Chemical Bonding — ICSE Class 10 Chemistry Important Questions

Answer: (b) Transfer of electrons.

An electrovalent bond forms when electrons are completely transferred from a metal atom to a non-metal atom, producing oppositely charged ions held together by electrostatic force.

Answer: (c) Three.

Each nitrogen atom ($2,5$2,5) needs 3 electrons to complete its octet, so the two atoms share three pairs of electrons, forming a triple bond, $N\equiv N$N N.

Answer: (c) $NH_4^{+}$NH_4^+.

In the ammonium ion, ammonia ($NH_3$NH_3) donates its lone pair to a $H^{+}$H^+ ion (which has no electrons), forming a coordinate bond where both shared electrons come from nitrogen.

Answer: (b).

In a solid ionic compound the ions are fixed in a rigid crystal lattice and cannot move, so no conduction occurs. On melting or dissolving, the lattice breaks and the ions become mobile, allowing them to carry current.

Answer: (a) Both statements are true and R correctly explains A. In covalent compounds only weak intermolecular forces act between molecules, so little energy is needed to separate them, giving low melting and boiling points.

Oxygen ($2,6$2,6) shares one electron pair with each of two hydrogen atoms, completing its octet and each hydrogen's duplet. Oxygen retains two lone pairs.

Each shared pair between O and H is a covalent bond, so water has two O-H single covalent bonds and two lone pairs on oxygen.

1. Bond formation / physical state: Electrovalent compounds are formed by transfer of electrons and are usually hard crystalline solids; covalent compounds are formed by sharing of electrons and are usually gases, liquids or soft solids.

2. Conductivity and melting point: Electrovalent compounds have high melting/boiling points and conduct electricity when molten or in solution; covalent compounds have low melting/boiling points and generally do not conduct electricity (they are non-electrolytes).

Magnesium ($2,8,2$2,8,2) has 2 valence electrons; each chlorine ($2,8,7$2,8,7) needs 1 electron to complete its octet.

Magnesium loses its 2 valence electrons, one to each of two chlorine atoms:

$Mg \rightarrow Mg^{2+} + 2e^{-}$Mg → Mg^2+ + 2e^-

$2Cl + 2e^{-} \rightarrow 2Cl^{-}$2Cl + 2e^- → 2Cl^-

The magnesium ion $Mg^{2+}$Mg^2+ ($2,8$2,8) and two chloride ions $Cl^{-}$Cl^- ($2,8,8$2,8,8) all attain stable octets. The oppositely charged ions are held by strong electrostatic attraction, giving the electrovalent compound $MgCl_2$MgCl_2.

(i) Ammonia, $NH_3$NH_3: Nitrogen ($2,5$2,5) shares one electron pair with each of three hydrogen atoms, forming three N-H single covalent bonds and leaving one lone pair on nitrogen.

(ii) Carbon dioxide, $CO_2$CO_2: Carbon ($2,4$2,4) shares two electron pairs with each oxygen atom, forming two double bonds, $O=C=O$O=C=O. Each atom attains a complete octet.

An ammonia molecule, $NH_3$NH_3, has nitrogen with three N-H covalent bonds and one lone pair of electrons.

A hydrogen ion, $H^{+}$H^+, has no electrons and needs a pair to complete its duplet. The nitrogen atom donates its lone pair to the $H^{+}$H^+ ion, forming a fourth N-H bond:

$NH_3 + H^{+} \rightarrow NH_4^{+}$NH_3 + H^+ → NH_4^+

Because both shared electrons of this fourth bond come from nitrogen alone, it is a coordinate (dative) bond. Thus $NH_4^{+}$NH_4^+ contains three normal covalent bonds and one coordinate bond. Once formed, all four N-H bonds are identical. The overall $+$+ charge is carried by the ion.

(a) A covalent bond is a bond formed by the mutual sharing of one or more pairs of electrons between two atoms, each atom usually contributing an equal number of electrons to the shared pair.

(b)

(i) Methane $CH_4$CH_4: carbon ($2,4$2,4) shares one pair with each of four hydrogen atoms, forming four C-H single bonds; carbon attains an octet and each H a duplet.

(ii) Nitrogen $N_2$N_2: each nitrogen ($2,5$2,5) shares three pairs, forming a triple bond $N\equiv N$N N; each atom has one lone pair.

(iii) Hydrogen chloride $HCl$HCl: hydrogen shares one pair with chlorine ($2,8,7$2,8,7), forming one H-Cl single bond; chlorine has three lone pairs.

(c) $HCl$HCl is a polar covalent molecule. Chlorine is much more electronegative than hydrogen, so the shared pair is pulled towards chlorine, making it slightly negative and hydrogen slightly positive, creating a permanent dipole.

(i) In $NaCl$NaCl the $Na^{+}$Na^+ and $Cl^{-}$Cl^- ions are held in a rigid lattice by strong electrostatic forces; a large amount of heat energy is needed to break these forces, so the melting point is high.

(ii) $CCl_4$CCl_4 is a covalent compound made of neutral molecules with no free ions or electrons, so there are no mobile charge carriers to conduct electricity.

(iii) Water is a polar solvent that strongly attracts and separates the ions of an ionic compound, so they dissolve; organic solvents are non-polar and cannot pull the ions apart, so ionic compounds are insoluble in them.

(iv) In a coordinate bond both electrons come from one atom, so the donor becomes slightly positive and the acceptor slightly negative, giving the bond a partial ionic (polar) character in addition to its covalent nature; hence it is called semi-polar.

(v) Both chlorine atoms are identical non-metals with equal electronegativity, so neither can transfer an electron to the other; they attain octets by sharing one pair, forming a covalent bond $Cl-Cl$Cl-Cl.

(i) Compound X is electrovalent (ionic) and compound Y is covalent.

(ii) X has a high melting point and conducts electricity in the molten state; both are characteristic of ionic compounds, which are made of ions held in a strong lattice and become mobile when molten.

(iii) In X the particles are oppositely charged ions held together by strong electrostatic (ionic) forces of attraction.

(iv) Y is made of neutral molecules with no ions or free electrons; even when molten it contains only whole molecules, so there are no mobile charge carriers and it cannot conduct electricity.