CBSE Class 9 Science Chapter 08 Journey Inside The Atom MCQs Set 02

Answer: (B) What is everything made up of?

Explanation:
The chapter explicitly states that Acharya Kanada, Leucippus, and Democritus pondered 'the same fundamental question that has continued to inspire human inquiry across centuries — What is everything made up of?' This shared curiosity across distant civilisations led them to theorise about matter's basic building blocks.

Answer: (C) He proposed a sphere of positive charge with electrons distributed throughout it

Explanation:
Thomson's plum pudding model was his solution to the charge-balancing problem. By picturing the atom as a positively charged sphere with electrons embedded throughout it, like seeds in a watermelon, he could account for the atom's electrical neutrality while incorporating his newly discovered negatively charged particles.

Answer: (B) Alpha particles should pass straight through with minimal deflection

Explanation:
Since Thomson's model had positive charge spread evenly throughout the atom, the positively charged alpha particles should experience only weak, distributed repulsive forces. Most particles would be expected to pass through essentially undeflected — which is exactly what happened, but some were sharply deflected or bounced back, contradicting the model entirely.

Answer: (B) It showed that atoms must be composed of smaller particles, contradicting the idea that they were indivisible

Explanation:
The chapter states that radioactivity was 'a phenomenon known as radioactivity. This showed that atoms must be composed of smaller particles, proving that they were not indivisible as previously believed.' This discovery fundamentally challenged Dalton's assertion that atoms were the ultimate indivisible units of matter.

Answer: (B) It could not explain why negatively charged electrons did not spiral into the positively charged nucleus due to energy loss

Explanation:
• A charged particle moving in a circular path continuously accelerates (changes direction)
• An accelerating charged particle loses energy through radiation
• Losing energy causes the electron to spiral inward toward the nucleus
• Yet atoms are stable — this contradiction required a new model to resolve it.

Answer: (B) In certain allowed orbits, electrons can move without losing energy despite being in motion

Explanation:
Bohr's breakthrough was proposing that electrons could occupy specific orbits—stationary states—where they maintained constant energy while orbiting the nucleus. This postulate directly answered the stability puzzle: unlike Rutherford's model, Bohr's electrons did not radiate away energy as they moved, so they did not collapse into the nucleus.

Answer: (B) Atoms with 8 electrons (or 2 in case of helium) in the outermost shell are largely unreactive and stable

Explanation:
The chapter observes that 'elements with complete octet of electrons (8 electrons), or 2 electrons in the case of helium in their valence shell are largely unreactive and more stable. On the other hand, atoms with incomplete valence shells are usually more reactive.' This pattern explains why noble gases are inert and why other atoms seek to gain, lose, or share electrons.

Answer: (A) 8

Explanation:
A neutral atom contains equal numbers of protons and electrons so that the positive and negative charges balance out. Since this atom has 8 protons, it must have 8 electrons to achieve electrical neutrality, regardless of how many neutrons are present.

Answer: (B) Atomic number is the count of protons only; mass number is the sum of protons and neutrons

Explanation:
Atomic number (Z) counts only the protons in the nucleus and determines the element's identity. Mass number (A) equals the total count of nucleons—protons plus neutrons—and accounts for the atom's overall mass. This distinction is why isotopes of the same element share the same atomic number but differ in mass number.

Answer: (B) Neutrons help reduce mutual repulsion among protons and strengthen the force binding the nucleus together

Explanation:
The chapter explains that 'Neutrons being neutral help reduce this repulsion by intervening and increasing the distance between protons, and also by strengthening the force, called the nuclear force, that binds all particles together. So, heavier atoms need many more neutrons to hold everything in the nucleus tightly bound.'

Answer: (B) Isobars

Explanation:
The chapter defines isobars as 'atoms of different elements have the same mass number, but different atomic numbers.' An example given is calcium (Z=20), potassium (Z=19), and argon (Z=18), all with mass number 40. This contrasts with isotopes, which are atoms of the same element with different mass numbers.

Answer: (C) Chlorine-35 is far more abundant (about 75%) than Chlorine-37 (about 25%), so a weighted average is needed

Explanation:
A simple average of 35 and 37 gives 36, but isotopes don't occur in equal ratios. The chapter demonstrates that multiplying each isotope's mass by its percent abundance (35 × 75/100 + 37 × 25/100 = 35.5 u) gives the weighted average, which accurately reflects how chlorine actually exists in nature. This matters because it accounts for the relative abundances of the two isotopic forms.

