Metals and Non-metals

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 tendency of an atom to take part in chemical combination is determined by the number of valence electrons (electrons in the outermost shell of an atom). The atoms acquire the stable noble gas configuration of having eight electrons in the outermost shell (called octect rule) during chemical combination.

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,

Properties of Ionic Compounds

Property	Electrovalent or ionic compounds
Structure of charged ions	They consist of oppositely charged molecules.
Physical state and hardness	The inter-atomic attraction is high, hence they are brittle, hard, crystalline solids.
Melting and boiling points	Due to strong attraction between the particles, high temperatures are required to melt or boil them.
Solubility	They are usually soluble in water, but insoluble in organic solvents.
Passage of electricity	Ionic compounds do not conduct electricity in the solid state because movement of ions in the solid is not possible due to their rigid structure. In the molten form or in aqueous solution form, since the electrostatic forces of attraction between the oppositely charged ions are overcome they allow the flow of electricity, and get decomposed by it.
Rate of reaction	Their reaction usually occurs with high speeds.
Dissociation in solution	Since electrovalent compounds are made up of charged ions, they dissociate to give negative and positive ions in solution.
Electrolysis	These compounds can undergo electrolysis. The cations get discharged at cathode and anions at anode.