The Periodic Table

Science — First Preparatory

The Periodic Table

Finding an element’s position from its electron arrangement

To find the position of an element
write its electron arrangement and apply these two rules:

Period number = number of levels that contain electrons.
group number = number of electrons in the outer level. For groups 13 to 18, add 10 to that number. For example, aluminium has 3 outer electrons and is in group 13.

Element	Atomic number	Electron arrangement	Period	Group
Sodium Na	11	2, 8, 1	3	1
Oxygen O	8	2, 6	2	16
Neon Ne	10	2, 8	2	18
Chlorine Cl	17	2, 8, 7	3	17

Note→applying the group rule to aluminium (atomic number 13, arrangement 2, 8, 3) gives group 13, because it has 3 outer electrons, and it lies in period 3.

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