2.1.2Atoms, Elements and Compounds
Group 1- Reactivity and Electronic Configurations — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 26 Unit quiz
Part 1 of 5Electronic Structure
Group 1 atoms have a single outer electron, and the number of filled shells increases down the group.
Outer electron
- Every Group 1 atom has one electron in its outer shell, and this is the electron lost to form a 1+ ion.
Noble gas arrangement
- After losing the outer electron, the ion has a full outer shell, the same arrangement as the noble gas before it.
Extra shells
- Each step down the group adds one more filled electron shell, so the outer electron lies further from the nucleus.
Shielding
- The inner electron shells block part of the nucleus's attraction, so the outer electron feels a weaker pull.
Part 2 of 5Explaining the Trend
1
Lithium, sodium and potassium each have one outer electron, which they lose to form 1+ ions.
2
Going down the group, the outer electron lies in a shell further from the nucleus.
3
The electrostatic attraction between the nucleus and the outer electron becomes weaker as the distance increases and shielding grows.
4
Less energy is needed to remove the outer electron, so the atom loses it more easily.
5
Reactivity therefore increases down Group 1, and francium is the most reactive member of the group.
Part 3 of 5Predictions from the Trend
Predictions from the Trend
Melting point
- Melting points fall down the group, so caesium is expected to melt at a lower temperature than sodium.
Reactions
- Heavier Group 1 metals react more vigorously with water and oxygen, which allows the behaviour of rubidium to be predicted.
Limits of trends
- Trends describe overall patterns rather than strict rules, so predictions should be stated as likely rather than certain.
| Element | Electron arrangement | Outer electrons |
|---|---|---|
| Lithium | 2,1 | 1 |
| Sodium | 2,8,1 | 1 |
| Potassium | 2,8,8,1 | 1 |
Part 4 of 5Worked Example
Question Explain why potassium is more reactive than sodium.
- Potassium has the arrangement 2,8,8,1 and sodium has 2,8,1.
- Potassium's outer electron is in the fourth shell, while sodium's is in the third shell, so it is further from the nucleus.
- Potassium has more inner shells to shield the nucleus, so the attraction on its outer electron is weaker.
- A weaker attraction means less energy is needed to remove the outer electron, so potassium loses it more readily.
Answer Potassium's outer electron is further from the nucleus and more shielded, so it is lost more easily.
Part 5 of 5Exam Checklist & Tips
2.1.2 – Key Points for the Exam
- Electron shell A layer of electrons at a fixed distance from the nucleus of an atom.
- Shielding The reduction in attraction on outer electrons caused by inner electron shells.
- Electronic configuration The arrangement of electrons in shells around an atom's nucleus.
- Reactivity How readily a substance undergoes a chemical reaction.
- Electrostatic attraction The force between a positive nucleus and negative electrons.
Common Mistakes
- Explaining the trend by saying the atoms are bigger so they want to react;
- refer to the attraction on the outer electron.
- Stating that Group 1 atoms gain electrons; they lose their single outer electron.
- Forgetting that each step down the group adds one more filled shell.
- Saying the nucleus attracts the outer electron more strongly lower down;
- the attraction is weaker because of distance and shielding.
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