Both Group I and Group VII change in a regular way going down, but the directions are opposite because one group loses an electron and the other gains one. A good explanation names what the atom does, then says how distance and shielding change that.
This is the reasoning step in periodic patterns, following the evidence skills in the previous lesson.
What is the explanation built from?
Three ideas make every trend explanation in these two groups.
- Shells increase down a group. Each step down adds an electron shell.
- The outer electron is further from the nucleus and more shielded by the inner shells. The attraction between the nucleus and the outer electron gets weaker.
- What the atom does decides the direction. A metal that loses an electron reacts more easily when that attraction is weaker. A non-metal that gains an electron reacts less easily, because the electron it needs to attract is further away.
The cause is the same in both groups. The consequence differs because losing and gaining are opposite processes.
Worked example
Question: explain why potassium is more reactive than sodium, and why chlorine is more reactive than bromine.
Potassium and sodium. Sodium is 2,8,1 and potassium is 2,8,8,1. Both lose one outer electron to form a 1+ ion.
Potassium’s outer electron is in the fourth shell, further from the nucleus and screened by more inner shells. The attraction is weaker, so it is lost more easily. Potassium is more reactive.
Chlorine and bromine. Chlorine’s outer shell is the third shell; bromine’s is the fourth. Both need one more electron to fill the outer shell.
In chlorine, the incoming electron goes into a shell closer to the nucleus with less shielding, so it is attracted more strongly and gained more easily. In bromine the outer shell is further away, so the attraction is weaker. Chlorine is more reactive.
Two elements, one cause (distance and shielding), two directions.
What mistake goes wrong here?
The frequent error is using one rule for both groups.
Mistaken answer: “Bromine is more reactive than chlorine because it has more electron shells, and more shells make an element more reactive.”
This copies the Group I reasoning. More shells make atoms lose electrons more easily but make them gain electrons less easily.
The correction: first ask “does this atom lose or gain electrons when it reacts?” Metals lose, halogens gain. Then apply the distance idea in the right direction.
The same care applies to physical trends. Down Group I, melting points fall (lithium melts at a higher temperature than caesium). Down Group VII, melting and boiling points rise: chlorine is a gas, bromine a liquid and iodine a solid at room temperature.
Check yourself
1. Explain why rubidium is more reactive than lithium.
Show answer
Both lose one outer electron. In rubidium the outer electron is in a shell much further from the nucleus and screened by more inner shells, so the attraction is weaker and the electron is lost more easily. Rubidium is more reactive.
2. A student says “Fluorine is the least reactive halogen because it is first in the group.” Correct the statement and give the reason.
Show answer
Fluorine is the most reactive halogen. Its incoming electron goes into the second shell, close to the nucleus with little shielding, so the attraction is strong and gaining an electron is easy.
3. Astatine is below iodine in Group VII. Predict its state at room temperature and its reactivity compared with iodine.
Show answer
Melting and boiling points rise down the group, so astatine is predicted to be a solid. Reactivity falls down the group, so astatine is predicted to be less reactive than iodine. Both are predictions from the trend.
Where this leads next
Next, distinguishing a metal property from an exception asks you to decide when a trend or a typical property does not apply. Practise the whole skill with the periodic patterns practice set, and keep the equation balance reasoning trainer close for the equations.
If you write the right idea but in the wrong direction, a teacher in online one-to-one Chemistry tuition can catch it in your own wording.