An electron arrangement lists how many electrons sit in each shell, starting from the one closest to the nucleus. For the first twenty elements the shells hold 2, then 8, then 8, then the remainder. The last number tells you the group, and the number of shells tells you the period, so the arrangement is your evidence in any pattern explanation.
This follows determining particles in a simple ion and explains why isotopes of one element react identically. It feeds into bonding and structure.
How do I write an electron arrangement?
- Find the number of electrons (the proton number for a neutral atom).
- Put 2 in the first shell.
- Put up to 8 in the second shell.
- Put up to 8 in the third shell, then start the fourth.
- Check the numbers add up to the electron count.
| Element | Electrons | Arrangement | Group | Period |
|---|---|---|---|---|
| Lithium | 3 | 2,1 | I | 2 |
| Sodium | 11 | 2,8,1 | I | 3 |
| Potassium | 19 | 2,8,8,1 | I | 4 |
| Chlorine | 17 | 2,8,7 | VII | 3 |
| Argon | 18 | 2,8,8 | 0 | 3 |
Worked example
Explain why lithium, sodium and potassium are in the same group, and why potassium reacts more readily than lithium when each loses its outer electron.
Step 1, arrangements: Li is 2,1. Na is 2,8,1. K is 2,8,8,1.
Step 2, same group: each has one outer-shell electron, so each is in Group I and they react in a similar way, by losing that electron to form a 1+ ion.
Step 3, period: the number of shells increases from 2 to 3 to 4, so lithium is in Period 2 and potassium is in Period 4.
Step 4, difference in reactivity: in potassium the outer electron is in the fourth shell, further from the nucleus and more shielded by the inner electrons. It is held less strongly, so it is lost more easily.
Answer: the same outer electron count explains the similarity. The greater distance of the outer shell explains why potassium is more reactive than lithium.
The mistake to watch for
A common slip is to read the group from the number of shells, or to overfill a shell.
Mistaken answer for potassium: 2,8,9. Group IX.
The student kept filling the third shell past 8 and then treated the last number as a group.
The correction is to fill 2, 8, 8 first for elements up to calcium, then start the next shell. Potassium has 19 electrons: 2 + 8 + 8 = 18, leaving 1 electron, so 2,8,8,1 in Group I.
Another slip is to write “Group I has one shell”. It has one outer electron. The number of shells gives the period.
Check yourself
1. Write the electron arrangement of sulfur (16 electrons) and state its group and period.
Show answer
2 + 8 = 10, remaining 6. Arrangement 2,8,6. Six outer electrons gives Group VI. Three shells gives Period 3.
2. An atom has the arrangement 2,8,3. How many protons does it have if it is neutral, and which group is it in?
Show answer
Electrons = 2 + 8 + 3 = 13, so protons = 13 (this is aluminium). Three outer electrons gives Group III.
3. Why do neon (2,8) and argon (2,8,8) both show very little tendency to react?
Show answer
Both have a full outer shell, so they have no need to lose, gain or share electrons. A full outer shell is a stable arrangement.
Where this leads next
The next lesson separates an element from a compound using the atoms they contain: distinguishing an element from a compound sample. Test the pattern skills in the mixed practice set, and use the equation balance reasoning trainer when arrangements lead into formulas.
If your arrangements are right but your explanation still reads like a list of numbers, a teacher in online one-to-one Chemistry tuition can help you turn each number into a reason.