Ionic or Electrovalent BondIonic bond is formed by the complete transfer of one or two valence electrons from one atom to another. As a result, both the combining atoms acquire positive and negative charges. These two oppositely charged ions are held by a strong electrostatic force of attraction. This force of attraction binding the two ions together is known as ionic bonding.
Electrovalency: The number of electrovalent or ionic bonds an atom can form is called its electrovalency. The electrovalency of an element is therefore equal to the number of electrons lost or gained by the atom to form an ion.
The chemical bond formed between the two atoms by mutual sharing of electrons between them is known as a covalent bond.For example, Cl2, CCl4, H2O, NH3 etc. Atoms linked by covalent bonds are called molecules.
The ability of an atom to pull the electron pair(s) in a bond towards itself in a molecule is called the elecÂtronegativity of the atom. It is a relative concept which has no unit.
Depending on the number of electron pairs shared between the two atoms, the types of covalent bonds formed may be a
- single covalent bond,
- double covalent bond or
- triple covalent bond
- Single covalent bond: A single covalent bond is formed when one electron pair is shared between the two atoms. Some compounds which possess single covalent bond include H2, CH4, NH3, Cl2, F2 and H2O.
- For the sake of understanding the force of attraction between atoms, a molecule is represented with lines joining the symbol of atoms.
In reality, no such thing exists.
- Whenever an electron pair is shown to be held between atoms, it does not mean that it remains in a fixed position between the two atoms. In reality, they would be revolving around the nucleus of both the atoms.
- Double covalent bond: A double covalent bond is formed when two pairs of electrons are shared between the two combining atoms. The compounds which exhibit double covalent bond are ethylene (C2H4), CO2 and O2.
- Triple covalent bond: A triple covalent bond is formed when three pairs of electrons are shared between the two combining atoms. Some compounds which possess triple covalent bonds include acetylene (C2H2), hydrogen cyanide (HCN) and nitrogen N2.
Shows the differences between the polar and non-polar covalent bonds.
Table: Differences between Polar and Non-polar Covalent Compounds
|Polar Covalent Compounds||Non-polar Covalent Compounds|
|Formed between atoms of different elements wherein the bonded electron pair(s) is not shared equally between the atoms.||Formed between atoms of the same elements wherein the bonded electron pair(s) is shared equally among the atoms.|
|Exhibit ionic characteristics, e.g. HF, H2O, NH3, Hd+ Fd- etc.||Do not exhibit ionic characteristics, e.g. H2, O2, N2 etc.
|Elements on the right of the periodic table are more electronegative than those on the left. The decreasing order of the electronegative elements can be represented as follows:
F > O > N > Cl > S > P
Hydrogen BondingIt is the electrostatic force of attraction between the hydrogen atom which is covalently bonded to highly electronegative atoms such as F, O or N in a polar covalent molecule such as H2O, NH3 or HF and the electronegative atom of either the same molecule or another molecule. A hydrogen bond is weaker than covalent, ionic and metallic bonds.
- If aqueous hydrogen bonds were stronger, then water would behave similar to glass, whereas if they are weaker, then water would be a gas and exists only as a liquid at sub-zero temperature.
- The high density of liquid water is also due to the cohesive nature of the hydrogen-bonded network.
- Even a covalent substance such as sugar dissolves in water because of hydrogen bonding.
Metallic BondingThe force that binds together the atoms of metals is known as a metallic bond. To account for bonding in metals, Lorentz proposed a model known as an electron gas model or an electron sea model.
|Ionisation energy is the minimum energy required to remove the outermost electron from a neutral gaseous atom of an element to form a gaseous ion.|
Van der Waals Forces
This is the attractive or repulsive force between molecules other than those due to covalent bonds or electrostatic interaction of ions with one another or with neutral molecules. The attraction is much weaker than a chemical bond.