1.4.4Principles of Chemistry
Electronic Configuration and Reactivity — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 21 of 50
Part 1 of 5Groups and outer electrons
Group number rule
- Group number rule For the main groups, the group number equals the number of electrons in the outer shell.
- Sodium is in Group 1 and has the configuration 2,8,1, so it has one outer electron.
Similar chemistry
- Similar chemistry Elements in the same group have the same number of outer electrons,
- so they lose, gain or share electrons in the same way and show similar reactions.
Helium is an exception
- Helium is an exception Helium is in Group 0 but has only two electrons, filling its first shell, instead of eight.
Part 2 of 5Predicting from groups
1Same outer electrons
- Same outer electrons Elements in a group have the same number of electrons in their outer shell.
2Same type of reaction
- Same type of reaction They therefore react in a similar way, forming similar compounds.
3Use one to predict others
- Use one to predict others Observing how one element in a group reacts allows you to predict how the others will behave.
4Trends in reactivity
- Trends in reactivity Reactivity changes steadily down a group: Group 1 metals get more reactive going down, while Group 7 elements get less reactive.
Part 3 of 5Why noble gases are unreactive
Group 0: full outer shells
Full outer shell
- Full outer shell Noble gases already have a complete outer shell of electrons: 2 for helium and 8 for the others.
- Atoms react to achieve a full outer shell, so these atoms have no need to lose, gain or share electrons.
Configurations
- Configurations Helium is 2, neon is 2,8 and argon is 2,8,8.
Properties
- Properties They are non-metals that exist as single atoms (monatomic), and they are colourless gases at room temperature that do not burn.
- Their inertness makes them useful where a non-reactive atmosphere is needed.
Part 4 of 5Configurations of selected elements
| Element | Electronic configuration | Group | Reactivity |
|---|---|---|---|
| Lithium | 2,1 | 1 | Reactive metal, loses one electron |
| Sodium | 2,8,1 | 1 | More reactive than lithium |
| Fluorine | 2,7 | 7 | Very reactive non-metal, gains one electron |
| Chlorine | 2,8,7 | 7 | Less reactive than fluorine |
| Neon | 2,8 | 0 | Unreactive, full outer shell |
Part 5 of 5Writing a configuration
1
Find the atomic number, which is the number of electrons in the neutral atom.
2
Place up to 2 electrons in the first shell.
3
Place up to 8 electrons in the second shell and then up to 8 in the third shell.
4
Write the numbers separated by commas, such as 2,8,3 for aluminium.
5
Check that the last number matches the group number for main-group elements.
Key terms
| Term | Meaning |
|---|---|
| Outer shell | The highest energy level of an atom that contains electrons. |
| Electronic configuration | The arrangement of electrons in the shells of an atom, such as 2,8,1. |
| Noble gases | The unreactive elements in Group 0 with full outer shells. |
| Monatomic | Existing as single, separate atoms. |
| Inert | Unreactive and not taking part in chemical reactions. |
Avoid these mistakes
1
Saying noble gases have 8 outer electrons in every case. Helium has only 2, which is a full first shell.
2
Explaining the similarity of group reactions by 'the same number of shells'. It is the same number of outer electrons.
3
Saying noble gases are unreactive because they are gases. The reason is the full outer shell of electrons.
4
Forgetting that reactivity trends go opposite ways: Group 1 increases down the group and Group 7 decreases.
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