Think of the periodic table as a city map. The horizontal rows are streets (periods), and the vertical columns are neighbourhoods (groups). Every element lives at a specific address determined by one number alone — its atomic number, which is simply the count of protons in its nucleus.
The Modern Periodic Table arranges all 118 known elements in order of increasing atomic number. The genius of this arrangement is that elements in the same group share nearly identical chemical behaviour because they have the same number of valence electrons — the electrons in the outermost shell that do all the chemical work.
Here is the analogy that makes this stick: imagine valence electrons as the hands an element uses to grab other elements. Sodium has one hand (valency 1), magnesium has two (valency 2), and nitrogen has three or five depending on how you count. Elements in the same group have the same number of hands, which is why they react similarly.
The table has 7 periods (rows) and 18 groups (columns). As you move down a group, each element gains an extra electron shell — like adding a new floor to a building. As you move across a period left to right, protons increase but no new shell is added, so the nuclear pull gets stronger and the atom shrinks. That single insight — nuclear charge increases across a period — explains almost every periodic trend question CDS throws at you.
The broad classification: s-block (Groups 1 and 2) are your reactive metals, p-block (Groups 13–18) contains metals, metalloids, and non-metals, d-block (Groups 3–12) are the transition metals, and the two rows pulled below the main table — lanthanoids (atomic numbers 57–71) and actinoids (89–103) — form the f-block.
For CDS GK specifically, the examiners love testing element-to-group matching, periodic trend direction (which way does radius go?), and whether you can identify which block/series an element belongs to. The good news: this entire topic reduces to about 15 facts and 4 trend rules.
The four blocks tell you where an element's last electron is being filled:
| Block | Groups | Key Series | |-------|--------|------------| | s-block | 1, 2 | Alkali metals (Gr 1), Alkaline earth metals (Gr 2) | | p-block | 13–18 | Noble gases (Gr 18), Halogens (Gr 17) | | d-block | 3–12 | Transition metals | | f-block | — (below main table) | Lanthanoids (57–71), Actinoids (89–103) |
Alkali metals (Group 1): Li, Na, K, Rb, Cs, Fr. One valence electron. Extremely reactive, soft, silvery. Caesium (Cs) is here — remember this for the match-list PYQ.
Alkaline earth metals (Group 2): Be, Mg, Ca, Sr, Ba, Ra. Two valence electrons. Less reactive than Group 1 but still fairly reactive. Metallic character increases as you go down: Be is the least metallic, Ba is the most — confirmed by CDS 2026.
Transition metals (d-block, Groups 3–12): These are your Fe, Cu, Zn, Ni, Mn crowd. They form coloured compounds, show variable valency, and act as catalysts. Osmium (Os) is a transition metal in Group 8, Period 6 — that fact appeared directly in a CDS PYQ.
Lanthanoids (57–71): Also called rare earth elements. Lanthanum (La, 57) starts the series. Samarium (Sm) is element 62 — a lanthanoid. They are all f-block elements tucked into Period 6.
Actinoids (89–103): Actinium (Ac, 89) starts the series. Neptunium (Np) is element 93 — an actinoid. Most actinoids are radioactive. Uranium (92), Plutonium (94) are famous members.
These four trends are where CDS questions are constructed. Learn the direction and the reason — then wrong options become obviously wrong.
1. Atomic Radius
The CDS 2026 paper tested Statement 1 ("radius increases left to right across a period") — that is false. It decreases. Don't get caught.
2. Ionisation Energy
Energy needed to remove the outermost electron.
3. Electronegativity
The tendency to attract shared electrons in a bond.
4. Metallic Character
The tendency to lose electrons and form cations.
This directly explains why, among Group 2 elements, metallic character follows Be < Ca < Sr < Ba — confirmed correct in CDS 2026.
Isotopes are atoms of the same element with the same atomic number but different mass numbers (different neutrons). Because periodic position is determined by atomic number (protons), not mass number, isotopes always occupy the same position in the periodic table.
