2.3.4 Electron Arrangement in Atom

1. We have learnt that electrons occupy orbits with definite energy level of an atom, as suggested by Neils Bohr.
2. These orbits with definite energy level are known as the shell.
3. Every single shell is capable of holding up to certain amount of electrons.
4. The first shell can hold up to two electrons. This is called a duplet.
5. The second shell can hold up to eight electrons. This is called an octet.
6. The third shell can hold up to eighteen electrons.
   
   
7. However, with the third shell, when eight electrons are present, extra stability is gained. The additional electrons go into the fourth shell before the third shell is completely filled.
8. The way in which the electrons are distributed in the shells of an atom is called the electron arrangement of the atom
9. The examples below show the electron arrangement of some elements:

Atom	Notes	Electrons Arrangement
	Lithium has 3 protons and 3 neutrons and three electrons as well. All the three electrons are arrange as follows: Two electrons are filled in the first shell. One electron is filled in the second shell. The electron arrangement of carbon is 2.1	2.1
	Chlorine has 17 protons and 18 neutrons and 17 electrons. All the three electrons are arrange as follows: Two electrons are filled in the first shell. Eight electrons are filled in the second shell. Seven electrons are filled in the third shell. The electron arrangement of chlorine is 2.8.7.	2.8.7
	Calcium has 20 protons and 20 neutrons and 20 electrons. All the three electrons are arrange as follows: Two electrons are filled in the first shell. Eight electrons are filled in the second shell. Eight electrons are filled in the third shell. Two electrons are filled in the forth shell. The electron arrangement of carbon is 2.8.8.2.	2.8.8.2

Table below shows the arrangement of electrons of the first 20 elements in the periodic table. We shall learn periodic table in chapter 4.