Almost everything around you — salt, water, oxygen, even your bones — exists because atoms bond together. NDA Chemistry quietly rewards students who understand why and how atoms combine. The Cavalier turns valency and chemical bonding into plain language, simple rules and exam-style practice, so you bank these marks without memorising blindly.
Why Valency and Bonding Matter in NDA
Open any NDA General Ability Test chemistry section and you will spot questions on chemical bonding, the nature of a compound, or the valency of an element. These are conceptual one-liners — no heavy calculation, just clear understanding.
Topics that repeat year after year include the octet rule, the difference between ionic and covalent compounds, writing formulas, and properties like melting point and electrical conductivity. Master these and you collect sure marks while others guess.
Most bonding questions test a single idea: does the compound transfer electrons or share them? Decide that first, and the rest of the answer usually follows.
Why Do Atoms Combine at All?
Atoms bond because they are chasing stability. The most stable atoms in nature are the noble gases (helium, neon, argon…) which already have a full outermost shell — and notice how they almost never react.
Every other atom tries to copy that arrangement. It does so by losing, gaining or sharing electrons until its outermost shell is complete. This drive to fill the outer shell is the single reason chemical bonds form.
Octet rule: atoms tend to combine so that each has 8 electrons in its outermost shell (like a noble gas).
Hydrogen, lithium and a few light atoms aim for a duplet — just 2 electrons — matching helium.
The electrons in the outermost shell that take part in bonding are called valence electrons. They are the stars of this entire chapter. Inner-shell electrons sit too close to the nucleus and almost never get involved, so whenever you analyse a bond you only need to look at the outermost shell of each atom.
What Is Valency?
Valency is the combining capacity of an element — the number of electrons an atom loses, gains or shares to complete its octet.
Finding valency from valence electrons
- If the outermost shell has 1, 2 or 3 electrons, the atom usually loses them. Valency = number of valence electrons.
- If it has 5, 6 or 7 electrons, the atom usually gains electrons. Valency = 8 − (valence electrons).
- If it has 4 electrons (like carbon), it usually shares, giving a valency of 4.
Sodium (Na) has 1 valence electron → valency 1. Oxygen has 6 → valency 8 − 6 = 2. Aluminium has 3 → valency 3.
Some elements show variable valency — iron can be Fe2+ or Fe3+, copper Cu1+ or Cu2+. Roman numerals like iron(II) and iron(III) tell you which one is meant. This usually happens with transition metals, where electrons from more than one shell can take part in bonding, so the same metal can form different compounds.
Valency has no sign; it is just a number. The charge on an ion (like Na+ or O2−) carries a sign. Don't confuse the two in your answers.
The Three Main Types of Chemical Bond
Once you know why atoms bond, the how falls into three neat categories:
- Ionic (electrovalent) bond — electrons are transferred from one atom to another.
- Covalent bond — electrons are shared between atoms.
- Metallic bond — electrons are pooled and shared by many metal atoms together.
Quick rule of thumb: metal + non-metal → ionic; non-metal + non-metal → covalent; metal + metal → metallic. This one line answers many NDA questions instantly.
Ionic (Electrovalent) Bonds
An ionic bond forms when one atom gives away electrons and another accepts them. The atom that loses electrons becomes a positive cation; the atom that gains becomes a negative anion. Opposite charges attract, holding the compound together.
The classic example: common salt
Sodium (Na) has 1 outer electron; chlorine (Cl) needs 1 to complete its octet. Sodium donates its electron to chlorine:
Na → Na+ + 1 electron, and Cl + 1 electron → Cl−
The Na+ and Cl− ions attract to form NaCl (sodium chloride, table salt).
Properties of ionic compounds
- Usually hard, crystalline solids with high melting and boiling points.
- Conduct electricity when molten or dissolved in water (free ions can move), but not as a dry solid.
- Generally soluble in water, less so in organic solvents like kerosene.
Ionic compounds conduct electricity only when the ions are free to move — that is, in the molten or aqueous state, never as a solid block.
Covalent Bonds
A covalent bond forms when two atoms share one or more pairs of electrons. This happens mostly between non-metals, which would rather share than fully transfer electrons.
Single, double and triple bonds
- Single bond: one shared pair — e.g. H−H in hydrogen gas, or H−Cl.
- Double bond: two shared pairs — e.g. O=O in oxygen gas.
- Triple bond: three shared pairs — e.g. N≡N in nitrogen gas.
In water (H2O), oxygen shares electrons with two hydrogen atoms. In methane (CH4), carbon shares with four hydrogens. Both are covalent.
Properties of covalent compounds
- Usually liquids or gases (or soft solids) with low melting and boiling points.
- Generally poor conductors of electricity (no free ions or electrons).
- Often insoluble in water but soluble in organic solvents.
Not every covalent compound is a non-conductor — but most are. Compared with ionic compounds, the giveaway signs are low melting point and poor conductivity.
Metallic Bonds
In a metal, the outer electrons are so loosely held that they leave their atoms and form a “sea” of free electrons moving among fixed positive metal ions. The attraction between this electron sea and the positive ions is the metallic bond.
