Atoms, Periodic Table & Radioactivity: Tricks, Chart, MCQs

Everything around us – air, water, rocks, plants and our own bodies – is made of tiny particles called atoms. An atom is so small that about a crore of them placed side by side would cover only about a millimetre. Yet inside each atom there is a tiny, heavy nucleus surrounded by fast-moving electrons. The way these particles are arranged decides whether a substance is a shiny metal, a poisonous gas or a life-giving element like oxygen.

Scientists arranged all known elements in a chart called the periodic table, which shows clear patterns in their properties. Some atoms are unstable and break down on their own, giving out powerful radiation – this is radioactivity, the basis of nuclear power, cancer treatment and carbon dating. This topic is a favourite for SSC, Railways, WBCS, UPSC and State PSC exams. This guide explains atomic structure, the full periodic table with easy tricks, periodic trends and radioactivity in simple language, with diagrams, a quick revision box and 20 MCQs with answers.

➤ Atom: Nucleus (protons + neutrons) surrounded by electrons
➤ Discoverers: Electron – J.J. Thomson (1897) · Nucleus – Rutherford (1911) · Neutron – Chadwick (1932)
➤ Modern periodic table: 118 elements · 18 groups · 7 periods · arranged by atomic number (Moseley)
➤ Radioactivity: Discovered by Henri Becquerel (1896) · Alpha, beta and gamma rays
➤ Key formula: Maximum electrons in a shell = 2n² · Energy E = mc²

Part A: Discovery of the Atom – Step by Step

The Three Basic Particles

Almost all the mass of an atom is in its nucleus, but the nucleus occupies only a tiny fraction of the atom's volume. The hydrogen atom (protium) is the only atom with no neutron.

Part B: Structure of the Atom

Atomic Number and Mass Number

• Atomic number (Z) = number of protons (= number of electrons in a neutral atom). It identifies the element
• Mass number (A) = number of protons + number of neutrons
• Number of neutrons = A − Z
• Written as ^A_ZX, e.g., ²³₁₁Na has 11 protons, 11 electrons and 12 neutrons

Electronic Configuration (Bohr–Bury Rules)

• Shells are named K, L, M, N (n = 1, 2, 3, 4) starting from the nucleus
• Maximum electrons in a shell = 2n² → K = 2, L = 8, M = 18, N = 32
• The outermost shell cannot hold more than 8 electrons
• Electrons in the outermost shell are valence electrons; they decide how an element reacts (its valency)

Figure 1: Bohr model of a sodium atom – 11 protons and 12 neutrons in the nucleus; electrons arranged 2, 8, 1 in the K, L and M shells.

Valency rule: If valence electrons are 1–4, valency = number of valence electrons. If 5–8, valency = 8 − valence electrons.

Isotopes, Isobars and Isotones

Heavy water (D₂O), made with deuterium, is used as a moderator in nuclear reactors to slow down neutrons.

Part C: History of the Periodic Table

Modern Periodic Law: The properties of elements are a periodic function of their atomic numbers. 2019 was celebrated as the International Year of the Periodic Table (150 years of Mendeleev's table).

Part D: The Modern Periodic Table

• 118 elements are now known and named. The last four were named in 2016: Nihonium (113), Moscovium (115), Tennessine (117) and Oganesson (118)
• 18 vertical columns = groups; elements in a group have the same number of valence electrons and similar properties
• 7 horizontal rows = periods; elements in a period have the same number of shells
• The two rows at the bottom are the lanthanides (57–71) and actinides (89–103), together called inner transition elements (f-block)
• Blocks: s-block (groups 1–2), p-block (groups 13–18), d-block (groups 3–12, transition elements), f-block (lanthanides and actinides)

The Periodic Table (All 118 Elements)

Each box shows the atomic number and symbol. Scroll sideways on mobile to see the full table.

Alkali metals Alkaline earth metals Transition metals Post-transition metals Metalloids Other non-metals Halogens Noble gases Lanthanides Actinides

Figure 2: The modern periodic table of 118 elements, coloured by type of element.

Number of Elements in Each Period

Trick: "2-8-8-18-18-32-32" – the numbers pair up after the first.

Important Groups and Their Families

Tricks to Remember the Groups

Part E: Periodic Trends – How Properties Change

Figure 3: How key properties change across a period and down a group.

Note: Noble gases are usually left out of size comparisons along a period because their radii are measured differently.