Chlorine-35 and Chlorine-37 both have atomic number 17 — same address, same neighbourhood. CDS directly tested this in 2026 and it is Statement 3 — correct.
Given atomic number, write the electron configuration using the 2, 8, 8, 18, 18, 32... shell filling pattern. The number of electrons in the outermost (valence) shell tells you the group:
For atomic number 11 (Sodium): configuration is 2, 8, 1. One valence electron → Group 1 → alkali metal → valency 1. Symbol Na (from Latin natrium). This was tested directly in CDS 2025.
The diagonal band from Boron to Astatine contains metalloids: B, Si, Ge, As, Sb, Te, Po. They have properties intermediate between metals and non-metals. Germanium (Ge) and Tellurium (Te) are metalloids — not transition metals. CDS 2026 Statement 2 claimed they were transition metals — that is false and was a direct trap.
When asked which block/series an element belongs to, check atomic number ranges:
For a match-list question with 4 elements, this lets you place all four in under 20 seconds. Standard approach (recall each element individually): ~60 seconds. This range-check method: ~15 seconds.
Four trends, one mnemonic: A-R-I-E (Atomic radius, then Ionisation energy, then Electronegativity, each reversing direction from the previous trend across a period).
Across a period left to right: Atomic radius decreases, Ionisation energy increases, Electronegativity increases.
Flip all three down a group: radius increases, ionisation energy decreases, electronegativity decreases.
Metallic character mirrors atomic radius. When a CDS statement says "increases across a period," you know it is wrong for radius, ionisation energy, and electronegativity — it only increases for metallic character going down a group.
This eliminates wrong statements in multi-statement questions in one sweep. Standard read-and-recall: 4 separate checks at ~15 seconds each = 60 seconds. ARIE pattern check: ~10 seconds per statement.
For s-block and p-block elements, the group number directly gives valence electrons:
So if the question gives you atomic number 11: write 2,8,1 — count the last digit (1) — Group 1 — valency 1 — done. No formula needed, no periodic table memorisation beyond shell filling. 3 steps versus the standard 6-step full configuration write-out; saves roughly 30 seconds.
CDS frequently tests symbols that do not match English names:
| Element | Symbol | Latin Origin | |---------|--------|--------------| | Sodium | Na | Natrium | | Potassium | K | Kalium | | Iron | Fe | Ferrum | | Copper | Cu | Cuprum | | Gold | Au | Aurum | | Silver | Ag | Argentum | | Lead | Pb | Plumbum | | Mercury | Hg | Hydrargyrum |
Memorise these 8 pairs as a fixed list — they appear regularly in option traps where one option gives the correct element name but a wrong symbol. Knowing these 8 removes that option in 5 seconds flat.
Any question asking whether isotopes occupy the same or different positions in the periodic table has one answer: same position, always, because position is determined by atomic number (proton count) alone, and isotopes share the same proton count by definition.
When you see "isotopes" in a statement, mark it correct without further analysis. This is a 3-second elimination versus reading the full justification (~20 seconds).
In the exam hall, periodic table questions in CDS GK fall into three types. Identify the type first, then apply the corresponding path:
Type 1 — Match the element to its group/series Check atomic number. Apply the block ranges from the Block Decoder trick. If atomic number is not given, check the symbol against your known lists (alkali metals: Li Na K Rb Cs Fr; lanthanoids include Sm, Eu, Gd; actinoids include Np, Pu, Am).
Type 2 — Multi-statement periodic trend question Apply ARIE. Any statement claiming radius/ionisation energy/electronegativity increases across a period → false. Any statement claiming metallic character increases down a group → true. Any statement about isotopes occupying the same periodic position → true. Eliminate wrong statements, then check which option matches your true/false pattern.
Type 3 — Identify element from atomic number
Write the electron configuration using 2, 8, 8, 18... shells. Count valence electrons. Map to group. Confirm name and symbol. For atomic numbers 1–20, this takes under 30 seconds if you know the shell pattern.
Do not waste time on elements you do not recognise. In a match-list, place the elements you know with confidence first — process of elimination will handle the rest.
