Every Rule About Metals Has a Famous Exception
Compare metals and non-metals on lustre, malleability, ductility, conductivity, sonority and melting point, classify an unknown element from its stated properties, and name the six exceptions that break the rules.
Can you always tell a metal from a non-metal by looking at it?
Most of the time, yes. A metal is shiny, hard, heavy, rings when struck, conducts heat and electricity, and can be hammered into a sheet or drawn into a wire. A non-metal is dull, brittle, a poor conductor, and breaks rather than bends.
Then you meet mercury, which is a metal that pours out of a bottle like water. And graphite, a non-metal that conducts electricity well enough to be used as an electrode. And sodium, a metal soft enough to cut with a knife and light enough to float on water.
So the physical properties are a good guide and not a definition. Every one of them has at least one well-known exception, and those exceptions are examined more often than the rules themselves — because naming an exception proves you know the rule it breaks.
What finally separates the two classes is chemical behaviour, which is the subject of the next part of this chapter: metals lose electrons and form basic oxides, while non-metals gain electrons and form acidic oxides. That test has no exceptions of the kind you meet below.
This page covers the first part of the CBSE Class 10 Science chapter on metals and non-metals: the physical properties compared, classifying an element from its stated properties, and the anomalous cases.
Then you meet mercury, which is a metal that pours out of a bottle like water. And graphite, a non-metal that conducts electricity well enough to be used as an electrode. And sodium, a metal soft enough to cut with a knife and light enough to float on water.
So the physical properties are a good guide and not a definition. Every one of them has at least one well-known exception, and those exceptions are examined more often than the rules themselves — because naming an exception proves you know the rule it breaks.
What finally separates the two classes is chemical behaviour, which is the subject of the next part of this chapter: metals lose electrons and form basic oxides, while non-metals gain electrons and form acidic oxides. That test has no exceptions of the kind you meet below.
This page covers the first part of the CBSE Class 10 Science chapter on metals and non-metals: the physical properties compared, classifying an element from its stated properties, and the anomalous cases.
How do metals and non-metals compare on each physical property?
Take the properties one at a time, and note the everyday object that shows each one.
Lustre. Metals have a characteristic shine when freshly cut, called metallic lustre — visible on a cut surface of aluminium or on polished silver. Non-metals are dull.
Malleability. Metals can be beaten into thin sheets. Aluminium foil for packing food and silver foil on sweets are both made this way, and gold can be beaten thinner than any other metal. Non-metals are brittle: strike a piece of sulphur or coal with a hammer and it shatters into powder.
Ductility. Metals can be drawn into wires — copper for electrical wiring, tungsten for a bulb filament. Gold and silver are the most ductile, and a very small mass of gold can be drawn into a remarkably long wire.
Conductivity of heat and electricity. Metals conduct both. Silver and copper are the best conductors, which is why copper is used for wiring and for the base of a cooking vessel; lead and mercury are the poorest among metals. Non-metals do not conduct, which is why plastic and rubber are used to insulate the same wires.
Sonority. Metals produce a ringing sound when struck — a school bell, a temple bell, a steel plate dropped on the floor. Non-metals give a dull thud, which is why nobody makes a bell out of wood or coal.
Melting and boiling points. Metals generally melt only at high temperatures, which is why they are used for cooking vessels and machine parts. Non-metals generally have low melting points and several are gases at room temperature.
Hardness and density. Metals are generally hard and dense — iron, copper and lead all are. Non-metals are soft or brittle, and many are gases with almost no density at all.
Worked comparison — why copper and not plastic for a cooking vessel. A vessel needs to conduct heat to the food, survive the flame without melting, and keep its shape when knocked. Copper satisfies all three; a plastic vessel fails the first two, and a glass one fails the third. The handle, meanwhile, needs the opposite of conductivity, which is why it is made of a non-metal such as bakelite. One object, two materials, chosen by two opposite properties.
The property that is really doing the work. Conductivity, lustre, malleability and sonority all come from the same feature: the outer electrons of a metal are free to move through the whole solid rather than being tied to one atom. Free electrons carry current, carry heat, reflect light to give lustre, and let the atoms slide past one another when hammered instead of snapping apart. Four properties, one explanation — and it is why the next part of the chapter can predict chemical behaviour from the same idea.
