LIWA

Chemical Bonding

ICSE Class 10 · Chemistry · Chapter 2

What this chapter covers

Key definitions

A chemical bond is the force of attraction that holds two or more atoms together in a molecule or in a compound.

Chemical bonding is the process by which atoms combine with one another, by losing, gaining or sharing electrons, to form chemical bonds.

The octet rule is the tendency of atoms to lose, gain or share electrons so that each atom ends up with eight electrons in its outermost shell, like the nearest noble gas.

The duplet rule is the tendency of very small atoms, such as hydrogen, lithium and beryllium, to attain two electrons in their outermost shell, like the noble gas helium.

Valence electrons are the electrons present in the outermost shell of an atom, and they decide how the atom combines with other atoms.

An electron dot structure is a representation of a molecule or an ion in which the symbol of each element is surrounded by dots and crosses that stand for the valence electrons of its atoms.

An ion is an atom or a group of atoms that carries an electric charge because it has lost or gained one or more electrons.

A cation is a positively charged ion formed when an atom loses one or more electrons.

An anion is a negatively charged ion formed when an atom gains one or more electrons.

Electrovalent bonding is the formation of a chemical bond by the complete transfer of one or more electrons from one atom to another, so that oppositely charged ions are formed and held together by electrostatic attraction.

An electrovalent compound is a compound in which the constituent particles are ions held together by electrovalent bonds.

Electrovalency is the number of electrons that an atom loses or gains while forming an electrovalent bond.

Key points

Questions answered

Define an electrovalent bond. Why does an electrovalent bond form between a metal and a non-metal?
An electrovalent bond is the electrostatic attraction between oppositely charged ions formed by the complete transfer of electrons from one atom to another. A metal has few valence electrons and a low ionisation potential, so it loses them easily; a non-metal has a high electron affinity and gains them readily, so both reach a stable octet by transfer.
Describe the electron dot structure of magnesium chloride.
Mg (2, 8, 2) transfers one electron to each of two Cl atoms (2, 8, 7). The product is [Mg]2+ with no outer electrons shown, between two [Cl]- ions, each with eight electrons (seven dots and one cross).
Why does solid sodium chloride not conduct electricity, while molten sodium chloride does?
In the solid the Na+ and Cl- ions are held in fixed positions in the lattice and cannot move. On melting the ions dissociate and become free to move towards the electrodes, so they carry the current.
Give a reason why electrovalent compounds have high melting points.
Their oppositely charged ions are held in the lattice by strong electrostatic forces acting in all directions, and a large amount of heat energy is needed to overcome them.
Define a covalent bond and state the covalency of nitrogen in N2.
A covalent bond is the chemical bond formed by the sharing of a pair of electrons between two atoms, each atom contributing one electron. Nitrogen shares three electrons in N2, so its covalency is three.
How many shared pairs and lone pairs are there in a molecule of oxygen?
Two shared pairs (a double bond) and four lone pairs, two on each oxygen atom.
What is electronegativity? Why is HCl a polar molecule while Cl2 is not?
Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. In Cl2 both atoms are identical, so the pair is shared equally. In HCl, chlorine is more electronegative than hydrogen and pulls the pair closer, so Cl gets a partial negative charge and H a partial positive charge.
Carbon tetrachloride contains polar bonds but is a non-polar compound. Explain.
The four C–Cl bonds are arranged symmetrically in a tetrahedron, so their equal pulls cancel, and the molecule has no overall positive or negative end.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Assertion (A): Ionic compounds have high melting points. Reason (R): A strong electrostatic force of attraction exists between the oppositely charged ions.

  1. (A) is true but (R) is false.
  2. (A) is false but (R) is true.
  3. Both (A) and (R) are true, and (R) is the correct explanation of (A).
  4. Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Show answer
Both (A) and (R) are true, and (R) is the correct explanation of (A).

ICSE 2027 sample paper · 2 marks

A compound commonly used in the laboratory has formula H₂P. H represents hydrogen, P represents a non-metal. Draw a dot and cross structure for the formation of the compound H₂P.

Show answer
P has 6 valence electrons (e.g. oxygen, H₂P = H₂O). Each H atom (one electron, •) shares its electron with one of P's electrons (×): H• + P (six ×, two lone pairs) + •H → H •× P ו H, with two lone pairs (××) remaining on P. Two single covalent bonds are formed; P attains an octet and each H a duplet.

ICSE 2027 sample paper · 2 marks

The structures of two atoms P and Q are: Atom P: nucleus with 11p and 12n; electrons arranged 2, 8, 1 in three shells. Atom Q: nucleus with 8p and 8n; electrons arranged 2, 6 in two shells. What is the formula and mass of one mole of the compound formed when P and Q react together? [At. wt of P is 23 and Q is 16]

Show answer
P (2, 8, 1) loses one electron, valency 1; Q (2, 6) gains two electrons, valency 2. Formula: P₂Q Mass of one mole = 2 × 23 + 16 = 46 + 16 = 62 g

ICSE 2027 sample paper · 3 marks

Draw the electron dot structure of: (a) ammonium ion (also label the co-ordinate bond) (b) nitrogen molecule [Atomic No.: N = 7, H = 1]

Show answer
(a) Ammonium ion, [NH₄]⁺: N (2, 5) shares three electrons with three H atoms (three single covalent bonds, each a pair of N's × and H's •) and has one lone pair; this lone pair is donated to an H⁺ ion, forming the co-ordinate bond (labelled N→H). The whole is enclosed in square brackets with a + charge. (b) Nitrogen molecule, N₂: each N contributes three electrons to form three shared pairs (a triple bond, N≡N) between the atoms, and each N keeps one lone pair; :N:::N:

ICSE 2026 board paper · 1 mark

Which of the following element pairs will form an ionic bond? | Pair | | |---|---| | P | Elements of Group 1 & Group 2 | | Q | Elements of Group 14 & Group 16 | | R | Elements of Group 2 & Group 17 | | S | Elements of Group 15 & Group 18 |

  1. P
  2. Q
  3. R
  4. S
Show answer
R

ICSE 2026 sample paper · 1 mark

Identify the ion that contain one lone pair of electrons.

  1. OH⁻¹
  2. H₃O⁺
  3. NH₄⁺
  4. H⁺
Show answer
H₃O⁺

ICSE 2026 board paper · 1 mark

An atom of X forms an ion according to the equation X → X⁺². The atomic number of the atom X is:

  1. 16
  2. 10
  3. 12
  4. 14
Show answer
12

ICSE 2026 board paper · 1 mark

The compound that has the highest melting point amongst the following is:

  1. Methane
  2. Sodium chloride
  3. Ammonia
  4. Ethanol
Show answer
Sodium chloride