Four Bonds per Atom Is Why Carbon Has So Many Compounds
See why carbon shares electrons instead of transferring them, draw electron dot structures for common molecules, tell saturated from unsaturated hydrocarbons, and count the structural isomers of butane and pentane.
Why does carbon share electrons instead of giving them away?
Carbon has four electrons in its outermost shell, so it needs four more to reach the stable eight. It has two obvious ways to get there and it takes neither.
It could lose all four electrons and become a ion. But removing four electrons from an atom with only six protons takes an enormous amount of energy — far more than any reaction can supply.
It could gain four electrons and become . But then six protons would have to hold ten electrons in place, and that nucleus is simply not strong enough to keep them.
So carbon does the third thing: it shares. Four of its electrons pair up with four electrons from other atoms, giving every atom involved a full outer shell without anyone becoming an ion. A bond formed by sharing a pair of electrons is a covalent bond, and every compound in this chapter is built from them.
That one decision has enormous consequences. Because carbon makes four bonds and because a carbon atom is small, carbon-to-carbon bonds are unusually strong and stable. Carbon can therefore link to itself in long chains, in branched chains and in rings — a property called catenation — and each of those skeletons can carry different atoms at different positions. Four bonds plus self-linking is why carbon compounds outnumber the compounds of every other element.
This page covers the first part of the CBSE Class 10 Science chapter on carbon and its compounds: covalent bonding, electron dot structures, saturated and unsaturated hydrocarbons, and structural isomerism.
It could lose all four electrons and become a ion. But removing four electrons from an atom with only six protons takes an enormous amount of energy — far more than any reaction can supply.
It could gain four electrons and become . But then six protons would have to hold ten electrons in place, and that nucleus is simply not strong enough to keep them.
So carbon does the third thing: it shares. Four of its electrons pair up with four electrons from other atoms, giving every atom involved a full outer shell without anyone becoming an ion. A bond formed by sharing a pair of electrons is a covalent bond, and every compound in this chapter is built from them.
That one decision has enormous consequences. Because carbon makes four bonds and because a carbon atom is small, carbon-to-carbon bonds are unusually strong and stable. Carbon can therefore link to itself in long chains, in branched chains and in rings — a property called catenation — and each of those skeletons can carry different atoms at different positions. Four bonds plus self-linking is why carbon compounds outnumber the compounds of every other element.
This page covers the first part of the CBSE Class 10 Science chapter on carbon and its compounds: covalent bonding, electron dot structures, saturated and unsaturated hydrocarbons, and structural isomerism.
How do you draw an electron dot structure?
Count the outer electrons of each atom, pair up as many as are needed to complete every octet, and show each shared pair as a bond. A hydrogen atom needs only two electrons, so one shared pair completes it.
**Hydrogen, . Each hydrogen brings one electron, so one shared pair gives both atoms the two they need: a single bond**.
**Oxygen, . Each oxygen has six outer electrons and needs two more, so two pairs are shared: a double bond. Each atom also keeps two unshared pairs, called lone pairs**.
**Nitrogen, . Each nitrogen has five outer electrons and needs three more, so three pairs are shared: a triple bond**, with one lone pair left on each atom.
**Water, . Oxygen forms one single bond to each hydrogen and keeps two lone pairs.
Methane, . Carbon's four electrons each pair with a hydrogen electron, giving four single bonds and no lone pairs at all.
Carbon dioxide, .** Carbon forms a double bond to each oxygen, using all four of its electrons.
**Ammonia, . Nitrogen forms three single bonds and keeps one lone pair.
Simple hydrocarbons follow the same rules.
- Ethane, — a single carbon-to-carbon bond, with three hydrogens on each carbon
- Ethene, — a double carbon-to-carbon bond, with two hydrogens on each carbon
- Ethyne, — a triple carbon-to-carbon bond, with one hydrogen on each carbon
The check to run on every structure you draw. Count the bonds at each atom: carbon must have exactly four, nitrogen three, oxygen two and hydrogen one. If any atom has the wrong number, the structure is wrong however tidy it looks. That single count catches nearly every error in a dot-structure question.
Why the triple bond in nitrogen matters. Three shared pairs make a very strong bond, and that is why nitrogen gas is so unreactive despite making up most of the air. The bond strength explains a property — which is the pattern this whole chapter follows.
