IGCSE Chemistry 0620: Finding Period and Group from Electron Configuration

A teacher gives four students the electron configurations of four elements, J, K, L and M, and asks each of them to identify that element's position in the Periodic Table.

J: 2,8,2
K: 2,8,7
L: 2,8,8
M: 2,8,1

(a) State the period and the group of element J, explaining how each is found from its electron configuration. [2]

(b) State the period and the group of element K, explaining how each is found from its electron configuration. [2]

(c) Element L has a full outer electron shell. State the group that L belongs to, and identify whether elements in this group are reactive or unreactive. [2]

(d) State the period and the group of element M, and predict whether M is a metal or a non-metal, giving a reason based on its number of outer-shell electrons. [2]

The two rules

Two simple rules connect an electron configuration to a position in the Periodic Table:

Period $=$ the number of occupied electron shells.
Group $=$ the number of electrons in the outer shell.

Part (a): Element J (2,8,2)

J has three numbers in its configuration, so it has 3 occupied shells $\rightarrow$ J is in Period 3.

The last (outer) shell holds 2 electrons $\rightarrow$ J is in Group II.

Part (b): Element K (2,8,7)

K also has three occupied shells $\rightarrow$ K is in Period 3.

The outer shell holds 7 electrons $\rightarrow$ K is in Group VII.

Part (c): Element L (2,8,8)

L’s outer shell holds 8 electrons, which is a full outer shell. A full outer shell places an element in Group VIII (also written Group 0).

Elements with a full outer shell have no tendency to lose, gain or share electrons, so elements in Group VIII are unreactive. These are the noble gases.

Part (d): Element M (2,8,1)

M has three occupied shells $\rightarrow$ Period 3. The outer shell holds only 1 electron $\rightarrow$ Group I.

An atom with only 1 outer-shell electron can reach a stable, full outer shell most easily by losing that single electron to form a $1+$ ion. Losing electrons to form positive ions is characteristic of metals, so M is predicted to be a metal.

Final answers

(a) J: Period 3, Group II.
(b) K: Period 3, Group VII.
(c) L: Group VIII (Group 0); elements in this group are unreactive.
(d) M: Period 3, Group I; M is a metal, because its single outer electron is readily lost.