Journey Inside the Atom - 1 - Free MCQ Practice Test with solutions,

Acharya Kanada, an ancient Indian philosopher, proposed that if matter (dravya) is divided repeatedly, you will eventually reach the smallest particle that cannot be divided further. He called these particles parmanu. His ideas are recorded in the Sanskrit text Vaisesika Sutras. A parmanu is infinitely small and cannot be perceived by the senses. Combinations of parmanus form dyads (groups of two) and triads (groups of three), from which the entire material universe is created. This concept was proposed more than 2,000 years ago, long before modern experimental atomic theory.

In 1897, J.J. Thomson studied conduction of electric current through gases at very low pressure. He used a glass tube with two electrodes — a cathode (negative) and an anode (positive) — and applied high voltage. He observed rays moving from the cathode (negative electrode) to the anode (positive electrode). By studying these cathode rays in electric and magnetic fields, he concluded they are streams of negatively charged particles with mass much smaller than that of atoms. These particles were later called electrons. The nature of cathode rays was found to be independent of the cathode material and the gas used, proving electrons are a fundamental component of all atoms.

According to the Bohr-Bury rule, the maximum number of electrons in any shell is given by the formula 2n², where n is the shell number. For the K-shell (n = 1): Maximum electrons = 2 × 1² = 2 × 1 = 2 electrons. For comparison: L-shell (n=2) = 2 × 4 = 8 electrons, M-shell (n=3) = 2 × 9 = 18 electrons, N-shell (n=4) = 2 × 16 = 32 electrons. Additionally, regardless of the formula, the outermost shell of any atom can hold a maximum of only 8 electrons. Electrons fill shells in order starting from the K-shell closest to the nucleus.

When J.J. Thomson discovered electrons, he faced a key puzzle — since atoms are electrically neutral, where is the positive charge? To answer this, he proposed the plum pudding model: the atom is a sphere of uniformly distributed positive charge with electrons embedded throughout it — just like plums distributed in a pudding. A more familiar comparison is a watermelon, where the red pulp represents the positive charge and the seeds represent the electrons. This was the first genuine attempt to explain how positive and negative charges are balanced in an atom. However, Rutherford's gold foil experiment later disproved this model entirely.

In Rutherford's famous gold foil experiment (1911), alpha particles were used as projectiles. An alpha particle is actually the nucleus of a helium atom, containing 2 protons and 2 neutrons. It carries a charge of +2 and has a relatively large mass compared to electrons. Alpha particles are emitted from certain radioactive elements. When these positively charged, heavy particles were fired at the thin gold foil, most passed through (confirming atoms are mostly empty space), some were deflected at large angles, and a very few bounced back — indicating the presence of a small, dense, positively charged nucleus at the centre of the atom.

Rutherford's model proposed that electrons revolve around the nucleus like planets around the Sun. However, this model had a critical flaw. A charged particle moving in a circular path is constantly changing direction, which means it is accelerating. According to classical physics, an accelerating charged particle must continuously emit energy as radiation. As the electron loses energy, it would slow down, spiral inward, and eventually collapse into the positively charged nucleus — making atoms unstable. But in reality, atoms are perfectly stable and matter exists around us intact. This major contradiction showed that Rutherford's model was incomplete and a new explanation was urgently needed.

The formula to find the number of neutrons is: Number of neutrons = Mass number (A) − Atomic number (Z). Here, Mass number = 35 and Atomic number = 17. Therefore, Number of neutrons = 35 − 17 = 18 neutrons. This element is Chlorine (Cl). Its nucleus contains 17 protons and 18 neutrons, and it has 17 electrons revolving around the nucleus (since the atom is electrically neutral). Its electronic configuration is 2, 8, 7. Chlorine has 7 valence electrons and needs 1 more to complete its octet, giving it a valency of 1.

To overcome the limitation of Rutherford's model, Niels Bohr proposed in 1913 that electrons do not revolve randomly but follow fixed circular paths called stationary states, orbits, or shells. In each shell, an electron has a definite, fixed amount of energy — hence these shells are also called energy levels. The shells are labelled K, L, M, N (or n = 1, 2, 3, 4...). The K-shell (n=1) is closest to the nucleus with the least energy. Energy increases as we move outward. The key postulate is that while revolving in a fixed shell, an electron does not lose energy, which explains why atoms are stable and electrons don't spiral into the nucleus.