Lustre. Metals have a characteristic shine when freshly cut, called metallic lustre — visible on a cut surface of aluminium or on polished silver. Non-metals are dull.
Malleability. Metals can be beaten into thin sheets. Aluminium foil for packing food and silver foil on sweets are both made this way, and gold can be beaten thinner than any other metal. Non-metals are brittle: strike a piece of sulphur or coal with a hammer and it shatters into powder.
Ductility. Metals can be drawn into wires — copper for electrical wiring, tungsten for a bulb filament. Gold and silver are the most ductile, and a very small mass of gold can be drawn into a remarkably long wire.
Conductivity of heat and electricity. Metals conduct both. Silver and copper are the best conductors, which is why copper is used for wiring and for the base of a cooking vessel; lead and mercury are the poorest among metals. Non-metals do not conduct, which is why plastic and rubber are used to insulate the same wires.
Sonority. Metals produce a ringing sound when struck — a school bell, a temple bell, a steel plate dropped on the floor. Non-metals give a dull thud, which is why nobody makes a bell out of wood or coal.
Melting and boiling points. Metals generally melt only at high temperatures, which is why they are used for cooking vessels and machine parts. Non-metals generally have low melting points and several are gases at room temperature.
Hardness and density. Metals are generally hard and dense — iron, copper and lead all are. Non-metals are soft or brittle, and many are gases with almost no density at all.
Worked comparison — why copper and not plastic for a cooking vessel. A vessel needs to conduct heat to the food, survive the flame without melting, and keep its shape when knocked. Copper satisfies all three; a plastic vessel fails the first two, and a glass one fails the third. The handle, meanwhile, needs the opposite of conductivity, which is why it is made of a non-metal such as bakelite. One object, two materials, chosen by two opposite properties.
The property that is really doing the work. Conductivity, lustre, malleability and sonority all come from the same feature: the outer electrons of a metal are free to move through the whole solid rather than being tied to one atom. Free electrons carry current, carry heat, reflect light to give lustre, and let the atoms slide past one another when hammered instead of snapping apart. Four properties, one explanation — and it is why the next part of the chapter can predict chemical behaviour from the same idea.
How do you classify an unknown element from its stated properties?
Check conductivity and malleability first — they are the two most reliable indicators. Then confirm with lustre, sonority and melting point.
Worked example 1. Element X is dull, brittle, breaks into powder when hammered, does not conduct electricity, and melts at a low temperature. Classify it.
Every property points one way: dull rather than lustrous, brittle rather than malleable, non-conducting, low melting point. X is a non-metal. Sulphur fits this description exactly.
Worked example 2. Element Y is shiny when freshly cut, can be beaten into a sheet, conducts electricity, and rings when struck. Classify it.
Lustre, malleability, conductivity and sonority together make Y a metal — aluminium would fit.
Worked example 3 — mixed evidence. Element Z is lustrous and brittle, and does not conduct electricity. Classify it.
Two properties disagree. Lustre suggests a metal, but brittleness and non-conductivity suggest a non-metal. Conductivity is the decisive test, so Z is a non-metal — and iodine is exactly this case: a shiny, greyish-black, brittle non-metal.
Worked example 4 — the reverse mix. Element W is a good conductor of electricity, feels greasy, is soft and marks paper black. Classify it.
Conductivity says metal; the softness and the black mark say otherwise. W is graphite, a non-metal that conducts — the one case where the conductivity test fails.
So no single property is conclusive, and that is the real lesson of this section. A sound answer classifies on the balance of the evidence and names the property that disagrees:
- Z is a non-metal, although it is lustrous, because it is brittle and does not conduct electricity
Two more tests that settle it chemically, and these you will use in Part 2:
- The oxide test. A metal's oxide dissolves in water to give a basic solution that turns red litmus blue; a non-metal's oxide gives an acidic solution
- The electron test. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions
The oxide test has no exceptions of the physical kind. Graphite may conduct and iodine may shine, but carbon dioxide and iodine's oxides are still acidic, and sodium oxide is still basic. Chemistry classifies; physics only suggests.
Worked example 1. Element X is dull, brittle, breaks into powder when hammered, does not conduct electricity, and melts at a low temperature. Classify it.
