Materials & Prep
Prepare a blank periodic table handout for each student, a periodic table displayed on the wall or board, and a short practice set. Students need colored pencils or highlighters. No special equipment is required.
Opening
Display the wall periodic table and ask students to silently inspect periods 1, 2, and 3. Without writing electron configurations, answer: What repeats from one row to the next? Why does period 1 contain 2 elements while periods 2 and 3 contain 8? Students write one observation and one prediction, then compare with a partner.
Ask“What can the shape of the table tell us before we know the notation?” Accept observations such as row lengths, repeated vertical groups, and the two detached rows at the bottom. Reveal that students have already identified the visible result of electrons filling different kinds of orbitals. Today, electron configurations will explain why the table has this shape.
Direct instruction
ConnectOn the displayed periodic table, outline the left two columns, the middle block, the right six columns, and the detached bottom rows. Label these the s-block, d-block, p-block, and f-block. Explain that the width of each block matches the number of electrons that sublevel can hold: s holds 2, p holds 6, d holds 10, and f holds 14.
ModelUse atomic number as the electron count for a neutral atom. For oxygen, Z = 8, so eight electrons must be placed. Fill 1s2, then 2s2, then 2p4, giving 1s2 2s2 2p4. Think aloud: “The superscripts must total 8. The last occupied sublevel is 2p, so oxygen appears in the p-block, period 2. The 2 tells me the principal energy level, and the p tells me the block.”
ConfrontAsk students to predict whether 3d fills before 4s. Show the filling sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Clarify that filling order is not the same as simply counting upward through the principal energy levels. A common error is writing 3d before 4s because 3 is smaller than 4. For the first-row transition element scandium, Z = 21, the configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d1, often written [Ar] 4s2 3d1.
ExplainEach new period begins when a new principal energy level starts filling. Period 2 has 2s and 2p available, so it contains 2 + 6 = 8 elements. Period 4 includes 4s, 3d, and 4p, so its 18 elements create the long row. The table is shaped by the sequence and capacities of sublevels, not by arbitrary formatting.
Guided practice
Have students use colored pencils to shade the s-, p-, d-, and f-blocks on the blank periodic table. They annotate each block with its capacity: s = 2, p = 6, d = 10, f = 14.
Worked exampleBuild the configuration for sulfur, Z = 16. Students first predict the final block and period. Fill 1s2 2s2 2p6 3s2 3p4. Check the total: 2 + 2 + 6 + 2 + 4 = 16. The final sublevel is 3p, so sulfur is in period 3 and the p-block.
Check for understandingStudents hold up or point to the block where the final electron enters for calcium, Z = 20, and explain the choice. Expected response: s-block, because calcium is [Ar] 4s2. Then ask students to identify the error in “phosphorus, Z = 15, ends in 3d3.” Students should state that 3d does not begin until after 4s and that phosphorus is [Ne] 3s2 3p3.
ScaffoldProvide the filling sequence with blank superscripts and a sentence frame: “The configuration ends in , so the element is in period and the -block.” Allow students to cross off each electron from the atomic number as they fill sublevels.
Independent work or discussion
Students complete and justify the following set on paper: (1) Write the full electron configuration for fluorine, Z = 9. (2) Write the shorthand configuration for potassium, Z = 19. (3) Write the full configuration for iron, Z = 26. (4) For each element, identify its period, block, and the sublevel receiving the last electron. (5) In three to four sentences, explain why period 4 is longer than period 3.
CirculateCheck that students total the superscripts to the atomic number, use 4s before 3d, and distinguish the final occupied sublevel from the highest number appearing anywhere in the configuration. Ask students to support each table location with configuration evidence rather than simply naming a position.
ExtensionStudents compare chromium, Z = 24, with the expected filling pattern and investigate the accepted configuration [Ar] 4s1 3d5. They state how this exception changes the configuration without changing chromium’s period or d-block location.
Closing
Exit ticketWrite the electron configuration for chlorine, Z = 17, then answer: “How does this configuration explain chlorine’s location in the periodic table?” A complete response includes 1s2 2s2 2p6 3s2 3p5, identifies period 3 and the p-block, and connects the final 3p sublevel and p-block capacity to the table’s structure.
Collect responses to determine whether students can both perform the filling procedure and explain the structural connection, rather than merely reproduce a memorized sequence.