Metals, Non-metals and Metalloids

• Metals (left and centre): shiny, malleable, ductile, good conductors. Mercury is the only metal that is liquid at room temperature. Gallium and caesium melt just above room temperature – gallium can melt in your hand
• Non-metals (top right): poor conductors, brittle if solid. Bromine is the only liquid non-metal. Graphite is a non-metal that conducts electricity
• Metalloids (along the zig-zag line): boron, silicon, germanium, arsenic, antimony, tellurium – properties of both; silicon and germanium are semiconductors

Record-Holding Elements

Elements with Unusual Symbols (Latin Names)

Trick: "Nice Kids Find Cute Angels And Happy Puppies Sitting With Sheep" – Na, K, Fe, Cu, Ag, Au, Hg, Pb, Sn, W, Sb.

Part F: Radioactivity

Radioactivity is the process in which an unstable nucleus breaks down on its own, giving out radiation. It is a nuclear process – it is not affected by temperature, pressure or chemical reactions.

• Discovered by Henri Becquerel (1896) while studying uranium salts
• Marie and Pierre Curie discovered polonium and radium (1898). Marie Curie won two Nobel Prizes – Physics (1903, shared with Becquerel and Pierre Curie) and Chemistry (1911) – the first person to win Nobels in two different sciences
• Elements with atomic number above 82 (lead) are generally radioactive
• SI unit of radioactivity: becquerel (Bq) = one decay per second. Older unit: curie (Ci) = 3.7 × 10¹⁰ Bq. Radiation dose to the body is measured in sievert (Sv)
• Radiation is detected by a Geiger–Müller (GM) counter

Alpha, Beta and Gamma Rays

Figure 4: Alpha rays are stopped by paper, beta rays by aluminium, and gamma rays are only weakened by thick lead.

Changes During Radioactive Decay

Group displacement trick (Soddy–Fajans): After alpha decay, the new element moves 2 places to the left in the periodic table; after beta decay, it moves 1 place to the right.

Half-Life

The half-life (T_½) is the time taken for half of the radioactive nuclei in a sample to decay. After n half-lives, the amount left is N = N₀ × (1/2)ⁿ.

Figure 5: Radioactive decay curve – the amount halves after every half-life.

Example: A sample of 80 g has a half-life of 10 days. After 30 days (3 half-lives), the amount left = 80 × (1/2)³ = 10 g.

Nuclear Fission and Fusion

Figure 6: Nuclear fission – a neutron splits a uranium-235 nucleus into smaller nuclei, releasing energy and more neutrons.

Energy in both processes comes from the loss of a small amount of mass, given by Einstein's equation E = mc². In a nuclear reactor, control rods (cadmium or boron) absorb extra neutrons to control the chain reaction, a moderator (heavy water or graphite) slows neutrons down, and a coolant carries away heat.

Uses and Dangers of Radioactivity

Dangers: Radiation can damage cells and DNA, causing burns, cancer and genetic defects. Major accidents include Chernobyl (Ukraine, 1986) and Fukushima (Japan, 2011). Workers wear a film badge or dosimeter to measure their exposure.

Indian Scientists and Kolkata's Connection

• Meghnad Saha developed the famous Saha ionisation equation (1920), used to study the temperature of stars; the Saha Institute of Nuclear Physics in Kolkata is named after him
• Satyendra Nath Bose worked with Einstein on Bose–Einstein statistics; particles called bosons are named after him
• Homi J. Bhabha founded India's nuclear programme; India's three-stage nuclear plan aims to use thorium, of which India has large reserves
• The Variable Energy Cyclotron Centre (VECC), Kolkata, is a major centre for nuclear physics research

India's leading research labs, including those of CSIR, work on materials and chemistry, while the VAIBHAV Fellowship brings Indian-origin scientists abroad into partnership with Indian institutions. Space science outreach, such as the planned ISRO Space Gallery at Darbhanga, also explains how stars shine through nuclear fusion.

20 MCQs on Atoms, Periodic Table and Radioactivity

Q1. Who discovered the electron?
(a) Ernest Rutherford
(b) J.J. Thomson
(c) James Chadwick
(d) Niels Bohr

Answer: (b) – He discovered it in 1897 using cathode rays.

Q2. Rutherford's gold foil experiment showed that:
(a) Electrons revolve in fixed orbits
(b) The atom has a small, dense, positive nucleus
(c) Atoms are indivisible
(d) Neutrons are present in the nucleus

Answer: (b) – Most alpha particles passed straight through, so the atom is mostly empty space.