Every property points one way: dull rather than lustrous, brittle rather than malleable, non-conducting, low melting point. X is a non-metal. Sulphur fits this description exactly.
Worked example 2. Element Y is shiny when freshly cut, can be beaten into a sheet, conducts electricity, and rings when struck. Classify it.
Lustre, malleability, conductivity and sonority together make Y a metal — aluminium would fit.
Worked example 3 — mixed evidence. Element Z is lustrous and brittle, and does not conduct electricity. Classify it.
Two properties disagree. Lustre suggests a metal, but brittleness and non-conductivity suggest a non-metal. Conductivity is the decisive test, so Z is a non-metal — and iodine is exactly this case: a shiny, greyish-black, brittle non-metal.
Worked example 4 — the reverse mix. Element W is a good conductor of electricity, feels greasy, is soft and marks paper black. Classify it.
Conductivity says metal; the softness and the black mark say otherwise. W is graphite, a non-metal that conducts — the one case where the conductivity test fails.
So no single property is conclusive, and that is the real lesson of this section. A sound answer classifies on the balance of the evidence and names the property that disagrees:
- Z is a non-metal, although it is lustrous, because it is brittle and does not conduct electricity
Two more tests that settle it chemically, and these you will use in Part 2:
- The oxide test. A metal's oxide dissolves in water to give a basic solution that turns red litmus blue; a non-metal's oxide gives an acidic solution
- The electron test. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions
The oxide test has no exceptions of the physical kind. Graphite may conduct and iodine may shine, but carbon dioxide and iodine's oxides are still acidic, and sodium oxide is still basic. Chemistry classifies; physics only suggests.
Which six elements break the rules, and why does each one do it?
Six exceptions cover almost every question on this topic, and each breaks exactly one rule.
Mercury — the only metal that is liquid at room temperature. Every other metal is a solid. Mercury's low melting point is why it was used in thermometers, and it remains a metal in every chemical sense: it is lustrous, it conducts, and it forms a basic oxide.
Bromine — the only non-metal that is liquid at room temperature. A dark reddish-brown liquid that gives off a choking vapour. Non-metals are otherwise gases or brittle solids.
Sodium and potassium — metals soft enough to cut with a knife. They break the metals are hard rule badly: a lump of sodium cuts like cheese, and both are so light that they float on water. They are also so reactive that they are stored under kerosene, which is why you never see them uncovered.
Iodine — a non-metal with lustre. Its crystals are shiny and greyish-black, looking much more like a metal than like sulphur. But it is brittle and non-conducting, so it is firmly a non-metal.
Graphite — a non-metal that conducts electricity. This is the most important exception in the chapter, and it has a real explanation. Carbon has four valence electrons, but in graphite each carbon atom is bonded to only three neighbours. The fourth electron is free to move through the layers, and free electrons are exactly what conduction needs. That is why graphite is used for electrodes in electrolysis and as a lubricant, while diamond — where all four electrons are used in bonding — does not conduct at all.
Diamond — a non-metal that is the hardest natural substance and has an extremely high melting point. Both facts come from the same structure: every carbon atom is bonded to four others in a rigid three-dimensional network, so nothing can slide and no bond can break without breaking many.
Diamond and graphite deserve a second look, because they are the same element with completely different properties. Diamond is the hardest substance known and an insulator; graphite is soft, greasy and a conductor. The element is identical and only the arrangement of the atoms differs — which is what allotropy means, and why a question about hardness or conductivity is really a question about structure.
A boundary case worth naming. Some elements sit genuinely between the two classes and have properties of both. Silicon and germanium are metalloids — lustrous like metals but brittle like non-metals, and conducting only slightly. They are not exceptions to a rule; they are a third category, and the reason the periodic table has a staircase boundary rather than a straight line.
Mercury — the only metal that is liquid at room temperature. Every other metal is a solid. Mercury's low melting point is why it was used in thermometers, and it remains a metal in every chemical sense: it is lustrous, it conducts, and it forms a basic oxide.
Bromine — the only non-metal that is liquid at room temperature. A dark reddish-brown liquid that gives off a choking vapour. Non-metals are otherwise gases or brittle solids.
Sodium and potassium — metals soft enough to cut with a knife. They break the metals are hard rule badly: a lump of sodium cuts like cheese, and both are so light that they float on water. They are also so reactive that they are stored under kerosene, which is why you never see them uncovered.
