With our new knowledge, we could now go through the range of atoms one by one and draw their atomic structure, and try to determine how reactive each one might be by looking at the number of electrons in its outer shell.

Remember: the first shell wants 2 electrons, the second, third and fourth want 8 electrons.


Protons
Electrons
Element Name
Symbol
Electrons in shells
Reactive ?
1
1
Hydrogen
H
1
Very
2
2
Helium
He
2
Not at all (ideal !)
3
3
Lithium
Li
2,1
Very
4
4
Beryllium
Be
2,2
Fairly
5
5
Boron
B
2,3
Some
6
6
Carbon
C
2,4
Some
7
7
Nitrogen
N
2,5
Some
8
8
Oxygen
O
2,6
Fairly
9
9
Flourine
F
2,7
Very
10
10
Neon
Ne
2,8
Not at all (ideal !)
11
11
Sodium
Na
2,8,1
Very
12
12
Magnesium
Mg
2,8,2
Fairly
13
13
Aluminium
Al
2,8,3
Some
14
14
Silicon
Si
2,8,4
Some
15
15
Phosphorus
P
2,8,5
Some
16
16
Sulphur
S
2,8,6
Fairly
17
17
Chlorine
Cl
2,8,7
Very
18
18
Argon
Ar
2,8,8
Not at all (ideal !)
19
19
Potassium
K
2,8,8,1
Very

Can you see a pattern? The numbers of electrons in the outer shell give different elements similar properties. Those with ideal outer shells are unreactive. The elements either side of this, that are nearly ideal are very reactive. Those further away from being ideal are not so reactive.

Lets reorder the elements in the table above, grouping them by level of reactivity. It is easy to see in the table below that elements with 8 electrons in the outer shell are all stable, those with 1 or 7 electrons in the outer shell are very reactive :

Outer shell electrons
1
2
3
4
5
6
7
8
Reactive ?
Very
Fairly
Some
Some
Some
Fairly
Very
Not
Shell 1
H
He*
Shell 2
Li
Be
Bo
C
N
O
Fl
Ne
Shell 3
Na
Mg
Al
Si
P
S
Cl
Ar
Shell 4
K
??
??
*although Helium has only 2 outer shell electrons, it is a full (ideal) number for shell 1, so we put it in with the other unreactive elements with ideal shells in column 8, not in column 2.

Once we have the elements in this format, we can actually go one step further and start guessing elements.

For example, we can predict that an element exists in the yellow cell in column 2 (next to Potassium [K]). It will have 20 protons and electrons (one more than the 19 protons and electrons that K has). This mystery element's electrons will be arranged 2,8,8,2 - thus with 2 in its outer shell it will be fairly reactive.

Bravo - you have just "discovered" an element. This element is Calcium [Ca] (2,8,8,2).

Further along the row, there is a mystery red cell in column 8 (under Argon [Ar]). We know that there will be an element with 26 electrons (six more than the Calcium that we just discovered) in the configuration 2,8,8,8. As this element has an "ideal" outer shell, it will be unreactive.
This element is Krypton [Kr] (2,8,8,8).

This method is how early scientists began to discover the elements. They loooked for gaps in this table, and realised that there must be an element to fill it.