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ICSE Class 10 Chemistry Question 16 of 28

Chemical Bonding — Question 10

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Question

Question 10

Compare :

(a) sodium atom and sodium ion

(b) chlorine atom and chloride ion

with respect to

  1. atomic structure
  2. electrical state
  3. chemical action
  4. toxicity
Answer

(a) Comparison between sodium atom and sodium ion:

PropertySodium AtomSodium Ion
Atomic StructureIt has one electron in M shell.It has 8 electrons in L shell.
Electrical StateIt is neutral.It is positively charged.
Chemical ActionIt is very Active.It is inactive.
ToxicityIt is poisonous.It is non-poisonous.

(b) Comparison between chlorine atom and chloride ion:

PropertyChlorine AtomChloride Ion
Atomic StructureIt has 7 electrons in M shell.It has 8 electrons in M shell.
Electrical StateIt is neutral.It is negatively charged.
Chemical ActionIt is very Active.It is inactive.
ToxicityIt is poisonous.It is non-poisonous.

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 BondElectrostatic force of attraction between oppositely charged ions formed by electron transfer
Covalent BondBond formed by mutual sharing of electron pairs between two atoms
Coordinate BondCovalent bond where both shared electrons are donated by one atom (donor → acceptor)
ElectrovalencyNumber of electrons lost or gained by an atom to form an ion
CovalencyNumber of electron pairs shared by an atom in covalent bond formation
Polar Covalent BondCovalent bond between atoms of different electronegativities; shared pair is displaced towards the more electronegative atom
Octet RuleAtoms tend to gain, lose, or share electrons to achieve 8 electrons in the outermost shell
Lone PairA 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 FormationElectron transferElectron sharing
Melting PointHighLow
SolubilitySoluble in waterSoluble in organic solvents
ConductivityConduct when molten/dissolvedDo not conduct
State at Room TempCrystalline solidsGases, 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).