Why this question: This is the highest-difficulty periodic table question CDS has asked — it requires knowing four elements across four different blocks simultaneously. It is a pure recall test of your block classification knowledge.
Solving path: Start with what you are most certain about. Cs (Caesium) is in the famous alkali metals list (Li Na K Rb Cs Fr) → matches Group 1 → alkali metal → B-2. Np (Neptunium) has atomic number 93 — that falls in the 89–103 actinoid range → D-3. Sm (Samarium) — if you know the lanthanoid series covers 57–71, Sm is element 62, firmly lanthanoid → A-4. Os (Osmium) is a d-block element in Group 8 → transition element → C-1. Pattern: A-4, B-2, C-1, D-3. Match to option (a): 4 2 1 3. Confirmed.
Why this question: This 4-statement question is a textbook CDS trap — it mixes one very common misconception (radius increases across a period) with a sneaky metalloid misclassification. If you know two trends and the metalloid list, you can eliminate three wrong options in under 45 seconds.
Solving path: Statement 1: "atomic radius increases left to right across a period" — apply ARIE: radius decreases across a period. Statement 1 is false. This eliminates options (b) which contains Statement 1. Statement 2: "Germanium, Tellurium, and Copper are transition metals" — Ge and Te are metalloids (on the diagonal border), only Cu is a transition metal. Statement 2 is false. This eliminates options (c). Statement 3: isotopes of Cl occupy the same position — always true by definition. Statement 3 is correct. Statement 4: metallic character of Group 2 elements follows Be < Ca < Sr < Ba — metallic character increases down a group, and these are all Group 2 in order of increasing period. Statement 4 is correct. Only Statements 3 and 4 are correct → option (d).
Why this question: This is the simplest possible periodic table question — but it comes with a symbol trap (Na versus Li) and a valency trap (could confuse with Group 2). It tests whether you know the Latin-origin symbol and can write the configuration quickly.
Solving path: Atomic number 11 → electron configuration: 2, 8, 1. Three shells filled, one valence electron in the outermost shell → Group 1 → alkali metal → valency 1. The element with 11 protons is Sodium. Symbol from Latin natrium → Na. Option (d): Sodium, Na, 1. The traps: option (a) offers Li (atomic number 3, not 11) and option (b) offers Mg (atomic number 12) — one proton off from Na, which is why atomic number precision matters. Option (c) Boron has atomic number 5 — not even close.
Confusing atomic radius trend direction. The single most-tested wrong statement in CDS is "atomic radius increases across a period." It decreases. Nuclear charge increases without adding a new shell, pulling electrons inward. If you ever see this in a statement, mark it false immediately.
Calling Germanium and Tellurium transition metals. Transition metals are the d-block elements (Groups 3–12). Ge (Group 14) and Te (Group 16) are both in the p-block and classified as metalloids. This exact trap appeared in CDS 2026.
Mixing up lanthanoids and actinoids. Lanthanoids are Period 6, atomic numbers 57–71. Actinoids are Period 7, atomic numbers 89–103. Remember: Lanthanoids come Lower in atomic number. A simple check: if the atomic number is 57–71, it is a lanthanoid; if 89–103, it is an actinoid.
Using mass number instead of atomic number for periodic position. The table is arranged by atomic number (proton count), not mass number. Isotopes differ in mass number (neutrons) but share atomic number, so they sit at the same periodic position. Confusing the two leads to wrong answers on isotope questions.
Forgetting Latin-origin symbols. Na for Sodium, K for Potassium, Fe for Iron, Cu for Copper, Au for Gold, Ag for Silver, Pb for Lead, Hg for Mercury. CDS options frequently pair a correct element name with a wrong symbol or vice versa — these eight are the ones that catch people.
Assuming valency equals group number directly for all elements. This works cleanly for Groups 1 and 2. For Group 17 elements (halogens), the valency is 1 (they need one electron to complete the octet, so they combine with one other atom). For Group 18 (noble gases), valency is 0. Do not mechanically subtract from 18 — check what makes chemical sense.