Iodine — a non-metal with lustre. Its crystals are shiny and greyish-black, looking much more like a metal than like sulphur. But it is brittle and non-conducting, so it is firmly a non-metal.
Graphite — a non-metal that conducts electricity. This is the most important exception in the chapter, and it has a real explanation. Carbon has four valence electrons, but in graphite each carbon atom is bonded to only three neighbours. The fourth electron is free to move through the layers, and free electrons are exactly what conduction needs. That is why graphite is used for electrodes in electrolysis and as a lubricant, while diamond — where all four electrons are used in bonding — does not conduct at all.
Diamond — a non-metal that is the hardest natural substance and has an extremely high melting point. Both facts come from the same structure: every carbon atom is bonded to four others in a rigid three-dimensional network, so nothing can slide and no bond can break without breaking many.
Diamond and graphite deserve a second look, because they are the same element with completely different properties. Diamond is the hardest substance known and an insulator; graphite is soft, greasy and a conductor. The element is identical and only the arrangement of the atoms differs — which is what allotropy means, and why a question about hardness or conductivity is really a question about structure.
A boundary case worth naming. Some elements sit genuinely between the two classes and have properties of both. Silicon and germanium are metalloids — lustrous like metals but brittle like non-metals, and conducting only slightly. They are not exceptions to a rule; they are a third category, and the reason the periodic table has a staircase boundary rather than a straight line.
Exam tip
What layout keeps a metals and non-metals answer complete?
Name the property, say what it means, and give an example or an exception. A one-word answer such as malleable earns less than malleable, meaning it can be beaten into sheets, as aluminium foil is.
- Define each property in the answer: ductile means drawn into wires, sonorous means it rings when struck. The definition is often the mark
- Give an everyday example, not just the element's name: copper wiring, aluminium foil, a school bell, a bakelite handle
- Name the exception with the rule it breaks: mercury is a metal, exceptional because it is liquid at room temperature
- Use the balance of evidence when classifying, and name the property that disagrees
- Say that conductivity is the more reliable test than lustre or hardness
- For graphite, give the reason — three bonds per carbon leave one free electron. Reciting the exception without the reason is a half answer
- **Keep malleable and ductile apart: sheets against wires. They are frequently swapped
- Remember that metalloids are a third class, not an exception
The misconception to name. Metals are not always hard and heavy. Sodium and potassium are soft enough to cut and light enough to float, while lithium is the least dense metal of all. Hardness and density are tendencies, not defining properties** — and a question offering soft, light, silvery, reacts violently with water is describing a metal, not a non-metal.
- Define each property in the answer: ductile means drawn into wires, sonorous means it rings when struck. The definition is often the mark
- Give an everyday example, not just the element's name: copper wiring, aluminium foil, a school bell, a bakelite handle
- Name the exception with the rule it breaks: mercury is a metal, exceptional because it is liquid at room temperature
- Use the balance of evidence when classifying, and name the property that disagrees
- Say that conductivity is the more reliable test than lustre or hardness
- For graphite, give the reason — three bonds per carbon leave one free electron. Reciting the exception without the reason is a half answer
- **Keep malleable and ductile apart: sheets against wires. They are frequently swapped
- Remember that metalloids are a third class, not an exception
The misconception to name. Metals are not always hard and heavy. Sodium and potassium are soft enough to cut and light enough to float, while lithium is the least dense metal of all. Hardness and density are tendencies, not defining properties** — and a question offering soft, light, silvery, reacts violently with water is describing a metal, not a non-metal.
Did you know
Why does a steel plate ring while a wooden one thuds?
Drop a steel plate in the kitchen and the whole house knows. Drop a wooden one and it lands with a flat thud. That difference has a name — sonority — and it is another consequence of free electrons holding the atoms of a metal together.
In a metal the atoms sit in a regular lattice bound by a shared sea of electrons, and that arrangement is springy: struck once, the whole plate vibrates for some time before the energy dies away, and those vibrations reach your ear as a ringing note. In wood or coal there is no such lattice; the blow is absorbed and scattered almost at once, so there is nothing left to ring.
That is why every bell in the country is metal. A temple bell, a school bell and a cycle bell are all chosen for one property, and the note depends on the alloy, the shape and the thickness — which is why the same blow on two bells gives two different notes.
