HSC Exam Practice

Sample response. Atomic radius is a measure of atomic size; in HSC questions, it is taken as the distance from the nucleus to the outermost occupied electron shell. Effective nuclear charge (Z_eff) is the net positive pull experienced by outer electrons after accounting for shielding by inner-shell electrons. A higher Z_eff pulls the outer electrons closer to the nucleus, reducing the atomic radius; a lower Z_eff allows the outer electrons to occupy a larger orbital, increasing the atomic radius.

Marking notes. 1 mark for a correct definition of atomic radius; 1 mark for a correct definition of effective nuclear charge (must reference shielding and net attraction); 1 mark for explicitly linking higher Z_eff to smaller atomic radius.

Sample response. Across a period (left to right): atomic radius decreases. Main reason: proton number increases while electrons enter the same main shell; shielding changes little, so effective nuclear charge rises and pulls outer electrons closer. Down a group (top to bottom): atomic radius increases. Main reason: each successive element has an additional occupied electron shell; outer electrons are further from the nucleus and more shielded by inner shells.

Marking notes. 1 mark for correctly stating the period trend (decreases); 1 mark for the structural reason (same shell, increasing nuclear charge); 1 mark for correctly stating the group trend (increases); 1 mark for the structural reason (additional electron shell).

Sample response. Na and Mg are in the same period (Period 3), so their outer electrons are in the same third electron shell. Na has 11 protons and Mg has 12 protons. Because the electrons added across the period enter the same shell, shielding barely changes. Mg therefore has a greater effective nuclear charge, pulling its outer electrons closer to the nucleus, which makes Mg smaller than Na. Conversely, Na’s lower effective nuclear charge allows its outer electron cloud to be larger.

Marking notes. 1 mark for noting both are in the same period / same electron shell; 1 mark for identifying that shielding barely changes but nuclear charge increases from Na to Mg; 1 mark for correctly concluding that higher Z_eff in Mg contracts the atom, making Na larger.

Sample response. Forming a cation makes the species smaller than the parent atom. Example: Na (186 pm) → Na⁺ (102 pm). Losing an electron removes the outermost valence electron, often resulting in one fewer occupied shell; electron–electron repulsion also decreases, so the remaining electrons are pulled closer by the same nuclear charge. Forming an anion makes the species larger than the parent atom. Example: Cl (99 pm) → Cl⁻ (181 pm). Gaining an electron increases electron–electron repulsion without adding protons, so the electron cloud expands.

Marking notes. 1 mark for cation smaller + mechanism (fewer electrons, less repulsion or one fewer shell); 1 mark for a named cation example with comparison (e.g. Na/Na⁺); 1 mark for anion larger + mechanism (more electrons, increased repulsion, same nuclear charge); 1 mark for a named anion example with comparison (e.g. Cl/Cl⁻). Accept other valid examples (Mg/Mg²⁺, O/O²⁻).

Sample response. K is in Period 4 and Li is in Period 2. K therefore has an additional fourth electron shell compared to Li’s second shell. The outer electron in K is farther from the nucleus and is shielded by three fully occupied inner shells (1s, 2s2p, 3s3p). Even though K has 19 protons compared to Li’s 3, the extra shell distance and increased shielding outweigh the higher nuclear charge, resulting in a larger atomic radius for K.

Marking notes. 1 mark for identifying K has more electron shells (Period 4 vs Period 2); 1 mark for explaining increased shielding by inner shells reduces the effective pull on the outermost electron; 1 mark for concluding that extra shell distance outweighs higher nuclear charge, making K larger.

Sample response. The student is incorrect. F and Cl are in the same group (Group 17), so the correct comparison is a group trend, not a period trend. The relevant factor is not atomic mass or number of electrons within a single atom, but the number of occupied electron shells. Cl (Period 3) has three electron shells while F (Period 2) has only two. The additional outer shell in Cl places its valence electrons farther from the nucleus, making Cl larger. The correct ranking is F < Cl (Cl has a larger atomic radius than F).

Marking notes. 1 mark for identifying the flaw (mass/electron count is irrelevant; must compare shells / group position); 1 mark for the correct structural explanation (Cl has an additional electron shell); 1 mark for the correct ranking (Cl > F / F is smaller than Cl).

