Chemical Bonding – NDA Chemistry Notes

Atoms rarely exist alone. They combine with other atoms to form molecules and compounds. The force that holds atoms together is called a chemical bond.

Why do atoms bond? Because bonding makes atoms more stable. Most atoms are more stable when their outermost shell is completely filled. For most elements, a full outer shell means 8 electrons. This is called the octet rule.

NDA tests this chapter through bond type identification, the CuSO₄·5H₂O all-four-bonds question, hydrogen bonds in water, covalent bonds are directional, octet rule exceptions, and the micelle and soap questions. The soap and micelle content appears in NDA papers alongside bonding questions, so it is covered here briefly, with full detail in Chapter 14.

1. The Octet Rule

The octet rule says that atoms tend to gain, lose, or share electrons to achieve 8 electrons in their outermost shell, like the nearest noble gas.

Noble gases (He, Ne, Ar) already have full outer shells. They are stable and do not react. All other atoms try to reach this stable configuration.

Helium is an exception. It needs only 2 electrons to be stable (its first shell is full with 2).

Octet rule exceptions (NDA tests this): Some molecules are stable even though the central atom does NOT have 8 electrons around it.

BF₃ (Boron trifluoride) is the most important example. Boron has only 6 electrons around it in BF₃, not 8. The octet rule is NOT valid for BF₃. [NDA 2007-I]

Other exceptions: PCl₅ (phosphorus has 10 electrons around it, expanded octet), SF₆ (sulphur has 12 electrons around it).

2. Types of Chemical Bonds

There are four main types of chemical bonds. Each forms under different conditions.

Ionic Bond (Electrovalent Bond)

An ionic bond forms when one atom transfers electrons to another atom. The atom that loses electrons becomes a positive ion (cation). The atom that gains electrons becomes a negative ion (anion). The opposite charges attract each other. This attraction is the ionic bond.

When does an ionic bond form? Between a metal and a non-metal. The metal loses electrons and becomes a cation. The non-metal gains electrons and becomes an anion.

Example: Sodium chloride (NaCl)

• Sodium (Na) loses 1 electron → Na⁺
• Chlorine (Cl) gains 1 electron → Cl⁻
• Na⁺ and Cl⁻ attract each other → ionic bond → NaCl

Properties of ionic compounds:

• Exist as crystals, arranged in a regular lattice
• High melting and boiling points
• Conduct electricity when dissolved in water or when melted
• Dissolve well in water
• Brittle, shatter when hit

Ionic bonds are non-directional. This means the attraction between ions acts equally in all directions. [NDA 2016-II]

When an ionic compound dissolves in water and produces m Aⁿ⁺ ions and n Bᵐ⁻ ions, the formula of the compound is AₙBₘ. [NDA 2006-II]

Water is a good solvent for ionic compounds. This is because water molecules are polar. They attract and surround the ions, pulling the ionic lattice apart. [NDA 2010-II]

Calcium oxide (CaO), Sodium nitride (Na₃N), and Zinc sulphide (ZnS) are ionic compounds. Silicon carbide (SiC) is a covalent compound, not ionic. [NDA 2022-I]

Element combinations that form ionic compounds: metal (low ionisation energy) with non-metal (high electron affinity). Mg (Z=12, metal) and Cl (Z=17, non-metal) will form an ionic compound. [NDA 2014-II]

Covalent Bond

A covalent bond forms when two atoms share electrons. Neither atom fully transfers electrons. Instead, both atoms share one or more pairs of electrons.

When does a covalent bond form? Between two non-metals. Both atoms want to gain electrons. Neither wants to give them up. So they share instead.

Example: Water (H₂O)

• Oxygen has 6 valence electrons. It needs 2 more to complete its octet.
• Each hydrogen has 1 valence electron. It needs 1 more.
• Oxygen shares 1 electron with each hydrogen → 2 covalent bonds form.