Magnesium has atomic number 12, meaning it has 12 electrons to be distributed across shells. Filling shells in order: K-shell (maximum 2): fills with 2 electrons → remaining = 10. L-shell (maximum 8): fills with 8 electrons → remaining = 2. M-shell: accommodates the remaining 2 electrons. So the electronic configuration of Magnesium is 2, 8, 2. Its outermost M-shell has 2 valence electrons. Since 2 < 4, Magnesium tends to lose 2 electrons to achieve the stable configuration of Neon (2, 8), giving it a valency of 2. This is why Magnesium forms MgO (magnesium oxide) and MgCl₂ (magnesium chloride).

Isobars are atoms of different elements that have the same mass number (A) but different atomic numbers (Z). A classic example is: ⁴⁰₁₈Ar (Argon, Z=18), ⁴⁰₁₉K (Potassium, Z=19), and ⁴⁰₂₀Ca (Calcium, Z=20) — all have mass number 40 but are completely different elements with different numbers of protons. Do not confuse isobars with isotopes: Isotopes are atoms of the same element with the same atomic number but different mass numbers (e.g., ³⁵Cl and ³⁷Cl). Isobars have the same total number of nucleons but distributed differently between protons and neutrons.

James Chadwick discovered the neutron in 1932 — a subatomic particle with no electrical charge (neutral) and mass nearly equal to that of a proton. It is represented by the symbol n⁰. Neutrons are found in the nucleus of all atoms except hydrogen (ordinary hydrogen/protium has no neutrons). The discovery of the neutron solved a major puzzle: Helium has 2 protons but is about 4 times heavier than Hydrogen (1 proton). The 2 neutrons in helium's nucleus account for the extra mass. Neutrons also help stabilise the nucleus by reducing repulsion between protons and strengthening the nuclear binding force. Chadwick received the Nobel Prize in Physics in 1935 for this discovery.

The symbol Hg for Mercury is derived from its Latin name 'Hydrargyrum', which itself comes from the Greek words hydor (water) and argyros (silver) — meaning "liquid silver," which reflects Mercury's appearance as a shiny, liquid metal at room temperature. This follows the IUPAC rule that some elements have symbols from their Latin, Greek, or German names. Other examples: Fe (Iron) from Latin Ferrum, Au (Gold) from Latin Aurum, K (Potassium) from Latin Kalium, Na (Sodium) from Latin Natrium, W (Tungsten) from German Wolfram. IUPAC standardises these symbols internationally so scientists worldwide can communicate clearly.

Chlorine (atomic number 17) has an electronic configuration of 2, 8, 7. Its outermost shell (M-shell) contains 7 valence electrons. Since 7 > 4, Chlorine tends to gain electrons (rather than lose them) to complete its octet. It needs just 1 more electron to reach 8 electrons in its outermost shell and achieve the stable configuration of Argon (2, 8, 8). Therefore, the valency of Chlorine is 1. This is why Chlorine forms compounds like HCl (hydrochloric acid), NaCl (sodium chloride/common salt), where it combines with 1 atom of hydrogen or sodium respectively. Chlorine is a highly reactive non-metal because of this tendency to gain 1 electron.

Isotopes are atoms of the same element with the same atomic number (same protons) but different mass numbers (different neutrons). Since atomic number = number of protons = number of electrons (in a neutral atom), all isotopes of an element have the same number of electrons and therefore the same electronic configuration. Since chemical properties depend mainly on the number and arrangement of valence electrons, all isotopes of an element show similar chemical properties. However, they differ in their physical properties (such as boiling point, melting point, density) because they have different masses. For example, all three isotopes of hydrogen — Protium (¹H), Deuterium (²H), and Tritium (³H) — react chemically in similar ways but have different physical properties.

⁶⁰₂₇Co (Cobalt-60) is a radioactive isotope used in radiation therapy for cancer. It emits high-energy gamma rays that are directed at cancerous tumours to destroy them. Different isotopes have different medical and industrial applications: ¹⁴₆C (Carbon-14) is used in radiocarbon dating to determine the age of ancient fossils and artefacts. ²³⁵₉₂U (Uranium-235) is used as fuel in nuclear reactors to generate electricity. ¹³¹₅₃I (Iodine-131) is used to treat goitre and thyroid cancer specifically. These applications of isotopes demonstrate the wide-ranging practical benefits of atomic science in medicine, energy, and archaeology.