Sample response (a). Within both Period 2 and Period 3, atomic radius decreases as atomic number increases (from left to right across the period). Period 3 starts higher (Na ≈ 186 pm) and ends at about 99 pm for Cl; Period 2 starts at about 152 pm for Li and ends at about 64 pm for F. Both curves show a generally smooth decrease, though the rate of decrease slows slightly towards the right side of each period. From the graph, the approximate atomic radius of Na is about 185–190 pm [1 — period trend described for both; 1 — approximate Na value read correctly; 1 — rate comment or quantitative comparison].

Sample response (b). Period 3 elements are consistently larger than their Period 2 counterparts in the same group because Period 3 elements have an additional third electron shell. For example, Na (Period 3) is much larger than Li (Period 2) even though both are Group 1; Na’s outer electron is in the third shell, farther from the nucleus and more shielded. Similarly, Cl (Period 3) is larger than F (Period 2) because Cl’s outer electrons occupy the third shell rather than the second. The extra shell distance and increased shielding consistently produce larger atomic radii in Period 3 [1 — identifies additional electron shell as cause; 1 — uses a named example correctly; 1 — links shell distance and shielding].

Sample response (c). The student’s claim is incorrect. The graph shows that when moving from Period 2 to Period 3 (i.e. from F at Z=9 to Na at Z=11), atomic radius increases sharply (from ~64 pm to ~186 pm), even though atomic number increases. This is because Na starts a new period with an additional electron shell, so the increase in size due to the new shell outweighs the increase in nuclear charge. Therefore, atomic radius does not always decrease as Z increases — it depends on whether you are moving within a period or to a new period [1 — identifies the counter-example (F to Na) from the graph; 1 — explains why (new shell, group trend dominates)].

Marking notes. Part (a): 3 marks as above. Part (b): 3 marks as above. Part (c): 2 marks as above.

Sample response. Atomic radius is determined by the interplay of two key factors: nuclear charge (number of protons) and electron configuration (number and arrangement of electron shells). These two factors interact differently depending on the direction of comparison on the periodic table, producing distinct and sometimes opposite trends. Across a period (left to right), nuclear charge is the dominant factor. As proton number increases, electrons are added to the same main shell, which shields the nucleus poorly. Because shielding changes little, effective nuclear charge (Z_eff) increases steadily. This stronger attractive force pulls the outer electrons closer to the nucleus, decreasing atomic radius. For example, Na (11 protons, 186 pm) is significantly larger than Cl (17 protons, 99 pm) despite both having outer electrons in the third shell; the 6-proton increase gives Cl a much higher Z_eff. Down a group, electron configuration is the dominant factor. Each successive element adds a new, fully occupied electron shell. This additional shell places the outer electrons further from the nucleus and provides substantially more shielding. Even though nuclear charge also increases down the group, the extra shell distance and shielding dominate: atomic radius increases. For example, K (19 protons, Period 4) is larger than Na (11 protons, Period 3) because the 4s outer electron in K is far further from the nucleus than Na’s 3s electron. Crucially, both factors (nuclear charge and shielding/shell distance) change simultaneously; it is their relative magnitude that determines the outcome. Across a period, Z_eff wins because shielding barely changes. Down a group, shell distance wins because each new shell dramatically increases the distance of the outermost electrons. A simple generalisation such as “atomic radius always decreases as atomic number increases” is therefore limited and incorrect. It holds within a single period but fails when comparing across periods — as demonstrated by the jump from F (~64 pm, Z=9) to Na (~186 pm, Z=11), where an extra electron shell more than doubles the atomic radius. Additional limitations include: isoelectronic species (same electron count) violate the generalisation (e.g. K⁺ at 138 pm vs Cl⁻ at 181 pm, same 18 electrons but different proton counts); and transition metals show more gradual radius changes because d-electrons provide additional shielding. In summary, atomic radius is best understood not as a simple function of atomic number, but as the result of competing nuclear attraction and electron shielding, with the dominant factor depending on whether comparison is within a period or down a group.

Marking criteria (7 marks). 1 = correctly identifies nuclear charge as dominant driver of period trend, with reference to same shell and Z_eff. 1 = correctly identifies electron shell addition as dominant driver of group trend, with reference to shielding and shell distance. 1 = named example for period trend with quantitative data (e.g. Na vs Cl or Li vs F). 1 = named example for group trend with reasoning (e.g. K vs Na, extra shell). 1 = explains the interaction: both factors change simultaneously; which one dominates determines the trend. 1 = identifies at least one valid limitation of a simple Z-based generalisation (e.g. F to Na jump across periods; isoelectronic species; transition metals). 1 = reaches an explicit evaluative judgement that integrates both factors and acknowledges that no single variable alone predicts atomic size across the whole table.