The same free electrons explain why a metal spoon left in hot tea becomes too hot to hold while a plastic one stays cool. The electrons carry energy through the metal quickly, so heat reaches the handle in seconds. A non-metal has no mobile electrons, so heat can only creep along by vibration, and a wooden or plastic handle stays comfortable. The utensil and its handle are deliberately made of materials on opposite sides of this chapter.
And the shine has the same origin. Free electrons at the surface reflect almost all the light that falls on them, which is what metallic lustre is. Cut a fresh surface on a piece of aluminium and it gleams; leave it a while and the shine dulls as a thin oxide layer forms over it. The lustre was never in the metal's bulk but in its surface electrons, which is why polishing restores it and why a tarnished layer hides it.
So four different properties — conductivity, sonority, lustre and malleability — are one fact wearing four disguises. Understanding that single fact is worth more than memorising the four separately, and it is exactly what makes the chemical behaviour in Part 2 predictable.
In a metal the atoms sit in a regular lattice bound by a shared sea of electrons, and that arrangement is springy: struck once, the whole plate vibrates for some time before the energy dies away, and those vibrations reach your ear as a ringing note. In wood or coal there is no such lattice; the blow is absorbed and scattered almost at once, so there is nothing left to ring.
That is why every bell in the country is metal. A temple bell, a school bell and a cycle bell are all chosen for one property, and the note depends on the alloy, the shape and the thickness — which is why the same blow on two bells gives two different notes.
The same free electrons explain why a metal spoon left in hot tea becomes too hot to hold while a plastic one stays cool. The electrons carry energy through the metal quickly, so heat reaches the handle in seconds. A non-metal has no mobile electrons, so heat can only creep along by vibration, and a wooden or plastic handle stays comfortable. The utensil and its handle are deliberately made of materials on opposite sides of this chapter.
And the shine has the same origin. Free electrons at the surface reflect almost all the light that falls on them, which is what metallic lustre is. Cut a fresh surface on a piece of aluminium and it gleams; leave it a while and the shine dulls as a thin oxide layer forms over it. The lustre was never in the metal's bulk but in its surface electrons, which is why polishing restores it and why a tarnished layer hides it.
So four different properties — conductivity, sonority, lustre and malleability — are one fact wearing four disguises. Understanding that single fact is worth more than memorising the four separately, and it is exactly what makes the chemical behaviour in Part 2 predictable.
Exam relevance
How do metal and non-metal properties feed into JEE and NEET?
This is foundation work whose ideas are reorganised in Class 11 around the periodic table.
Where it leads. Class 11 Classification of Elements and Periodicity turns metal and non-metal into a trend: metallic character increases down a group and decreases across a period, because it depends on how easily an atom loses an electron. The physical properties in this chapter become consequences of ionisation enthalpy and electronegativity, which are examined directly in JEE Main and NEET. The staircase boundary of metalloids is the line where those two trends cross.
Where the free-electron idea leads. The sea of electrons explanation of conductivity, lustre, malleability and sonority becomes metallic bonding in Class 11 Chemical Bonding, and then band theory in higher study. In Physics, the same free electrons carry the current in Class 12 Current Electricity, where resistivity is explained by how those electrons move and scatter.
Where graphite and diamond lead. They reappear in Class 11 the p-Block Elements as the allotropes of carbon, with their structures drawn and compared, and the conductivity of graphite explained by delocalised electrons. The three-bonds-and-one-free-electron reason you learn here is the same reason given there, so it is worth learning properly once.
Question types to expect. At this level: compare, classify, name the exception. In competitive papers: order elements by metallic character, match a property to an element, and assertion-reason items on the exceptions — graphite conducts electricity although it is a non-metal is a standard pairing.
The single trap that costs marks. Treating a physical property as a definition. A question describing a lustrous, brittle, non-conducting solid is describing iodine, and answering metal because of the lustre is the error the question was written to catch. The chemical test — does it lose electrons and form a basic oxide — is the one that decides.
A second trap. Swapping malleability and ductility. Sheets are malleability; wires are ductility. In a one-mark question the wrong word scores nothing even when the idea is right.
