Journey Inside the Atom - 2 - Free MCQ Practice Test with solutions,

Until the late 19th century, following Dalton's theory, atoms were believed to be the smallest indivisible units of matter. However, scientists discovered that certain elements spontaneously emit invisible energy and particles — a phenomenon called radioactivity. This was a revolutionary finding because it proved that atoms were not simply solid, indivisible spheres. They must contain smaller internal components (subatomic particles) that can be ejected. This directly contradicted Dalton's idea of the indivisible atom and opened the door to the discovery of: electrons (Thomson, 1897), protons (Rutherford), and neutrons (Chadwick, 1932), ultimately leading to modern atomic models.

Chlorine occurs naturally as a mixture of two isotopes: ³⁵Cl (mass = 35 u, natural abundance = 75%) and ³⁷Cl (mass = 37 u, natural abundance = 25%). A simple average would give (35 + 37) / 2 = 36 u, but this is inaccurate because the isotopes don't occur in equal proportions. The weighted average atomic mass accounts for natural abundance: = (35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5 u. This accurately reflects the mass of chlorine as it occurs in nature. No individual chlorine atom has a mass of 35.5 u — it is a statistical weighted average used in all chemical calculations involving Chlorine.

According to Bohr's model, electrons can exist only in fixed shells with definite energy levels. The energy of shells increases as we move away from the nucleus (K < L < M < N). When an electron jumps from a higher energy shell to a lower energy shell (moves closer to the nucleus), it must release a fixed amount of energy equal to the difference between the energies of the two levels. Conversely, when an electron moves from a lower to a higher shell, it must absorb that fixed amount of energy. This principle of energy absorption and emission by electrons explains the emission and absorption spectra of elements — each element produces a unique spectrum, like a fingerprint, based on its energy level differences.

The element with 2 protons has atomic number Z = 2, which identifies it as Helium (He). Its mass number A = protons + neutrons = 2 + 2 = 4, so it is ⁴₂He. Its electronic configuration is 2 — meaning both electrons are in the K-shell, which is completely filled (the K-shell maximum is 2). Since its outermost shell is completely filled, Helium has no tendency to gain, lose, or share electrons. Therefore, its valency is 0. Helium is a noble gas and is chemically unreactive (inert). It is used in weather balloons and as a coolant in MRI machines precisely because of its non-reactive nature.

The standard IUPAC notation for representing an atom places the mass number (A) at the top left and the atomic number (Z) at the bottom left of the element symbol. So for Carbon: Mass number = 12 (top left), Atomic number = 6 (bottom left), Symbol = C. This gives ¹²₆C. This notation tells us everything about the atom: it has 6 protons, 6 electrons, and 12 − 6 = 6 neutrons. This standard notation is universally used by scientists to represent specific isotopes of elements clearly and unambiguously, making it easy to identify any atom at a glance.

IUPAC — the International Union of Pure and Applied Chemistry — is the international scientific organisation responsible for approving and standardising the names and symbols of all chemical elements. The rules for writing chemical symbols include: the first letter is always a capital (uppercase) letter, and the second letter (if any) is always a small (lowercase) letter. For example: Aluminium = Al (not AL or aL), Cobalt = Co (not CO). IUPAC also governs nomenclature in organic chemistry, biochemistry, and other fields. These internationally recognised symbols allow scientists across the world to communicate clearly, regardless of language barriers, making global scientific collaboration possible.

Argon (Ar) has atomic number 18 and electronic configuration 2, 8, 8. Its outermost shell (M-shell) has 8 electrons — a complete octet. Since its valence shell is completely filled, Argon has absolutely no tendency to gain, lose, or share electrons with other atoms. Therefore, its valency is 0. Argon is a noble gas and is chemically inert. In contrast: Sodium (2,8,1) has valency 1 (loses 1 electron), Chlorine (2,8,7) has valency 1 (gains 1 electron), and Sulfur (2,8,6) has valency 2 (gains 2 electrons). Argon is used in electric light bulbs and welding to provide an inert atmosphere.

Ernest Rutherford was born in New Zealand and later worked with J.J. Thomson at the Cavendish Laboratory in Cambridge. He is known as the Father of Nuclear Physics and is best known for proposing the nuclear model of the atom in 1911 (based on the gold foil experiment). However, he actually won the Nobel Prize in Chemistry in 1908 — before proposing his atomic model — for his contributions to the chemistry of radioactive substances and his investigations into the disintegration of elements. He later discovered the nucleus and identified protons. J.J. Thomson won the Nobel Prize in Physics in 1906, Niels Bohr in Physics in 1922, and James Chadwick in Physics in 1935.