Answer: (B) The energy levels or shells where electrons can be found

Explanation:
In Bohr's model, K, L, M, and N denote the electron shells or energy levels at increasing distances from the nucleus. The K-shell (n=1) is closest and has the lowest energy, while N-shell (n=4) is farther away and has higher energy. Electrons fill these shells in order from innermost outward.

Answer: (A) Electronic configuration: 2,4; valence electrons: 4

Explanation:
With 6 protons, the atom has 6 electrons (for neutrality). Following the rule that K-shell holds max 2 and L-shell holds max 8, the configuration is 2 in K-shell and 4 in L-shell. Since L-shell is the outermost shell, it contains 4 valence electrons. This atom is carbon (C).

Answer: (B) No, the chapter explicitly states that later Bohr's model was found to have limitations and a quantum mechanical model was proposed

Explanation:
The chapter includes a 'Next Level Up' section stating 'Later, even Bohr's model was found to have limitations, and yet another model, the quantum mechanical model, was proposed. You will learn about it in higher grades.' This reflects the iterative nature of scientific progress—each model improved upon earlier ones but was itself eventually superseded by more refined understanding.

Answer: (B) Parmanu

Explanation:
The chapter explicitly states that Acharya Kanada called the smallest, indivisible particles parmanu, and his ideas were recorded in the Sanskrit text Vaisesika Sutras. Atomos is the Greek equivalent term, while nucleons and dyads refer to different concepts.

Answer: (A) Electrons were negatively charged but atoms showed no net charge

Explanation:
Thomson had discovered negatively charged electrons but knew atoms were electrically neutral overall. This meant positive charge had to be present somewhere in the atom, which his plum pudding model attempted to explain by distributing it throughout a sphere.

Answer: (B) The positive charge must be concentrated in a tiny central region

Explanation:
The particles that reversed direction indicated they had struck something extremely dense and positively charged head-on. If positive charge were spread throughout as Thomson proposed, particles would scatter more gradually. This sharp reversal proved a concentrated nucleus existed.

Answer: (B) It proposed that in these specific states, electrons maintain constant energy while orbiting, avoiding the energy loss predicted by classical physics

Explanation:
Rutherford's model predicted electrons would lose energy while accelerating in circular paths and spiral into the nucleus. Bohr's postulate stated that in stationary states, energy remains constant despite orbital motion—a revolutionary idea that saved atomic stability without classical explanation, though later refined in quantum mechanics.

Answer: (C) 18

Explanation:
For the M-shell, n = 3, so 2n² = 2(3)² = 2(9) = 18 electrons maximum. The chapter explicitly provides this formula and applies it to show that K holds 2, L holds 8, and M holds 18 electrons.

Answer: (B) Why helium atoms are heavier than their proton count would suggest

Explanation:
The chapter notes that while helium has two protons, its mass is about four times that of a hydrogen atom, not double. Chadwick's neutron discovery explained this extra mass: helium contains two neutrons in addition to its two protons, accounting for the four-fold mass difference.

Answer: (C) Mass number above, atomic number below the symbol

Explanation:
The chapter shows the standard notation with mass number (A) written above and atomic number (Z) written below the element symbol, as in the carbon example ¹²₆C where 12 is the mass number and 6 is the atomic number.

Answer: (B) Isotopes

Explanation:
Both atoms have the same atomic number (11 protons) but different mass numbers (23 versus 24). The chapter defines isotopes as atoms of the same element with identical atomic numbers but different mass numbers due to varying neutron counts. Isobars, by contrast, have the same mass number but different atomic numbers.

Answer: (A) 2

Explanation:
An atom with configuration 2, 8, 6 has six valence electrons in its outermost shell. Since it has fewer than four valence electrons... actually, it has more than four. Following the chapter's rule, atoms with more than four valence electrons tend to gain electrons to complete an octet. To gain two electrons reaches eight, so the valency is 2 (the number of electrons gained).

Answer: (B) They allowed international scientific communication without language barriers and created a standardized system

Explanation:
The chapter states that scientists use alphabetic symbols because they are internationally recognized and allow scientists worldwide to communicate clearly, regardless of language barriers. This was a significant practical improvement over Dalton's pictorial symbols and Latin-based names alone.