And why covalent compounds behave as they do. Because the molecules are neutral, the forces between molecules are weak even though the bonds inside them are strong. So covalent compounds have low melting and boiling points, and they are poor conductors of electricity: there are no ions and no free electrons to carry a current. Compare that with the ionic compounds of the previous chapter** — high melting points and conduction when molten — and the contrast comes entirely from whether electrons were transferred or shared.
**Hydrogen, . Each hydrogen brings one electron, so one shared pair gives both atoms the two they need: a single bond**.
**Oxygen, . Each oxygen has six outer electrons and needs two more, so two pairs are shared: a double bond. Each atom also keeps two unshared pairs, called lone pairs**.
**Nitrogen, . Each nitrogen has five outer electrons and needs three more, so three pairs are shared: a triple bond**, with one lone pair left on each atom.
**Water, . Oxygen forms one single bond to each hydrogen and keeps two lone pairs.
Methane, . Carbon's four electrons each pair with a hydrogen electron, giving four single bonds and no lone pairs at all.
Carbon dioxide, .** Carbon forms a double bond to each oxygen, using all four of its electrons.
**Ammonia, . Nitrogen forms three single bonds and keeps one lone pair.
Simple hydrocarbons follow the same rules.
- Ethane, — a single carbon-to-carbon bond, with three hydrogens on each carbon
- Ethene, — a double carbon-to-carbon bond, with two hydrogens on each carbon
- Ethyne, — a triple carbon-to-carbon bond, with one hydrogen on each carbon
The check to run on every structure you draw. Count the bonds at each atom: carbon must have exactly four, nitrogen three, oxygen two and hydrogen one. If any atom has the wrong number, the structure is wrong however tidy it looks. That single count catches nearly every error in a dot-structure question.
Why the triple bond in nitrogen matters. Three shared pairs make a very strong bond, and that is why nitrogen gas is so unreactive despite making up most of the air. The bond strength explains a property — which is the pattern this whole chapter follows.
And why covalent compounds behave as they do. Because the molecules are neutral, the forces between molecules are weak even though the bonds inside them are strong. So covalent compounds have low melting and boiling points, and they are poor conductors of electricity: there are no ions and no free electrons to carry a current. Compare that with the ionic compounds of the previous chapter** — high melting points and conduction when molten — and the contrast comes entirely from whether electrons were transferred or shared.
What is the difference between a saturated and an unsaturated hydrocarbon?
A saturated hydrocarbon has only single bonds between its carbon atoms; an unsaturated one has at least one double or triple bond.
A hydrocarbon contains only carbon and hydrogen, and the three families are:
- Alkanes — all single bonds, general formula , names ending in -ane
- Alkenes — at least one double bond, general formula , names ending in -ene
- Alkynes — at least one triple bond, general formula , names ending in -yne
Worked example — building the alkanes. Put into :
Check the hydrogen counts: , , , , as required.
The alkenes, from with upwards:
The alkynes, from :
There is no alkene or alkyne with one carbon, because a double or triple bond needs two carbon atoms to sit between. **The series therefore begin at **, and a question asking for methene has no answer.
Worked example — naming from a formula. A hydrocarbon has the formula . Which family is it in?
Test the general formulas with : an alkane would be , an alkene , an alkyne . So it is an alkene — pentene.
Two ways to tell them apart in the laboratory.
- The flame. Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow, sooty flame, because they contain a higher proportion of carbon and need more oxygen for complete combustion
- Bromine water. An unsaturated compound decolourises bromine water by adding across the double or triple bond; a saturated one leaves the colour unchanged. This is the standard test, and the reason is that unsaturated compounds undergo addition reactions while saturated ones do not
**The word saturated has a reason behind it.** An alkane is holding as many hydrogen atoms as its carbon skeleton can carry — it is full up. An alkene has room for two more and an alkyne for four, which is exactly what an addition reaction supplies. **So unsaturated is not a defect; it is the reason those compounds are chemically more useful.**
A hydrocarbon contains only carbon and hydrogen, and the three families are:
- Alkanes — all single bonds, general formula , names ending in -ane
- Alkenes — at least one double bond, general formula , names ending in -ene
- Alkynes — at least one triple bond, general formula , names ending in -yne
Worked example — building the alkanes. Put into :
Check the hydrogen counts: , , , , as required.
