Why is the periodic table shaped like that?
The periodic table has an odd shape: two tall columns on the left, ten shorter columns sunk into the middle, six tall columns on the right, and two separate rows of 14 elements hanging underneath. In lesson 1.4.2 you learned about periods and groups, and in lesson 2.2.5 you learned the aufbau principle. This lesson shows that they are really the same idea: the shape of the periodic table is the aufbau filling order, laid out row by row.
Each period starts with ns and ends with np
Following the aufbau principle, electrons fill 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, and so on. Break that sequence just before each ns, and you get the periods:
| Period | Sublevels filled, in order | Number of elements |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s, 2p | 2 + 6 = 8 |
| 3 | 3s, 3p | 2 + 6 = 8 |
| 4 | 4s, 3d, 4p | 2 + 10 + 6 = 18 |
| 5 | 5s, 4d, 5p | 2 + 10 + 6 = 18 |
| 6 | 6s, 4f, 5d, 6p | 2 + 14 + 10 + 6 = 32 |
| 7 | 7s, 5f, 6d, 7p | 2 + 14 + 10 + 6 = 32 |
- Each period starts with an alkali metal (period 1 starts with hydrogen), whose outermost electron is ns¹, and ends with a noble gas, ns²np⁶ (helium is 1s²). That repeating pattern of configurations is where the “periodic” in periodic table comes from.
- The period number is the highest occupied energy level, n.周期序数 = 最大能层序数 The 3d sublevel belongs to level 3, but it fills after 4s, so the 3d elements appear in period 4.
- The number of elements in a period is the total number of electrons its sublevels can hold.
The four blocks
Grouping elements by the sublevel their last electron goes into divides the table into four blocks区:
| Block | Groups | Valence configuration |
|---|---|---|
| s block | 1 and 2, plus helium | ns¹⁻² |
| p block | 13–18 (except helium) | ns²np¹⁻⁶ |
| d block | 3–12 | (n−1)d¹⁻¹⁰ns⁰⁻² (with exceptions) |
| f block | lanthanides and actinides | last electrons go into (n−2)f |
- The s- and p-block elements are the main-group (representative) elements. The d-block elements are the transition metals过渡金属, and the f-block elements are the inner transition metals.
- Chinese textbooks divide the table into five blocks instead. Groups 11 and 12, whose (n−1)d sublevel is already full, form a separate “ds block”. Helium goes in the p block with the rest of the noble gases, while many English books draw it in the s block because its configuration is 1s². Chinese books also label groups ⅠA–ⅦA, ⅠB–ⅦB, Ⅷ, and 0 instead of 1–18.
Reading position from a configuration
- Period: the highest occupied energy level, n.
- s block: the group number is the number of ns electrons (1 or 2).
- p block: the group number is 10 plus the number of ns and np electrons; for example, 2 + 5 = 7 valence electrons puts an element in group 17.
- d block: the group number is the number of (n−1)d and ns electrons added together.
Switch the coloring to Block, then click a few elements to see each one’s electron configuration and block:
The table is wide: swipe sideways to see all of it.
Click any element in the table.
Now work backward: you get only the configuration, and you choose the period, group, and block:
Electron configuration:
Example 1: Where is iron?
Iron has atomic number 26. Where is it on the periodic table?
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Write the configuration. Fill 26 electrons in order of energy:
In noble-gas form, [Ar]4s²3d⁶.
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Period = highest energy level. The highest level is n = 4, so iron is in period 4.
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Block = the sublevel filled last. The last electrons go into 3d, which isn’t full, so iron is in the d block.
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Group: count the valence electrons. For transition elements the valence electrons include the (n−1)d and ns electrons: 6 + 2 = 8, so iron is in group 8.
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A common mistake. Looking only at the outer 4s² and putting iron in group 2. For transition elements, count the 3d electrons too.
Example 2: From position to configuration
An element is in period 4, group 17. Write its valence configuration and its noble-gas configuration. Which element is it?
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The period gives the outer level. Period 4 means the outer level is n = 4.
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The group gives the valence electrons. Group 17 elements have 7 valence electrons, in 4s and 4p: 4s²4p⁵.
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Add the inner electrons. Period 4 elements fill 3d before 4p, so
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Atomic number. 18 + 2 + 10 + 5 = 35: it’s bromine (Br).
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Check. Bromine is in the p block with 7 valence electrons; the filled 3d¹⁰ doesn’t count as valence. A common mistake is leaving out 3d¹⁰ and landing on element 25.
- [Ne]3s¹
The last electron goes into 3s: this is sodium, in group 1 of the s block.
- [Ar]4s²3d¹⁰4p³
The last electrons go into 4p: this is arsenic, in group 15 of the p block.
- [Ar]4s¹3d⁵
This is chromium, an exception to the aufbau principle with a half-filled 3d⁵. It is in group 6 of the d block.
- [Ar]4s¹3d¹⁰
This is copper, another exception with a full 3d¹⁰. It is in group 11 of the d block, not group 1 (Chinese textbooks call this the ds block).
- [Ar]4s²3d¹⁰
This is zinc, in group 12 of the d block, not group 2 (Chinese textbooks call this the ds block).
- [Ar]4s²
The last electrons go into 4s: this is calcium, in group 2 of the s block.
- [Xe]6s²4f⁷
The last electrons go into 4f: this is europium (Eu), a lanthanide in the f block.
- [Ar]4s²3d⁶
This is iron. Its 3d sublevel is not full, so it is in the d block, group 8.
- 1s²
This is helium. By its configuration it belongs to the s block, even though it sits in group 18. (Chinese textbooks put it in the p block with the other noble gases.)
Common mistakes
Mistake 1: “An element with two electrons in its outermost s sublevel is in group 2.” Helium (1s²) is in group 18, iron ([Ar]4s²3d⁶) is in group 8, and zinc ([Ar]4s²3d¹⁰) is in group 12. Most transition metals have ns² outer electrons, so count the (n−1)d electrons too.
Mistake 2: “3d is in level 3, so the 3d elements are in period 3.” The period number is the highest occupied energy level. The 3d sublevel is higher in energy than 4s and fills after it, so the ten elements from scandium to zinc are all in period 4.
Mistake 3: “Every s-block element is a reactive metal.” Hydrogen is in group 1 and the s block, but it is a nonmetal. All the other s-block elements (helium aside) are reactive metals.
Quick check
An element has the configuration [Ar]4s¹3d¹⁰. Where is it on the periodic table?
The highest level is 4, so it is in period 4. Its 4s and 3d electrons add up to 1 + 10 = 11, so it is in group 11 of the d block. This is copper.
How many elements are in period 6?
Period 6 fills 6s, 4f, 5d, and 6p, which hold 2 + 14 + 10 + 6 = 32 electrons, so it has 32 elements.
Summary
- By the aufbau principle, each period starts with ns and ends with np. The period number is the highest occupied energy level, and the periods hold 2, 8, 8, 18, 18, 32, and 32 elements.
- The table has four blocks: s (groups 1–2 and helium), p (groups 13–18), d (groups 3–12, the transition metals), and f (the lanthanides and actinides). Chinese textbooks add a separate ds block for groups 11–12 and put helium in the p block.
- To find the group: in the s block, count ns electrons; in the p block, add 10 to the ns and np electrons; in the d block, add the (n−1)d and ns electrons.
On to AP (for later)
AP U1.5 asks you to write electron configurations for atoms and ions. The fastest way on the exam is to read them off the periodic table: walk across from hydrogen, filling ns as you cross the s block, (n−1)d in the d block, and np in the p block. AP numbers groups 1–18.