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there is one sublevel (s), the second one has two (s, p), at the third one there are

three (s, p, d), and the fourth one has four (s, p, d, f).

2
The maximum number of electrons at the s-sub level is two (s ), at the p-
6
10
sublevel is six (p ), at the d-sublevel is ten (d ), and at the f-sublevel is fourteen
14
(f ).
The order of electron filling of energy levels and sublevels

Period number Electron filling of energy levels and sublevels
1 1s 2

6
2
2 2s 2p
2
3 3s 3p 6
10
2
4 4s 3d 4p 6
10
2
5 5s 4d 5p 6
1
9
14
2
6 6s 5 d 4f 5d 6p 6
2
14
9
1
7 7s 6d 5f 6d 7p 6

Lithium, Sodium, Potassium, Cesium, Rubidium, Francium atoms at the last
energy level have one electron. All of them are the most active metals (alkaline).

The total electron formula for the last energy level for these elements is described

1
by the formula ns , where n is the period number. For example, the full electron
formula of Sodium is as follows:

2
2
6
11Na 1s 2s 2p 3s 1

The electrons of the last energy level are valence, which means that they are

involved in the formation of chemical bonds. For Sodium valence electrons are

1
3s -electrons.
There are two electrons at the last energy level in Beryllium, Magnesium,

Calcium, Strontium, Barium, and Radium atoms. All of them are active metals

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