explanation. Carbon dioxide consists of one carbon and two oxygen atoms, making it heteroatomic and triatomic with a total of three atoms.
Q8
Oxygen exists in two allotropic forms: oxygen gas and ozone. What are their respective atomicities?
- A. 1 and 2
- B. 2 and 3Correct
- C. 2 and 4
- D. 3 and 2
Explanation. Oxygen gas exists as diatomic O2 with an atomicity of two, while ozone exists as triatomic O3 with an atomicity of three.
Q9
What is the SI unit of Relative Molecular Mass?
- A. Grams
- B. Atomic mass units
- C. It has no unitCorrect
- D. Moles
Explanation. Relative Molecular Mass is defined as a ratio of the mass of a molecule to one-twelfth of a carbon-12 atom's mass, so it has no unit.
Q10
What is the gram molecular mass of hydrogen chloride (HCl) gas?
- A. 18 g
- B. 36.5 gCorrect
- C. 44 g
- D. 17 g
Explanation. The gram molecular mass of HCl is calculated by adding the mass of hydrogen (1 g) and chlorine (35.5 g), which equals 36.5 grams.
Q11
Which of the following statements represents a correct difference between atoms and molecules?
- A. Atoms exist in a free state, while molecules do not.
- B. Molecules do not possess chemical bonds, while atoms do.
- C. Except for noble gases, atoms are highly reactive, while molecules are less reactive.Correct
- D. Atom is the smallest particle of a compound, while molecule is the smallest particle of an element.
Explanation. Atoms of most elements are highly reactive and seek stability, whereas molecules are more stable and less reactive because their bonding shells are satisfied.
Q12
Which scientific hypothesis resolved the divisibility of atoms and molecules by showing that one molecule of hydrogen chloride gas is made of half a molecule of hydrogen and half a molecule of chlorine?
- A. Dalton's Atomic Theory
- B. Gay-Lussac's Law
- C. Avogadro's HypothesisCorrect
- D. Berzelius Hypothesis
Explanation. Avogadro's hypothesis showed that molecules can be subdivided during chemical reactions, aligning atomic theory with gas volume observations.
Q13
How is the relative molecular mass of a gas mathematically related to its vapour density?
- A. Relative Molecular Mass = Vapour Density / 2
- B. Relative Molecular Mass = 2 \\(\times\) Vapour DensityCorrect
- C. Relative Molecular Mass = Vapour Densit\(y^2\)
- D. Relative Molecular Mass = \\(\sqrt{Vapour Density}\)
Explanation. Relative molecular mass is exactly two times the vapour density of a gas, as derived using Avogadro's law.
Q14
According to the applications of Avogadro's law, what volume does one mole of any gas occupy at S.T.P.?
- A. 11.2 litres
- B. 2.24 litres
- C. 22.4 litresCorrect
- D. 44.8 litres
Explanation. Avogadro's law helps determine that one gram molar volume of any gas occupies exactly 22.4 litres at standard temperature and pressure (S.T.P.).
Q15
What is the gram molecular mass of calcium phosphate, Ca3(PO4)2? (Atomic mass: Ca = 40, P = 31, O = 16)
- A. 310 gCorrect
- B. 135 g
- C. 200 g
- D. 155 g
Explanation. Gram molecular mass of Ca3(PO4)2 is calculated as: 3 * 40 (Calcium) + 2 * [31 + 4 * 16] (Phosphate) = 120 + 2 * 95 = 310 g.
Frequently asked questions
How many MCQs are there in Atoms and Molecules?
This chapter has 15 book-back multiple-choice questions, each with the correct answer and a step-by-step explanation.
Are these 10th Standard Science MCQs free to practise online?
Yes. Every question, answer and explanation here is free, and you can also take them as a timed practice test.
Where can I find the Atoms and Molecules book-back answers?
The correct option for each question is highlighted on this page with a worked explanation, plus a quick answer-key summary at the top.