The alkenes, from with upwards:
The alkynes, from :
There is no alkene or alkyne with one carbon, because a double or triple bond needs two carbon atoms to sit between. **The series therefore begin at **, and a question asking for methene has no answer.
Worked example — naming from a formula. A hydrocarbon has the formula . Which family is it in?
Test the general formulas with : an alkane would be , an alkene , an alkyne . So it is an alkene — pentene.
Two ways to tell them apart in the laboratory.
- The flame. Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow, sooty flame, because they contain a higher proportion of carbon and need more oxygen for complete combustion
- Bromine water. An unsaturated compound decolourises bromine water by adding across the double or triple bond; a saturated one leaves the colour unchanged. This is the standard test, and the reason is that unsaturated compounds undergo addition reactions while saturated ones do not
**The word saturated has a reason behind it.** An alkane is holding as many hydrogen atoms as its carbon skeleton can carry — it is full up. An alkene has room for two more and an alkyne for four, which is exactly what an addition reaction supplies. **So unsaturated is not a defect; it is the reason those compounds are chemically more useful.**
How do you draw and count the structural isomers of butane and pentane?
Isomers are compounds with the same molecular formula but different structures. Draw every different carbon skeleton, fill in the hydrogens, and count the distinct ones.
**Butane, — two isomers.
- A straight chain** of four carbons: , called normal butane
- A branched chain of three carbons with the fourth attached to the middle one: 2-methylpropane, also called isobutane
Both have exactly ten hydrogens and four carbons, and every carbon in both has four bonds. They are different substances with different boiling points, even though a chemical analysis would give both the same formula.
**Pentane, — three isomers.
- The straight chain of five carbons: normal pentane
- A four-carbon chain with one branch on the second carbon: 2-methylbutane
- A three-carbon chain with two branches on the middle carbon: 2,2-dimethylpropane
How to be sure you have them all. Work down from the longest chain, shortening it by one carbon each time and placing the spare carbons as branches:
- Chain of five, no branch — one structure
- Chain of four, one branch — the branch can only go on the second carbon, since putting it on the end would just make a five-carbon chain again
- Chain of three, two branches — both must go on the middle carbon
- Chain of two — impossible, since three branches cannot fit on two carbons
That systematic search is the whole technique, and it is what stops you from missing an isomer or counting one twice.
The trap that makes students over-count. Putting a methyl branch on the third carbon of a four-carbon chain looks like a new structure, but turn the drawing round and it is the same as putting it on the second. A structure and its mirror image on paper are the same molecule, so count skeletons, not drawings. Butane has two isomers and not three, and pentane has three and not four.
Where isomerism begins. Methane, ethane and propane have only one possible structure each — there is no way to branch a chain of three without simply renaming the ends. Structural isomerism in the alkanes starts at four carbon atoms, and the number grows quickly after that: butane has two, pentane three, and hexane five.
Why isomers matter. Two isomers are genuinely different compounds with different melting points, boiling points and reactivities. The molecular formula names the atoms; only the structure names the substance** — which is why organic chemistry draws structures rather than writing formulas, and why the naming rules of the next part of this chapter are needed at all.
**Butane, — two isomers.
- A straight chain** of four carbons: , called normal butane
- A branched chain of three carbons with the fourth attached to the middle one: 2-methylpropane, also called isobutane
Both have exactly ten hydrogens and four carbons, and every carbon in both has four bonds. They are different substances with different boiling points, even though a chemical analysis would give both the same formula.
**Pentane, — three isomers.
- The straight chain of five carbons: normal pentane
- A four-carbon chain with one branch on the second carbon: 2-methylbutane
- A three-carbon chain with two branches on the middle carbon: 2,2-dimethylpropane
How to be sure you have them all. Work down from the longest chain, shortening it by one carbon each time and placing the spare carbons as branches:
- Chain of five, no branch — one structure
- Chain of four, one branch — the branch can only go on the second carbon, since putting it on the end would just make a five-carbon chain again
- Chain of three, two branches — both must go on the middle carbon
- Chain of two — impossible, since three branches cannot fit on two carbons
That systematic search is the whole technique, and it is what stops you from missing an isomer or counting one twice.