Q3. The neutron was discovered by:
(a) Goldstein
(b) Dalton
(c) Chadwick
(d) Moseley

Answer: (c) – James Chadwick discovered it in 1932.

Q4. An atom has 17 protons and 18 neutrons. Its mass number is:
(a) 17
(b) 18
(c) 35
(d) 1

Answer: (c) – A = 17 + 18 = 35; this is chlorine-35.

Q5. The maximum number of electrons in the M shell is:
(a) 8
(b) 18
(c) 32
(d) 2

Answer: (b) – Using 2n² with n = 3: 2 × 9 = 18.

Q6. Atoms of the same element with different mass numbers are called:
(a) Isobars
(b) Isotones
(c) Isotopes
(d) Isomers

Answer: (c) – Example: protium, deuterium and tritium.

Q7. Which isotope of hydrogen is radioactive?
(a) Protium
(b) Deuterium
(c) Tritium
(d) None

Answer: (c) – Deuterium is used to make heavy water.

Q8. The modern periodic table is based on:
(a) Atomic mass
(b) Atomic number
(c) Number of neutrons
(d) Density

Answer: (b) – This was shown by Henry Moseley in 1913; Mendeleev used atomic mass.

Q9. Mendeleev's "eka-aluminium" was later discovered as:
(a) Scandium
(b) Germanium
(c) Gallium
(d) Boron

Answer: (c) – Eka-silicon became germanium.

Q10. How many groups and periods are there in the modern periodic table?
(a) 18 groups, 7 periods
(b) 7 groups, 18 periods
(c) 8 groups, 7 periods
(d) 16 groups, 9 periods

Answer: (a) – It has 118 elements in total.

Q11. Elements of Group 17 are called:
(a) Alkali metals
(b) Noble gases
(c) Halogens
(d) Chalcogens

Answer: (c) – Halogen means "salt former"; Group 16 elements are chalcogens.

Q12. Which is the most electronegative element?
(a) Oxygen
(b) Chlorine
(c) Fluorine
(d) Nitrogen

Answer: (c) – It sits at the top right of the table (excluding noble gases).

Q13. Across a period from left to right, atomic size generally:
(a) Increases
(b) Decreases
(c) Remains the same
(d) First increases then decreases

Answer: (b) – Increasing nuclear charge pulls electrons closer.

Q14. Which non-metal is a liquid at room temperature?
(a) Mercury
(b) Bromine
(c) Iodine
(d) Chlorine

Answer: (b) – Mercury is a liquid metal, a common trap.

Q15. The chemical symbol 'Pb' stands for:
(a) Platinum
(b) Phosphorus
(c) Lead
(d) Potassium

Answer: (c) – From the Latin word "Plumbum".

Q16. Radioactivity was discovered by:
(a) Marie Curie
(b) Henri Becquerel
(c) Ernest Rutherford
(d) Albert Einstein

Answer: (b) – He discovered it in 1896 while studying uranium salts.

Q17. Which radiation has the highest penetrating power?
(a) Alpha
(b) Beta
(c) Gamma
(d) All are equal

Answer: (c) – Alpha has the highest ionising power but the lowest penetrating power.

Q18. When a nucleus emits an alpha particle, its atomic number:
(a) Increases by 1
(b) Decreases by 2
(c) Decreases by 4
(d) Does not change

Answer: (b) – Its mass number decreases by 4.

Q19. The half-life of a radioactive substance is 5 days. What fraction remains after 15 days?
(a) 1/2
(b) 1/4
(c) 1/8
(d) 1/16

Answer: (c) – 15 days is 3 half-lives, so (1/2)³ = 1/8.

Q20. The energy of the Sun is produced by:
(a) Nuclear fission
(b) Nuclear fusion
(c) Chemical burning
(d) Radioactive decay

Answer: (b) – Hydrogen nuclei fuse to form helium, releasing energy (E = mc²).

Conclusion

Atoms are the building blocks of everything, the periodic table is the map that shows how their properties repeat, and radioactivity reveals the enormous energy locked inside the nucleus. For exams, remember the discoverers and years, the atomic number and mass number rules, 2n², isotopes and isobars, the structure of the periodic table (118 elements, 18 groups, 7 periods), the main families and trends, record-holding elements, Latin symbols, and the properties of alpha, beta and gamma rays with half-life, fission and fusion. Revise the memory tricks above once more before your exam.

Reference: NCERT Science Class IX "Structure of the Atom"; NCERT Chemistry Class XI "Classification of Elements and Periodicity in Properties"; NCERT Physics Class XII "Nuclei".