Atomic Structure and Isotopes: Question 7

Worked solution

Part (a): Electronic configuration of aluminium and phosphorus

Electrons fill shells from the innermost outward, with a maximum of 2 in the first shell and up to 8 in the second and third shells (for these proton numbers).

• Aluminium (proton number $13$): $2, 8, 3$
• Phosphorus (proton number $15$): $2, 8, 5$

Part (b): Outer-shell electrons and group number

• Aluminium’s outer shell has $3$ electrons, so aluminium is in Group III.
• Phosphorus’s outer shell has $5$ electrons, so phosphorus is in Group V.

For Groups I to VII, the number of outer-shell electrons is equal to the group number.

Part (c): Electronic configuration of potassium

Potassium has proton number $19$. The first shell holds $2$ electrons and the second shell holds $8$, using $10$ electrons. The third shell can hold up to $8$ more, using a further $8$ electrons ($18$ so far). The $19$th electron cannot fit into the third shell (already full at $8$), so it starts a new, fourth shell:

$2, 8, 8, 1$

Potassium has $1$ outer-shell electron, so it is in Group I.

Final answers

• (a) Aluminium: $2,8,3$; phosphorus: $2,8,5$.
• (b) Aluminium is in $\boxed{\text{Group III}}$; phosphorus is in $\boxed{\text{Group V}}$.
• (c) Potassium: $2,8,8,1$, in $\boxed{\text{Group I}}$.