The trap that makes students over-count. Putting a methyl branch on the third carbon of a four-carbon chain looks like a new structure, but turn the drawing round and it is the same as putting it on the second. A structure and its mirror image on paper are the same molecule, so count skeletons, not drawings. Butane has two isomers and not three, and pentane has three and not four.
Where isomerism begins. Methane, ethane and propane have only one possible structure each — there is no way to branch a chain of three without simply renaming the ends. Structural isomerism in the alkanes starts at four carbon atoms, and the number grows quickly after that: butane has two, pentane three, and hexane five.
Why isomers matter. Two isomers are genuinely different compounds with different melting points, boiling points and reactivities. The molecular formula names the atoms; only the structure names the substance** — which is why organic chemistry draws structures rather than writing formulas, and why the naming rules of the next part of this chapter are needed at all.
Exam tip
What layout keeps a carbon-compounds answer complete?
Draw the structure, count the bonds at every atom, and name the family. Most marks in this chapter are for a correct structure rather than for a sentence.
- Check the valency at every atom: carbon four bonds, nitrogen three, oxygen two, hydrogen one. Run this check on every structure before moving on
- Show lone pairs when a question asks for an electron dot structure — oxygen in water has two, nitrogen in ammonia has one
- Write the general formula and substitute when identifying a family: for , test , and
- Name the bond type explicitly: single, double or triple, and say how many shared pairs it holds
- For isomers, work down from the longest chain and stop when the branches cannot fit
- Do not count a rotated drawing as a new isomer — check whether the longest chain is the same length
- Give both tests for unsaturation: the sooty yellow flame and the decolourising of bromine water
- **Say shared pair of electrons** when defining a covalent bond, not merely shared electrons
The misconception to name. A double bond is not two bonds that can each be broken separately, and it is not twice as long or twice as strong in a simple way. It is one link made of two shared pairs, and its chemical importance is that it can open up to accept two new atoms — which is what an addition reaction does and what bromine water detects.
- Check the valency at every atom: carbon four bonds, nitrogen three, oxygen two, hydrogen one. Run this check on every structure before moving on
- Show lone pairs when a question asks for an electron dot structure — oxygen in water has two, nitrogen in ammonia has one
- Write the general formula and substitute when identifying a family: for , test , and
- Name the bond type explicitly: single, double or triple, and say how many shared pairs it holds
- For isomers, work down from the longest chain and stop when the branches cannot fit
- Do not count a rotated drawing as a new isomer — check whether the longest chain is the same length
- Give both tests for unsaturation: the sooty yellow flame and the decolourising of bromine water
- **Say shared pair of electrons** when defining a covalent bond, not merely shared electrons
The misconception to name. A double bond is not two bonds that can each be broken separately, and it is not twice as long or twice as strong in a simple way. It is one link made of two shared pairs, and its chemical importance is that it can open up to accept two new atoms — which is what an addition reaction does and what bromine water detects.
Did you know
Why does a gas stove burn blue while a candle burns yellow?
Light a gas stove and the flame is blue and almost invisible at the base. Light a candle and the flame is yellow and bright, and holding a vessel over it leaves a black deposit. Both are burning hydrocarbons, so why the difference?
It comes down to oxygen supply and carbon content. The gas in a cylinder is a mixture of saturated hydrocarbons, and a stove is designed to mix it with plenty of air before it burns. Complete combustion results:
with a blue flame and no soot at all.
A candle burns wax, a much larger hydrocarbon, and the air reaches it only by drifting in from the sides. The supply is not enough for complete combustion, so some carbon is released as tiny glowing particles — and it is those particles, white hot, that make the flame yellow and bright. The soot on the vessel is unburnt carbon that escaped before it could react.
The same rule explains a kitchen warning sign. If a gas stove burns with a yellow flame instead of a blue one, the air holes are blocked. Less air means incomplete combustion, which means soot on the vessels and wasted fuel — and it also means carbon monoxide, which is why the holes must be cleaned rather than ignored.