This was one of the key puzzles that led to the discovery of the neutron. A Hydrogen atom has 1 proton and mass number = 1. A Helium atom has 2 protons but mass number = 4 — four times that of hydrogen, not twice. If only protons contributed mass, Helium should be only twice as heavy as Hydrogen. The answer is that Helium's nucleus contains 2 protons AND 2 neutrons. Since neutrons have mass nearly equal to that of a proton, they contribute significantly to the total mass. Mass number = protons + neutrons = 2 + 2 = 4. Electrons have negligible mass and are not counted. This principle applies to all atoms: mass number = total nucleons (protons + neutrons).

Carbon (atomic number 6) has an electronic configuration of 2, 4. Its outermost shell has 4 valence electrons. To achieve a stable octet, Carbon would need to either lose 4 electrons or gain 4 electrons — both of which require a very large amount of energy and are not energetically favourable. Therefore, Carbon takes a third route: it shares its 4 valence electrons with other atoms through covalent bonding, completing both its own octet and the octets of the atoms it bonds with. This gives Carbon a valency of 4. This unique property of Carbon — sharing 4 bonds — is the foundation of all organic chemistry and is the reason Carbon can form millions of different compounds, including the molecules of life.

Isotopes are atoms of the same element (same atomic number Z) with different mass numbers (different number of neutrons). Let us check each option: Option A — ⁴⁰₁₈Ar (Z=18) and ⁴⁰₂₀Ca (Z=20): different elements, same mass number → isobars, not isotopes. Option B — ¹²₆C (Z=6, mass=12) and ¹⁴₆C (Z=6, mass=14): same element Carbon (Z=6), different mass numbers → these are isotopes ✓. Option C — ⁴⁰₁₉K (Z=19) and ⁴⁰₂₀Ca (Z=20): different elements, same mass number → isobars. Option D — ¹H (Z=1) and ⁴He (Z=2): completely different elements → neither isotopes nor isobars.

The diameter of an atom is approximately 10⁻¹⁰ m, while the diameter of the nucleus is approximately 10⁻¹⁵ m. The ratio is: 10⁻¹⁰ / 10⁻¹⁵ = 10⁵ (one lakh). The nucleus is one lakh times smaller than the atom. If an atom were scaled up to the size of a cricket ground (100 m across), the nucleus at the centre would be only a few millimetres — like a tiny black pepper grain. The rest of the atom is essentially empty space through which electrons revolve at great distances from the nucleus. This analogy powerfully illustrates just how small the nucleus is relative to the whole atom, and why most alpha particles passed straight through the gold foil in Rutherford's experiment.

J.J. Thomson studied the conduction of electric current through gases at very low pressure using a cathode ray tube in 1897. By analysing cathode rays in electric and magnetic fields, he concluded they were streams of negatively charged particles with mass much smaller than atoms. These were the first identified subatomic particles, later called electrons. Thomson showed that the nature of cathode rays was the same regardless of the cathode material or gas used, proving electrons are present in all atoms. He was head of the Cavendish Laboratory and guided many scientists including Ernest Rutherford. He received the Nobel Prize in Physics in 1906 for his work on the electrical conductivity of gases.

Every proton in the nucleus carries a positive charge and repels every other proton through electrostatic repulsion. For small atoms, the nuclear force (which binds all nucleons) is sufficient to overcome this repulsion. But for larger atoms with many protons, the repulsion becomes enormous. Neutrons help in two important ways: (1) Being electrically neutral, they can sit between protons, effectively increasing the separation between them and reducing direct repulsion. (2) Neutrons also strengthen the nuclear binding force that holds all nucleons together. This is why heavier atoms need proportionally more neutrons — Iron (26 protons, 30 neutrons) and Uranium (92 protons, 146 neutrons) illustrate how the neutron-to-proton ratio increases for heavier elements.

Aluminium has atomic number 13, meaning it has 13 electrons to distribute. Filling shells in order: K-shell (maximum 2 electrons): 2 electrons → remaining = 11. L-shell (maximum 8 electrons): 8 electrons → remaining = 3. M-shell: 3 electrons → remaining = 0. So the electronic configuration of Aluminium is 2, 8, 3. Its valence shell (M-shell) has 3 valence electrons. Since 3 < 4, Aluminium tends to lose 3 electrons to achieve the stable Neon configuration (2, 8), giving it a valency of 3. This is why Aluminium forms compounds like Al₂O₃ (aluminium oxide) and AlCl₃ (aluminium chloride), where it bonds in a ratio reflecting its valency of 3.