Question 16
Explain with the help of electron dot diagrams the formation of the following molecules, stating the valency of each element involved.
(a) Hydrogen
(b) Chlorine
(c) Oxygen
(d) Nitrogen
(e) Water
(f) Methane
(g) Carbon tetrachloride
(h) Ammonia
(i) Carbon dioxide
[at. nos. H = 1, C = 6, N = 7, O = 8, Cl = 17]
(a) Formation of Hydrogen molecule (H2) — The valency of hydrogen element is 1. It needs one electron to attain stable duplet structure of nearest noble gas - He [2].
Each of the 'H' atoms contribute one electron so as to have one shared pair of electrons between them. Both atoms attain stable duplet structure resulting in the formation of a single covalent bond [H-H] between them.
Electron Dot Structure:
(b) Formation of Chlorine (Cl2) — The valency of chlorine element is 1. It needs one electron to attain stable octet structure of nearest noble gas - Ar [2, 8, 8].
Each of the 'Cl' atoms contribute one electron so as to have one shared pair of electrons between them. Both atoms attain stable octet structure resulting in the formation of a single covalent bond [Cl-Cl] between them.
Electron Dot Structure:
(c) Formation of Oxygen molecule (O2) — The valency of oxygen element is 2. It needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8].
Each of the two 'O' atoms contribute two electrons so as to have two shared pair of electrons between them. Both atoms attain stable octet structure resulting in the formation of a double covalent bond [O=O] between them.
Electron Dot Structure:
(d) Formation of Nitrogen Molecule (N2) — The valency of nitrogen element is 3. Nitrogen needs three electrons to attain stable octet structure of nearest noble gas - Ne [2,8].
Each of the two N atoms contributes three electron so as to have three shared pair of electrons between them. Both atoms attain stable octet structure, resulting in the formation of a triple covalent bond [N≡N] between them.
Electron Dot Structure:
(e) Formation of Water Molecule (H2O) — The valency of hydrogen element is 1 and that of oxygen is 2. Hydrogen needs one electrons to attain stable duplet structure of nearest noble gas - He [2] and oxygen needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
Each of the two hydrogen atoms shares an electron pair with the oxygen atom such that hydrogen acquires a duplet configuration and oxygen an octet configuration resulting in the formation of two single covalent bonds [H-O-H] in the molecule of water.
Electron Dot Structure:
(f) Formation of Methane Molecule (CH4) — The valency of hydrogen element is 1 and that of carbon is 4. Hydrogen needs one electron to attain stable duplet structure of nearest noble gas - He [2] and carbon needs four electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
One carbon atom shares four electron pairs one with each of the four atoms of hydrogen such that hydrogen acquires a duplet configuration and carbon an octet configuration resulting in the formation of four single covalent bonds.
Electron Dot Structure:
(g) Formation of Carbon tetrachloride Molecule (CCl4) — The valency of carbon element is 4 and that of chlorine is 1. Chlorine needs one electron to attain stable octet structure of nearest noble gas - Ar [2,8,8] and Carbon needs four electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
One carbon atom shares four electron pairs one with each of the four atoms of chlorine such that chlorine and carbon attain a stable octet configuration resulting in the formation of four single covalent bonds.
Electron Dot Structure:
(h) Formation of Ammonia Molecule (NH3) — The valency of hydrogen element is 1 and that of nitrogen is 3. Hydrogen needs one electrons to attain stable duplet structure of nearest noble gas - He [2] and nitrogen needs three electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
One nitrogen atom shares three electron pairs one with each of the three atoms of hydrogen such that hydrogen acquires a duplet configuration and carbon attain a stable octet configuration resulting in the formation of three single covalent bonds .
Electron Dot Structure:
(i) Formation of Carbon dioxide Molecule (CO2) — The valency of carbon element is 4 and that of oxygen is 2. Carbon needs four electrons to attain stable octet structure of nearest noble gas - Ne [2,8] and oxygen needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
One carbon atom shares two electron pairs one with each of the two atoms of oxygen such that oxygen and carbon attain a stable octet configuration resulting in the formation of two double covalent bonds.