And it explains why unsaturated compounds are sootier still. An alkene or an alkyne carries a higher proportion of carbon for the same number of hydrogens, so it needs proportionally more oxygen. Given the same air supply it burns less completely, which is exactly why the flame colour is a laboratory test for unsaturation.
One more thing the candle demonstrates. The brightest part of its flame is the least efficient, and the faint blue base is the hottest part. A luminous flame and a hot flame are not the same thing — which is why a laboratory burner is adjusted to be blue before anything is heated on it, and why a candle is a good source of light and a poor source of heat.
It comes down to oxygen supply and carbon content. The gas in a cylinder is a mixture of saturated hydrocarbons, and a stove is designed to mix it with plenty of air before it burns. Complete combustion results:
with a blue flame and no soot at all.
A candle burns wax, a much larger hydrocarbon, and the air reaches it only by drifting in from the sides. The supply is not enough for complete combustion, so some carbon is released as tiny glowing particles — and it is those particles, white hot, that make the flame yellow and bright. The soot on the vessel is unburnt carbon that escaped before it could react.
The same rule explains a kitchen warning sign. If a gas stove burns with a yellow flame instead of a blue one, the air holes are blocked. Less air means incomplete combustion, which means soot on the vessels and wasted fuel — and it also means carbon monoxide, which is why the holes must be cleaned rather than ignored.
And it explains why unsaturated compounds are sootier still. An alkene or an alkyne carries a higher proportion of carbon for the same number of hydrogens, so it needs proportionally more oxygen. Given the same air supply it burns less completely, which is exactly why the flame colour is a laboratory test for unsaturation.
One more thing the candle demonstrates. The brightest part of its flame is the least efficient, and the faint blue base is the hottest part. A luminous flame and a hot flame are not the same thing — which is why a laboratory burner is adjusted to be blue before anything is heated on it, and why a candle is a good source of light and a poor source of heat.
Exam relevance
How does carbon bonding feed into JEE and NEET Chemistry?
This is foundation work for the largest single block of the Class 11 and 12 Chemistry syllabus.
Where it leads. Class 11 Organic Chemistry: Some Basic Principles and Techniques rebuilds everything on this page with hybridisation: the four bonds of methane become orbitals at a fixed angle, the double bond of ethene becomes with a pi bond, and the triple bond of ethyne becomes and linear. The tetravalency you learn here is the reason all three exist, and the geometry explains why a double bond cannot rotate — which creates a second kind of isomerism in Class 11.
Where isomerism leads. Structural isomerism is examined directly in JEE Main, usually as how many isomers has this formula, and the systematic longest-chain-first search you use here is the method that scales. Class 11 adds chain, position, functional, metamerism and tautomerism, and then stereoisomerism — and candidates who over-count at Class 10 keep over-counting later.
Where the bonding contrast leads. Ionic against covalent — high melting point and conduction when molten against low melting point and no conduction — becomes a comparison question in Class 11 Chemical Bonding, extended with polarity, dipole moment and hydrogen bonding. The reason you give here, that intermolecular forces are weak even when bonds are strong, is the correct reason there too.
Where the tests lead. Bromine water decolourising and the sooty flame are still the standard distinguishing tests in Class 12 for unsaturation, and NEET asks them as identification questions.
Question types to expect. At this level: draw a dot structure, name the family, count isomers, distinguish saturated from unsaturated. In competitive papers: count isomers of a larger formula, identify hybridisation, and assertion-reason items on bond strength and reactivity.
The single trap that costs marks. Over-counting isomers by drawing the same skeleton twice in different orientations. Always identify the longest continuous chain first — if two drawings have the same longest chain and the same branch positions counted from the nearer end, they are one compound.
A second trap. Forgetting that carbon must have exactly four bonds. A structure with a three-bonded carbon is not a molecule, and in JEE a mechanism drawn with the wrong valency invalidates the whole answer. Count the bonds at every atom, every time.
Board versus competitive emphasis. The CBSE paper marks the drawn structure, the named family and the stated test; a competitive paper marks an isomer count or a hybridisation. The transferable habit is the valency check — it takes five seconds and it catches errors in every organic question you will ever attempt.
