# 2.3.2 Electrons Arrangement in Atom

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## 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.
 Element Proton Number Number of Electron Number of electron in Electron Arrangement 1st shell 2nd shell 3rd shell 4th shell Hydrogen 1 1 1 0 0 0 1 Helium 2 2 2 0 0 0 2 Lithium 3 3 2 1 0 0 2.1 Beryllium 4 4 2 2 0 0 2.2 Boron 5 5 2 3 0 0 2.3 Carbon 6 6 2 4 0 0 2.4 Nitrogen 7 7 2 5 0 0 2.5 Oxygen 8 8 2 6 0 0 2.6 Fluorine 9 9 2 7 0 0 2.7 Neon 10 10 2 8 0 0 2.8 Sodium 11 11 2 8 1 0 2.8.1 Magnesium 12 12 2 8 2 0 2.8.2 Aluminium 13 13 2 8 3 0 2.8.3 Silicon 14 14 2 8 4 0 2.8.4 Phosphorus 15 15 2 8 5 0 2.8.5 Sulphur 16 16 2 8 6 0 2.8.6 Chlorine 17 17 2 8 7 0 2.8.7 Argon 18 18 2 8 8 0 2.8.8 Potassium 19 19 2 8 8 1 2.8.8.1 Calcium 20 20 2 8 8 2 2.8.8.2

## Valence Electron

Valence electrons are the electrons in the outermost shell.
1. The electrons in the outermost shell of an atom are called valence electrons.
2. The valence electrons have great significance in determining the chemical properties of an atom.
3. Elements with the same number of valence electron have the same chemical properties.

Example:
Given that a sodium atom has 11 protons 12 neutrons. Find the number of valence electron in a sodium atom.Answer:
For an atom,

Number of electrons = number of protons = 11

Electron arrangement of sodium = 2.8.1

Therefore, sodium has 1 valence electron.