Electron Dot Structure:
Chapter Overview: Chemical Bonding
Chemical bonding explains how atoms combine to form molecules and compounds. Atoms bond to achieve a stable electronic configuration (octet or duplet). The three primary types of bonds are ionic (electrovalent), covalent, and coordinate (dative) bonds. Ionic bonds form by the transfer of electrons from a metal to a non-metal, creating oppositely charged ions held by electrostatic attraction. Covalent bonds form by the sharing of electron pairs between non-metal atoms. Coordinate bonds are a special type of covalent bond where both shared electrons come from one atom. The type of bond determines the physical properties of a substance: ionic compounds have high melting points and conduct electricity in molten/aqueous state, while covalent compounds generally have low melting points and are poor conductors. Students must draw electron dot (Lewis) structures, understand the octet rule and its exceptions, differentiate between polar and non-polar covalent bonds, and relate bonding type to physical properties. This chapter carries significant weightage and questions frequently involve drawing structures and explaining properties.
Key Definitions
| Term | Definition |
|---|---|
| Ionic Bond | Electrostatic force of attraction between oppositely charged ions formed by electron transfer |
| Covalent Bond | Bond formed by mutual sharing of electron pairs between two atoms |
| Coordinate Bond | Covalent bond where both shared electrons are donated by one atom (donor → acceptor) |
| Electrovalency | Number of electrons lost or gained by an atom to form an ion |
| Covalency | Number of electron pairs shared by an atom in covalent bond formation |
| Polar Covalent Bond | Covalent bond between atoms of different electronegativities; shared pair is displaced towards the more electronegative atom |
| Octet Rule | Atoms tend to gain, lose, or share electrons to achieve 8 electrons in the outermost shell |
| Lone Pair | A pair of valence electrons not involved in bonding |
Must-Know Concepts
- Na (2,8,1) transfers 1 electron to Cl (2,8,7) forming Na+Cl− (ionic bond)
- In H2O, oxygen shares 2 electron pairs with 2 hydrogen atoms (covalent, polar)
- In NH4+, the lone pair on nitrogen forms a coordinate bond with H+
- Single bond (1 shared pair), double bond (2 pairs, e.g. O2), triple bond (3 pairs, e.g. N2)
- Ionic compounds: crystalline solids, high MP/BP, conduct electricity when molten or in solution
- Covalent compounds: low MP/BP, generally insoluble in water, do not conduct electricity
- Electronegativity difference determines bond type: large difference = ionic, small = covalent
Ionic vs Covalent Compounds
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Bond Formation | Electron transfer | Electron sharing |
| Melting Point | High | Low |
| Solubility | Soluble in water | Soluble in organic solvents |
| Conductivity | Conduct when molten/dissolved | Do not conduct |
| State at Room Temp | Crystalline solids | Gases, liquids, or soft solids |
Important Diagrams to Practice
- Electron dot diagrams for NaCl, MgO, CaCl2 (ionic)
- Electron dot structures for H2O, NH3, CH4, CO2, C2H4, N2 (covalent)
- Coordinate bond formation in NH4+ and H3O+
Common Mistakes
- Drawing electron dot structures without showing lone pairs
- Confusing coordinate bond with ordinary covalent bond (arrow shows direction in coordinate bond)
- Saying ionic compounds conduct electricity in solid state (they do not - ions are fixed)
- Forgetting to show the charge on ions in ionic bond diagrams
- Not recognising that HCl is a polar covalent molecule (not ionic despite containing a metal-like H)
Scoring Tips
- Always show complete electron transfer/sharing with arrows in diagrams
- When comparing ionic and covalent, use a table format for clarity and full marks
- For coordinate bonds, clearly identify the donor (has lone pair) and acceptor (has empty orbital)
- Practice drawing at least 10 electron dot structures till they become automatic
Frequently Asked Questions
Why do ionic compounds have high melting points?
Ionic compounds consist of a lattice of positive and negative ions held by strong electrostatic forces. A large amount of energy is needed to overcome these forces, resulting in high melting points.
Can a molecule have both ionic and covalent bonds?
Yes. For example, NaOH has an ionic bond between Na+ and OH−, while the O-H bond within the hydroxide ion is covalent.
What is the difference between polar and non-polar covalent bonds?
In a non-polar covalent bond, electrons are shared equally (e.g., H2, Cl2). In a polar covalent bond, electrons are shared unequally due to a difference in electronegativity (e.g., HCl, H2O).