Where it leads. Class 11 Organic Chemistry: Some Basic Principles and Techniques rebuilds everything on this page with hybridisation: the four bonds of methane become orbitals at a fixed angle, the double bond of ethene becomes with a pi bond, and the triple bond of ethyne becomes and linear. The tetravalency you learn here is the reason all three exist, and the geometry explains why a double bond cannot rotate — which creates a second kind of isomerism in Class 11.
Where isomerism leads. Structural isomerism is examined directly in JEE Main, usually as how many isomers has this formula, and the systematic longest-chain-first search you use here is the method that scales. Class 11 adds chain, position, functional, metamerism and tautomerism, and then stereoisomerism — and candidates who over-count at Class 10 keep over-counting later.
Where the bonding contrast leads. Ionic against covalent — high melting point and conduction when molten against low melting point and no conduction — becomes a comparison question in Class 11 Chemical Bonding, extended with polarity, dipole moment and hydrogen bonding. The reason you give here, that intermolecular forces are weak even when bonds are strong, is the correct reason there too.
Where the tests lead. Bromine water decolourising and the sooty flame are still the standard distinguishing tests in Class 12 for unsaturation, and NEET asks them as identification questions.
Question types to expect. At this level: draw a dot structure, name the family, count isomers, distinguish saturated from unsaturated. In competitive papers: count isomers of a larger formula, identify hybridisation, and assertion-reason items on bond strength and reactivity.
The single trap that costs marks. Over-counting isomers by drawing the same skeleton twice in different orientations. Always identify the longest continuous chain first — if two drawings have the same longest chain and the same branch positions counted from the nearer end, they are one compound.
A second trap. Forgetting that carbon must have exactly four bonds. A structure with a three-bonded carbon is not a molecule, and in JEE a mechanism drawn with the wrong valency invalidates the whole answer. Count the bonds at every atom, every time.
Board versus competitive emphasis. The CBSE paper marks the drawn structure, the named family and the stated test; a competitive paper marks an isomer count or a hybridisation. The transferable habit is the valency check — it takes five seconds and it catches errors in every organic question you will ever attempt.
Key takeaways
What should you know about carbon compounds before functional groups?
One element, one kind of bond, and two reasons it makes so many compounds.
- Carbon has four outer electrons and neither loses nor gains four, because both would cost too much energy — so it shares
- A covalent bond is a shared pair of electrons, and carbon forms four of them
- Tetravalency and small size give strong carbon-to-carbon bonds, which allow catenation — chains, branches and rings
- Dot structures: one shared pair in , two in , three in ; has four single bonds and two double bonds
- Check the valencies: carbon four, nitrogen three, oxygen two, hydrogen one
- Covalent compounds have low melting points and do not conduct, because the forces between molecules are weak and there are no free ions
- **Alkanes are saturated; alkenes and alkynes are unsaturated, and both begin at two carbons
- Unsaturated compounds decolourise bromine water and burn with a sooty yellow flame; saturated ones do neither
- Isomers have the same molecular formula and different structures — butane has two, pentane three, hexane five
- Isomerism starts at four carbons**, and a rotated drawing is not a new isomer
The sharpest self-test is pentane. Draw all three isomers of from a blank page, check that every carbon has four bonds and every structure has twelve hydrogens, and then convince yourself there is no fourth.
- Carbon has four outer electrons and neither loses nor gains four, because both would cost too much energy — so it shares
- A covalent bond is a shared pair of electrons, and carbon forms four of them
- Tetravalency and small size give strong carbon-to-carbon bonds, which allow catenation — chains, branches and rings
- Dot structures: one shared pair in , two in , three in ; has four single bonds and two double bonds
- Check the valencies: carbon four, nitrogen three, oxygen two, hydrogen one
- Covalent compounds have low melting points and do not conduct, because the forces between molecules are weak and there are no free ions
- **Alkanes are saturated; alkenes and alkynes are unsaturated, and both begin at two carbons
- Unsaturated compounds decolourise bromine water and burn with a sooty yellow flame; saturated ones do neither
- Isomers have the same molecular formula and different structures — butane has two, pentane three, hexane five
- Isomerism starts at four carbons**, and a rotated drawing is not a new isomer
The sharpest self-test is pentane. Draw all three isomers of from a blank page, check that every carbon has four bonds and every structure has twelve hydrogens, and then convince yourself there is no fourth.