Board versus competitive emphasis. The CBSE paper marks the defined property with an example and the named exception; a competitive paper marks an ordering or a matching. The transferable asset is the free-electron explanation — it answers questions about conductivity, lustre, bonding and resistivity across three subjects.
Where it leads. Class 11 Classification of Elements and Periodicity turns metal and non-metal into a trend: metallic character increases down a group and decreases across a period, because it depends on how easily an atom loses an electron. The physical properties in this chapter become consequences of ionisation enthalpy and electronegativity, which are examined directly in JEE Main and NEET. The staircase boundary of metalloids is the line where those two trends cross.
Where the free-electron idea leads. The sea of electrons explanation of conductivity, lustre, malleability and sonority becomes metallic bonding in Class 11 Chemical Bonding, and then band theory in higher study. In Physics, the same free electrons carry the current in Class 12 Current Electricity, where resistivity is explained by how those electrons move and scatter.
Where graphite and diamond lead. They reappear in Class 11 the p-Block Elements as the allotropes of carbon, with their structures drawn and compared, and the conductivity of graphite explained by delocalised electrons. The three-bonds-and-one-free-electron reason you learn here is the same reason given there, so it is worth learning properly once.
Question types to expect. At this level: compare, classify, name the exception. In competitive papers: order elements by metallic character, match a property to an element, and assertion-reason items on the exceptions — graphite conducts electricity although it is a non-metal is a standard pairing.
The single trap that costs marks. Treating a physical property as a definition. A question describing a lustrous, brittle, non-conducting solid is describing iodine, and answering metal because of the lustre is the error the question was written to catch. The chemical test — does it lose electrons and form a basic oxide — is the one that decides.
A second trap. Swapping malleability and ductility. Sheets are malleability; wires are ductility. In a one-mark question the wrong word scores nothing even when the idea is right.
Board versus competitive emphasis. The CBSE paper marks the defined property with an example and the named exception; a competitive paper marks an ordering or a matching. The transferable asset is the free-electron explanation — it answers questions about conductivity, lustre, bonding and resistivity across three subjects.
Key takeaways
What should you know about metals and non-metals before their reactions?
Seven properties, four explanations, and six exceptions.
- Lustre — metals shine when freshly cut; iodine is a lustrous non-metal
- Malleability — beaten into sheets, as aluminium foil is; non-metals are brittle
- Ductility — drawn into wires; gold and silver are the most ductile
- Conductivity — metals conduct heat and electricity, silver and copper best; graphite is a conducting non-metal
- Sonority — metals ring when struck; non-metals thud
- High melting point, hardness and density are tendencies, not rules
- All of those come from free electrons moving through the metal lattice
- The six exceptions: mercury, the only liquid metal; bromine, the only liquid non-metal; sodium and potassium, soft and light enough to cut and to float; iodine, lustrous; graphite, conducting; diamond, the hardest natural substance
- Graphite conducts because only three of carbon's four valence electrons are bonded, leaving one free; diamond uses all four and does not conduct
- Metalloids such as silicon are a third class, with properties of both
- The decisive test is chemical: metals lose electrons and form basic oxides
The sharpest self-test is worked example 3. Describe an element as lustrous, brittle and non-conducting, decide what it is, and write the sentence that names the property which disagrees with your answer.
- Lustre — metals shine when freshly cut; iodine is a lustrous non-metal
- Malleability — beaten into sheets, as aluminium foil is; non-metals are brittle
- Ductility — drawn into wires; gold and silver are the most ductile
- Conductivity — metals conduct heat and electricity, silver and copper best; graphite is a conducting non-metal
- Sonority — metals ring when struck; non-metals thud
- High melting point, hardness and density are tendencies, not rules
- All of those come from free electrons moving through the metal lattice
- The six exceptions: mercury, the only liquid metal; bromine, the only liquid non-metal; sodium and potassium, soft and light enough to cut and to float; iodine, lustrous; graphite, conducting; diamond, the hardest natural substance
- Graphite conducts because only three of carbon's four valence electrons are bonded, leaving one free; diamond uses all four and does not conduct
- Metalloids such as silicon are a third class, with properties of both
- The decisive test is chemical: metals lose electrons and form basic oxides
The sharpest self-test is worked example 3. Describe an element as lustrous, brittle and non-conducting, decide what it is, and write the sentence that names the property which disagrees with your answer.