Today's lesson is a quick check-up on everything you've learned this month about atomic structure and the Periodic Table. Instead of new content, you'll test yourself on electron configurations, periods, groups, and blocks — the same ideas we've been building since lesson 1. Use this as a chance to spot any gaps before we move to bonding next month.
Quick Recap
Before you start, remind yourself of these key ideas:
- The period number tells you the highest occupied energy level (principal quantum number, n) for an atom's outer electrons (Talbot, p.233).
- Elements in the same group share the same number of valence electrons, which is why they behave similarly.
- The Periodic Table is divided into blocks (s, p, d, f) based on which sublevel is being filled last.
- Table of orbitals filled per period (Talbot, p.239):
| Period | Orbitals filled | Number of elements |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s, 2p | 8 |
| 3 | 3s, 3p | 8 |
| 4 | 4s, 3d, 4p | 18 |
| 5 | 5s, 4d, 5p | 18 |
| 6 | 6s, 4f, 5d, 6p | 32 |
| 7 | 7s, 5f, 6d, 7p | 32 |
- To write a condensed electron configuration, find the nearest noble gas before your element, put it in square brackets, then continue filling from there (Talbot, p.241).
Worked Example (Reminder)
Question: Predict the condensed electron configuration of tin (Sn).
Answer: Tin sits in period 5, second column of the p-block, so its configuration ends in 5p². The nearest earlier noble gas is krypton. So:
Sn: [Kr] 5s² 4d¹⁰ 5p²
(Talbot, p.241)
Practice Set
Try these five questions on your own before checking the answers. No peeking!
1. Which period and block does the element with electron configuration [Ar] 4s² 3d¹⁰ 4p³ belong to? Name the element.
2. How many elements are in period 6, and which four sublevels are filled across it?
3. Write the full (uncondensed) electron configuration for calcium (Ca).
4. Element X is in group 17. How many valence electrons does it have, and what does this tell you about its chemical behaviour compared to other group 17 elements?
5. Explain briefly why the lanthanoids and actinoids are usually shown separately at the bottom of the Periodic Table rather than within the main body of the table.
Answers
1. This is period 4, p-block. Counting protons (18 + 10 + 3 = 31 electrons) gives gallium (Ga).
2. Period 6 has 32 elements; the sublevels filled are 6s, 4f, 5d, and 6p (Talbot, p.239).
3. Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² (Talbot, p.241).
4. Group 17 elements (halogens) have 7 valence electrons. This means they all tend to gain one electron to complete an octet, giving them similar reactive, non-metallic chemical behaviour, though reactivity decreases going down the group.
5. They are separated purely for layout convenience — including all 14 f-block elements in periods 6 and 7 within the main table would make it far too wide to display neatly (Talbot, p.239).
Key Takeaways
- The period number matches the highest principal energy level occupied by electrons.
- Groups share valence electron counts, giving similar chemical properties.
- Blocks (s, p, d, f) are defined by the sublevel being filled last in an atom's configuration.
- Condensed electron configurations use the previous noble gas as a shorthand starting point.
- Lanthanoids and actinoids are moved below the table only for space reasons, not because they belong elsewhere chemically.