Periodic Table — First Look
Level 4 — Application (Novel Problems)
Time: 60 minutes Total Marks: 50
Answer all questions. Show reasoning. A periodic table is NOT provided — deduce from principles and the first-30 elements.
Q1. [10 marks] An element X has atomic number 15. Without a periodic table: (a) State its period and group, giving reasoning from its electron configuration. (4) (b) Classify X as metal, non-metal or metalloid, and predict whether its oxide is acidic or basic. Justify. (3) (c) Mendeleev left gaps in his table and predicted "eka-silicon." Element X sits immediately after silicon (Z = 14). Explain why Mendeleev's mass-based ordering would still place X correctly here, and give one situation in the first 30 elements where mass-based ordering FAILS but atomic-number ordering succeeds. (3)
Q2. [12 marks] Three unknown elements A, B, C from the first 30 have the following clues:
- A is a noble gas in Period 3.
- B is in the same group as A but one period above.
- C is the most reactive metal in Period 4 among the first 30 (i.e. the s-block element beginning Period 4).
(a) Identify A, B, C by symbol and atomic number. (6) (b) Arrange A, B, C in order of increasing atomic radius, with reasoning. (3) (c) State the block (s/p/d/f) of each of A, B, C. (3)
Q3. [10 marks] A newly "discovered" hypothetical element Q is claimed to have atomic number 31. (a) Using periodic trends and the structure of Period 4, predict Q's group number and block. (4) (b) Predict whether Q is a metal, non-metal or metalloid, and compare its metallic character with that of aluminium (Z = 13), which lies directly above it in the same group. (3) (c) The claim states Q has an atomic mass LOWER than zinc (Z = 30). Evaluate whether this is physically reasonable, referencing the modern periodic law. (3)
Q4. [10 marks] Match each described behaviour to ONE element from the first 30 and give its symbol. Justify each choice in one line. (a) A metalloid used in semiconductors, sitting between metals and non-metals in Period 3. (2) (b) The lightest element that is a gas and forms water with oxygen. (2) (c) A transition metal in Period 4 commonly magnetic (used in magnets), Z between 25 and 29. (2) (d) A Period 2 non-metal forming the backbone of organic molecules. (2) (e) A halogen among the first 30 (Period 3). (2)
Q5. [8 marks] (a) Mendeleev ordered elements by atomic mass; the modern table orders by atomic number. Argon (Ar, mass ≈ 40) comes before potassium (K, mass ≈ 39) in the modern table. Explain this "anomaly" and what it demonstrated about the correct basis of periodicity. (4) (b) Predict, with reasoning, which has the larger atomic radius: sodium (Z = 11) or chlorine (Z = 17). (4)
Answer keyMark scheme & solutions
Q1 [10]
(a) Z = 15 → configuration = 2,8,5. (1)
- Highest principal quantum number n = 3 → Period 3. (1.5)
- Valence electrons = 5 in s+p (2+3) → p-block group; group = 10 + 5 = Group 15. (1.5)
(b) Element is phosphorus (P), a non-metal. (1) Non-metals form acidic oxides (e.g. → phosphoric acid). (2)
(c) Mass increases with Z through Si(28)→P(31), so mass-order = atomic-number order here — placement identical. (1.5) Failure example: Ar (39.9) and K (39.1) — by mass Ar would come after K, wrongly; atomic number (Ar=18 < K=19) fixes the order. (1.5)
Q2 [12]
(a)
- A = noble gas in Period 3 → Argon, Ar, Z = 18. (2)
- B = same group (18), one period above → Period 2 noble gas → Neon, Ne, Z = 10. (2)
- C = s-block element starting Period 4 → Potassium, K, Z = 19. (2)
(b) Increasing atomic radius: Ne < Ar < K. (1) Reasoning: Ne and Ar both Group 18; radius increases down a group (more shells) so Ar > Ne. (1) K is an alkali metal starting Period 4 with one loosely-held outer electron and larger — biggest of the three. (1)
(c) Ne: p-block (noble gas). Ar: p-block. K: s-block. (3) (1 each)
Q3 [10]
(a) Z = 31 config: 2,8,18,3 → . Outer electron in 4p → p-block, Period 4. (2) Valence (s+p of shell 4) = 2+1 = 3 → group = 10 + 3 = Group 13. (2) (Element = gallium.)
(b) Q (gallium) is a metal. (1) Directly below Al in Group 13; metallic character increases down a group, so Q is more metallic than aluminium. (2)
(c) Not reasonable. (1) Modern periodic law orders by atomic number; generally mass increases with Z. Z=31 > Z=30, and there is no known mass inversion here (Ga ≈ 69.7 > Zn ≈ 65.4). (2)
Q4 [10]
(a) Si (silicon) — Period 3 metalloid, semiconductor. (2) (b) H (hydrogen) — lightest gaseous element, forms with O. (2) (c) Fe (iron, Z = 26) — Period 4 transition metal, ferromagnetic. (2) (d) C (carbon) — Period 2 non-metal, backbone of organic compounds. (2) (e) Cl (chlorine, Z = 17) — Period 3 halogen (Group 17). (2)
Q5 [8]
(a) By atomic mass Ar (40) > K (39), so mass-ordering would place K before Ar — wrong, since Ar (inert gas) must group with noble gases and K with alkali metals. (2) Ordering by atomic number (Ar=18, K=19) restores correct grouping. This showed periodicity depends on atomic number, not mass (modern periodic law). (2)
(b) Sodium has the larger radius. (2) Na (Z=11) and Cl (Z=17) are both in Period 3 (same number of shells). Across a period nuclear charge increases, pulling electrons in, so radius decreases left→right; Na (left) > Cl (right). (2)
[
{"claim":"P (Z=15) is in Group 15 via 10 + valence(5)","code":"valence=3+2; group=10+valence; result = (group==15)"},
{"claim":"Gallium Z=31 is Group 13, block p","code":"valence_sp=2+1; group=10+valence_sp; result = (group==13)"},
{"claim":"Argon Z=18 precedes Potassium Z=19 by atomic number despite higher mass","code":"z_Ar=18; z_K=19; m_Ar=39.9; m_K=39.1; result = (z_Ar<z_K) and (m_Ar>m_K)"},
{"claim":"Ne electron count 2,8 sums to 10 = Z of neon","code":"shells=[2,8]; result = (sum(shells)==10)"}
]