Types of covalent bonds by number of shared pairs:

• Single bond: one pair shared (H–H, H–Cl, C–H)
• Double bond: two pairs shared (O=O, C=O)
• Triple bond: three pairs shared (N≡N, C≡C)

Properties of covalent compounds:

• Usually exist as gases, liquids, or soft solids at room temperature
• Low melting and boiling points (generally)
• Poor conductors of electricity
• Often soluble in organic solvents, not in water (with exceptions like HCl, NH₃)

Covalent bonds are directional. This means a covalent bond points in a specific direction in space. This is why molecules have specific shapes. [NDA 2016-II]

This is the key difference from ionic bonds: ionic bonds are non-directional, covalent bonds are directional.

Counting covalent bonds (NDA tests this directly):

Total covalent bonds in methanol (CH₃OH):

• 3 C–H bonds + 1 C–O bond + 1 O–H bond = 5 covalent bonds [NDA 2021-II]

Total covalent bonds in chloropropane (C₃H₇Cl):

• 2 C–C bonds + 7 C–H bonds + 1 C–Cl bond = 10 covalent bonds [NDA 2020]

Valence electrons in O²⁻ ion:

• Oxygen normally has 6 valence electrons. O²⁻ has gained 2 extra electrons. So it has 8 valence electrons. [NDA 2014-I]

Coordinate Bond (Dative Bond)

A coordinate bond is a special type of covalent bond. In a normal covalent bond, each atom contributes one electron to the shared pair. In a coordinate bond, both electrons in the shared pair come from the same atom.

The atom that donates both electrons is called the donor. The atom that accepts is called the acceptor.

Example: NH₄⁺ (ammonium ion)

• NH₃ has a lone pair on nitrogen.
• H⁺ has an empty orbital.
• Nitrogen donates both electrons → coordinate bond forms → NH₄⁺

Once formed, a coordinate bond is identical in strength and nature to a normal covalent bond.

Hydrogen Bond

A hydrogen bond is a special attractive force. It is NOT a full chemical bond like ionic or covalent bonds. It is a weaker force, but it has enormous importance in daily life.

How does a hydrogen bond form? When hydrogen is bonded to a highly electronegative atom (F, O, or N), the hydrogen becomes slightly positive (δ+). This slightly positive hydrogen is attracted to a lone pair of electrons on another electronegative atom nearby. This attraction is the hydrogen bond.

Hydrogen bonds form between molecules that contain: H–F, H–O, or H–N bonds.

Most important example (water): Water molecules form hydrogen bonds with each other. Each water molecule can form up to 4 hydrogen bonds.

This is why water has unusual properties:

• Very high boiling point (100°C) for such a small molecule
• High heat of vaporisation
• High specific heat capacity
• Ice is less dense than liquid water

The bond between water molecules is a hydrogen bond. [NDA 2013-I]

Water has a high boiling point. This is because of hydrogen bonding between water molecules. [NDA 2012-I]

Water is NOT a non-polar molecule. Water IS polar: the oxygen is more electronegative than hydrogen, so the molecule has a permanent dipole. This polarity is what allows water to form hydrogen bonds. [NDA 2019-I]

Van der Waals Forces

Van der Waals forces are the weakest type of intermolecular forces. They exist between all molecules, even non-polar ones.

They arise from temporary uneven distribution of electrons creating temporary dipoles. These temporary dipoles attract neighbouring molecules weakly.

Van der Waals forces are responsible for:

• Noble gases being able to form liquid state at very low temperatures
• The weak forces between graphite layers (which allow layers to slide)

Graphite layers are held together by weak van der Waals forces, not by covalent bonds. [NDA 2018-I]

Property	Ionic Bond	Covalent Bond	Coordinate Bond	Hydrogen Bond
How formed	Electron transfer	Electron sharing	One atom donates both electrons	Attraction between δ+ H and lone pair
Between	Metal + Non-metal	Non-metal + Non-metal	Donor + Acceptor	Molecules with H–F, H–O, H–N
Strength	Strong	Strong	Strong (same as covalent once formed)	Weak
Directional?	No	Yes	Yes	Yes
Example	NaCl, MgO	H₂O, CO₂, CH₄	NH₄⁺, CuSO₄·5H₂O	Water molecules, DNA base pairs

3. All Four Bond Types in One Molecule: CuSO₄·5H₂O

This is the single most famous bonding question in NDA Chemistry.

