CBSE Class 9 Chemistry Chemical Bonding Notes Set 04

Why Atoms Combine

Noble gases (He, Ne, Ar, Kr, Xe) are chemically inert because their outermost shell is complete: 2 electrons for helium and 8 for the others (octet). Atoms of other elements react to reach this stable arrangement and, in doing so, to reach a state of lower energy and greater stability. The electrons in the outermost shell, called valence electrons, take part in bonding; inner electrons do not.

Octet Rule and Lewis Dot Symbols

The tendency of atoms to gain, lose or share electrons so as to have eight electrons in the valence shell is called the octet rule (hydrogen follows the duet of two). G. N. Lewis (1916) represented valence electrons by dots around the symbol of the element; such a picture is called an electron dot (Lewis) symbol.

Ionic (Electrovalent) Bond

An ionic bond is formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom. The metal becomes a positive ion (cation) and the non-metal becomes a negative ion (anion). The oppositely charged ions are held together by electrostatic attraction.
Example: sodium chloride. Sodium (2, 8, 1) loses one electron to give Na⁺ (2, 8), the neon arrangement. Chlorine (2, 8, 7) gains one electron to give Cl⁻ (2, 8, 8), the argon arrangement. The ions attract each other to form crystalline NaCl.
Other examples: MgCl₂ (Mg²⁺ and two Cl⁻), CaO (Ca²⁺ and O²⁻) and AlF₃ (Al³⁺ and three F⁻). In every ionic compound the total positive charge equals the total negative charge, so the compound is neutral overall. The number of electrons lost or gained by an atom is its valency.

Properties of Ionic Compounds

1. Physical state: hard, brittle crystalline solids because ions are packed in a regular lattice.
2. Solubility: dissolve in polar solvents like water but not in non-polar solvents like benzene.
3. Melting and boiling points: high, since strong electrostatic forces hold the ions.
4. Electrical conductivity: solid ionic compounds do not conduct because ions cannot move; they conduct in the molten state and in aqueous solution, when ions are free.

Energy Changes in the Formation of Sodium Chloride

The formation of one mole of NaCl crystal from sodium metal and chlorine gas can be split into steps.
1. Sodium solid to gaseous atoms: 108 kJ absorbed.
2. Gaseous sodium atoms to Na⁺ ions (ionisation energy): 496 kJ absorbed.
3. Half a mole of Cl₂ to gaseous chlorine atoms (dissociation): 121 kJ absorbed.
4. Chlorine atoms to Cl⁻ ions (electron affinity): 349 kJ released.
5. Gaseous ions to crystal (lattice energy): 787 kJ released.
Net energy released = (349 + 787) − (108 + 496 + 121) = 1136 − 725 = 411 kJ per mole. The large release of energy, mostly from the lattice energy, makes the crystal stable.

Covalent Bond

A covalent bond is formed when two atoms, usually of non-metals, share pairs of electrons so that each attains a stable noble gas arrangement. A shared pair is written as a dash.
1. H₂: each hydrogen atom shares its one electron, forming a single bond (H–H) and getting the helium arrangement.
2. Cl₂: each chlorine atom shares one electron to complete its octet. Each atom also has three pairs of electrons not used in bonding (lone pairs).
3. O₂: two pairs are shared, so there is a double bond (O=O).
4. N₂: three pairs are shared, so there is a triple bond (N≡N).
5. H₂O, NH₃ and CH₄: the central atom shares electrons with 2, 3 and 4 hydrogen atoms respectively, so O has two lone pairs, N has one lone pair and C has none.
A few compounds, such as SF₆ and PCl₅, have more than eight electrons round the central atom and are exceptions to the octet rule.

Properties of Covalent Compounds

1. They may be gases, liquids or soft solids.
2. They have low melting and boiling points because the forces between molecules are weak.
3. They usually dissolve in non-polar organic solvents and are insoluble in water (many exceptions exist, such as sugar and alcohol).
4. They do not conduct electricity as they have no free ions or electrons.

Polar Covalent Bond and Electronegativity

When the two bonded atoms are alike, as in H₂ or Cl₂, the shared pair lies midway and the bond is non-polar. When the atoms are different, as in HCl, the shared pair is pulled towards the atom with the greater tendency to attract electrons. This tendency is called electronegativity. In HCl, chlorine gets a small negative charge (δ−) and hydrogen a small positive charge (δ+), forming a dipole; the bond is polar covalent.
Electronegativity increases across a period and decreases down a group. Fluorine is the most electronegative element (4.0 on Pauling's scale). Some values: H 2.1, C 2.5, N 3.0, O 3.5, Cl 3.0, Na 0.9.

Predicting the Type of Bond

The larger the difference in electronegativity between two atoms, the greater the ionic character of the bond. A difference of about 1.7 or more gives a bond that is mostly ionic; a very small difference (for example C–H, 0.4) gives an almost non-polar covalent bond. Some compounds, such as NaOH, NH₄Cl, KMnO₄ and CaSO₄, contain both ionic and covalent bonds: the covalent bonds hold the atoms of the polyatomic ion together and the ionic bond holds the ions together.