Free, mobile electrons explain why metals are good conductors of heat and electricity, and why they are malleable (can be hammered into sheets) and ductile (can be drawn into wires).
This model also explains the typical shiny lustre of metals and their generally high melting points — it takes a lot of energy to pull the strongly bonded structure apart. It even explains why metals can be bent and reshaped without breaking: the layers of metal ions slide over one another while the electron sea keeps holding everything together, so the metal stays intact instead of shattering like a brittle ionic crystal.
Writing Chemical Formulas Using Valency
Valency lets you write the correct formula of a compound using the simple criss-cross method:
- Write the symbols side by side.
- Write each element's valency above it.
- Criss-cross the valencies to become the subscripts of the other element.
- Simplify the ratio if possible.
Calcium (valency 2) + Chlorine (valency 1): criss-cross gives CaCl2. Aluminium (3) + Oxygen (2) gives Al2O3.
The same trick works with polyatomic ions — groups that act as a unit, such as sulphate (SO4, valency 2), nitrate (NO3, valency 1) and carbonate (CO3, valency 2). For example, calcium + carbonate gives CaCO3 (limestone).
When you criss-cross onto a polyatomic ion and need more than one of it, put the ion in brackets: calcium + nitrate → Ca(NO3)2, never CaNO32.
Ionic vs Covalent — Quick Comparison
NDA frequently asks you to tell ionic and covalent compounds apart from their properties. Keep this contrast crystal clear:
Ionic: electrons transferred, high melting point, conducts when molten/dissolved, usually water-soluble.
Covalent: electrons shared, low melting point, usually non-conducting, often water-insoluble.
A handy memory hook
- Think of salt (NaCl) as the model ionic compound — hard, high melting, dissolves and conducts in water.
- Think of sugar or water as the model covalent compound — melts/boils easily, does not conduct.
Students assume “dissolves in water = conducts electricity.” Sugar dissolves but does not conduct because it forms molecules, not ions. Only compounds that release free ions conduct.
Worked Example
Aluminium (atomic number 13) reacts with oxygen (atomic number 8). Find the valency of each element and write the formula of the compound formed.
So aluminium and oxygen combine in the ratio 2:3 to form Al2O3. Because a metal (Al) bonds with a non-metal (O), the bond is ionic.
Previous-Year Style Practice
Q. Which of the following is held together by a covalent bond, and which by an ionic bond: NaCl and CH4?
Answer: NaCl is held by an ionic bond — sodium (a metal) transfers an electron to chlorine (a non-metal). CH4 (methane) is held by covalent bonds — carbon and hydrogen, both non-metals, share electrons.
NDA loves “identify the bond type” questions because they test one clear idea. Always check the elements first: a metal-plus-non-metal pairing points to ionic, while two non-metals point to covalent.
If a question gives a property instead of a formula — say “high melting point, conducts when molten” — that is the fingerprint of an ionic compound.
Quick Revision
- Why bond: to complete the outer shell — the octet rule (duplet for H, Li).
- Valency: electrons lost, gained or shared; from outer shell, valency = N or 8 − N.
- Ionic bond: electrons transferred (metal + non-metal); high melting point, conducts when molten/dissolved.
- Covalent bond: electrons shared (non-metal + non-metal); low melting point, usually non-conducting.
- Metallic bond: sea of free electrons; good conductor, malleable, ductile.
- Formulas: criss-cross the valencies; bracket polyatomic ions, e.g. Ca(NO3)2.
If you fix just one idea firmly — transfer → ionic, share → covalent — you can reason out most bonding questions in the exam hall on the spot.
Frequently asked questions
What is the difference between valency and the charge on an ion?
Valency is just a number — the combining capacity of an atom (electrons lost, gained or shared). The charge on an ion is that number plus a sign, such as Na+ (1+) or O2− (2−). Valency 2 can give either a 2+ or a 2− ion depending on whether the atom loses or gains electrons.
Why do ionic compounds conduct electricity only when molten or dissolved?
Electricity needs charged particles that are free to move. In a solid ionic compound the ions are locked in a rigid lattice. Melting it or dissolving it in water frees the ions, so they can move and carry current.
What is the octet rule?
It states that atoms combine in order to have eight electrons in their outermost shell, copying the stable arrangement of noble gases. Light atoms like hydrogen and lithium aim for a duplet of just two electrons instead.
Why are metals good conductors of electricity?
In a metal the outer electrons break free and form a 'sea' of mobile electrons around fixed positive ions. These free electrons drift easily when a voltage is applied, allowing metals to conduct both electricity and heat very well.
How do I quickly decide if a bond is ionic or covalent?
Check the elements involved. A metal combined with a non-metal usually forms an ionic bond (electron transfer), while two non-metals usually form a covalent bond (electron sharing). Properties help too: high melting point and conductivity in solution indicate ionic.
How many marks can Valency and Chemical Bonding fetch in NDA?
This topic typically contributes a few sure marks each year in the General Ability Test chemistry section, usually as direct conceptual one-liners on bond type, valency or compound properties — making it a high-yield, easy area to prepare.
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