CuSO₄·5H₂O (blue vitriol / copper sulphate pentahydrate) contains ALL FOUR types of bonds:

• Electrovalent (ionic) bond: Between Cu²⁺ and SO₄²⁻
• Covalent bond: Between S and O inside the SO₄²⁻ ion
• Coordinate (dative) bond: Between Cu²⁺ and 4 water molecules (Cu²⁺ accepts lone pairs from water)
• Hydrogen bond: Between the 4 coordinated water molecules and the 5th water molecule

What are the types of bonds present in CuSO₄·5H₂O? Answer: Electrovalent, covalent, coordinate covalent, and hydrogen bonds, all four types. [NDA 2006-II]

4. Polar and Non-Polar Covalent Bonds

When two atoms of the same element share electrons, they share equally. The bond is non-polar. Example: H–H, Cl–Cl, O=O.

When two different atoms share electrons, the more electronegative atom attracts the shared electrons more strongly. The bond is polar. The electronegative atom becomes slightly negative (δ−) and the other atom becomes slightly positive (δ+). Example: H–Cl, H–O.

Polar molecules: Molecules where the individual bond dipoles do not cancel out. Water (H₂O) is polar: bent shape means dipoles add up.

Non-polar molecules: Molecules where bond dipoles cancel out perfectly. Carbon dioxide (CO₂) is non-polar despite having polar C=O bonds: the linear shape means the two bond dipoles cancel exactly.

5. Soap, Micelles and Emulsions

This section introduces key bonding-related concepts about soap. Full detail is in Chapter 14.

What is soap? Soap is the sodium or potassium salt of a long chain fatty acid. [NDA 2016-I, NDA 2011-II]

The sodium salt of fatty acid makes hard soap (solid bars). The potassium salt makes soft soap (liquid or soft). [NDA 2019-II]

Structure of a soap molecule: A soap molecule has two distinct ends:

• Hydrophilic head: the ionic end (–COO⁻Na⁺). This end loves water.
• Hydrophobic tail: the long carbon chain. This end hates water but loves oil and grease.

How soap cleans (micelle formation): When soap is added to water, soap molecules arrange themselves around oil droplets. The hydrophobic tails point inward, towards the oil. The hydrophilic heads point outward, towards the water. This spherical arrangement is called a micelle.

The oil is trapped inside the micelle. The micelle is water-soluble because its outer surface is ionic. The dirt is lifted away when you rinse with water.

Key NDA Confusion about micelles: The IONIC end of the soap molecule faces OUTWARD towards the water, NOT towards the oil droplet. The non-polar tail faces inward towards the oil. [NDA 2025-I, NDA 2021-I]

The cleaning action of soap and detergent in water is due to the formation of micelles. [NDA 2015-I]

Soap removes dirt by forming aggregates (micelles) and taking the dirt into the core of the aggregates.

Soap with water forms a lyotropic liquid crystal, not a simple solution. The ordered arrangement of micelles in water is a type of liquid crystal where order is caused by concentration (not temperature). [NDA 2024-I]

Emulsion: An emulsion is a colloidal solution of two liquids. One liquid is dispersed as tiny droplets inside the other. Example: milk (fat droplets in water). [NDA 2016-II]

Turmeric as an indicator (NDA special): Turmeric is yellow in neutral and acidic conditions. Soap is alkaline. When soap touches turmeric, turmeric turns reddish-brown. When washed with water (neutral), turmeric returns to yellow. Sequence: Yellow → Reddish-brown → Yellow. [NDA 2020]

(For complete coverage of soap, detergents, cleansing action, hard water, antiseptics and disinfectants, see Chapter 14.)