We are familiar with the fact that the atoms of all elements, except noble gases, have an incomplete outermost shell. Consequently, most elements are reactive and they try to achieve the stability of the noble or inert gases by electron transfer or by electron sharing. After this combination, the molecule becomes much less reactive. Noble gases have their outermost shell complete and hence they are unreactive or "inert".
Elements that can donate electrons are called metals. They form positive ions by losing electrons. The elements that accept electrons are called non-metals. They form negative ions by gaining electrons. Metals have 1 to 3 electrons in the outermost shell of their atom and non-metals have 4 to 8 electrons in the outermost shell. There are two exceptions to this rule: Hydrogen and helium. Hydrogen is a non-metal having 1 electron in the valence shell and helium too is a non-metal having 2 electrons in the valence shell.The electronic configuration of some of the elements is shown here.
| Type of Elements | Element | Atomic Number | Number of Electrons in Shells | |||
| K | L | M | N | |||
| Noble Gases | Hellium (He) | 2 | 2 | |||
| Neon (Ne) | 10 | 2 | 8 | |||
| Argon (Ar) | 18 | 2 | 8 | 8 | ||
| Metals | Sodium (Na) | 11 | 2 | 8 | 1 | |
| Magnesium (Mg) | 12 | 2 | 8 | 2 | ||
| Aluminium (Al) | 13 | 2 | 8 | 3 | ||
| Potassium (K) | 19 | 2 | 8 | 8 | 1 | |
| Calcium (Ca) | 20 | 2 | 8 | 8 | 2 | |
| Non-metals | Nitrogen (N) | 7 | 2 | 5 | ||
| Oxygen (O) | 8 | 2 | 6 | |||
| Fluorine (F) | 9 | 2 | 7 | |||
| Phosphorus (P) | 15 | 2 | 8 | 5 | ||
| Sulphur (S) | 16 | 2 | 8 | 6 | ||
| Chlorine (Cl) | 17 | 2 | 8 | 7 |
Metals that donate electrons gain positive charge equal to the number of electrons donated. For example, atomic number of aluminium is 13, so the electronic configuration of Al is 2, 8, and 3. Aluminium has 3 electrons in the valence shell; it loses 3 electrons to form Al3+.
For example,
Non-metals gain electrons gain negative charge equal to the number of electrons accepted.
For example,
The combination of atoms occurs in two ways: either by electrovalent bonding or covalent bonding. In all chemical reactions, it is the electrons from the outermost shell of an atom that are involved in interacting with other atoms, either by their transfer or by sharing.
Electrovalent Bonding
When an atom donates one, two or three electrons from its valence shell to another atom, which has the ability to accept these electrons, it is known as electrovalency. As a result of electrovalency, both these atoms achieve the structure of an inert gas. When the chemical bond occurs by the transfer of electrons from the atom of an element to the atom or atoms of another it is called Ionic or Electrovalent bond.
Thus, the electrovalency of sodium is 1+, and that of chlorine is 1- in NaCl. Similarly, calcium, magnesium in their chloride exhibits an electrovalency of 2+. There are many elements, which show different electrovalencies in different compounds. This phenomenon is called 'variable electrovalency' e.g., iron exists as Fe2+ and Fe3+ in ferrous sulphate and ferric sulphate respectively.Formation of Sodium Chloride
During the formation of an ionic bond between the metal sodium and the non-metal chlorine, sodium loses one electron to complete its octet as it has only one electron in its valence shell. It acquires a noble gas configuration of neon (2, 8). While the chlorine atom has seven electrons in its valence shell and gains one electron to complete its octet and also acquires stable electronic configuration of argon.
Formation of Magnesium Chloride
Magnesium, whose atomic number is 12, has 2, 8, 2 configuration. Its valence shell has two electrons. The electronic configuration of chlorine (At. no. 17) is 2, 8, 7. It has seven valence electrons. Since, magnesium has two electrons in excess of the neon configuration (2, 8), and chlorine is one electron short of the argon configuration (2,8,8), hence one atom of magnesium will look for two atoms of chlorine to transfer its two electrons to (one to each) as shown below:
The Mg2+ and the two Cl- so formed, then form ionic bonds between them.
In terms of Lewis dot structure,
