So, the first 20 in the start with **H** and end with **Ca**. The quickest way to remember the number of is to form a relationship with the number of the group the element is located in.

I’ll skip noble gases, since these are elements which have a stable , and as a result are usually not a part of chemical reactions.

Starting with the second row:

Group 1 – **Li** – has 1 valence electron (v.e);Group 2 – **Be** – has 2 v.e;Group 13 – **B** – has 3 v.e;Group 14 – **C** – has 4 v.e;Group 15 – **N** – has 5 v.e;Group 16 – **O** – has 6 v.e;Group 17 – **F** – has 7 v.e;

The group number jumps from 2 to 13 because the first 20 elements are not found in groups 3-12.

Then the pattern repeats itself in the third row. Again,

Group 1 – **Na** – has 1 v.e;Group 2 – **Mg** – has 2 v.e;Group 13 – **Al** – has 3 v.e;Group 14 – **Si** – has 4 v.e;

and so on.

So, if you can examine a periodic table, the number of can be determined if you look at group number.

The digit that represents the units (1 in 1, 3 in 13, 4 in 14, 6 in 16, and so on) is equal to the number of .

**K** and **Ca** follow the same pattern. For row 4,

Group 1 – **K** – has 1 v.e;Group 2 – **Ca** – has 2 v.e;

So, always think group number ##->## units digit ##->## valence electrons.