Physical Properties of Metals and Non-metals
Pick up two objects from a school lab tray — a piece of copper wire and a lump of sulphur — and your hands already know the chemistry chapter before you open the book. The copper is shiny, bends without snapping, and feels cool because it drains heat from your fingers. The sulphur is dull, crumbles if you press it, and feels the same temperature as the air. That everyday contrast is the entire foundation of the metals-and-non-metals unit: elements split into two broad physical families — lustrous, malleable, ductile, conducting metals and dull, brittle, poorly conducting non-metals — before any reaction is even considered. The catch CBSE loves to test is that the split is not perfectly clean: a short list of named exceptions (sodium, potassium, mercury, lead, iodine, graphite, diamond) breaks the "typical" pattern for their class, and exam questions are built precisely around spotting them.
Metals are typically lustrous, malleable, ductile, sonorous, and good conductors of heat and electricity with high melting points and density, while non-metals are typically dull, brittle, non-sonorous, and poor conductors with low melting points and density — except for a fixed list of named exceptions (Na, K, Hg, Pb, iodine, graphite, diamond) that CBSE expects by name.
The core idea
The eight physical tests. Chemists sort an unknown element into "metal" or "non-metal" by running it through a short checklist of physical properties — none of these involve a chemical reaction, only how the sample looks, feels, and responds to being hammered, pulled, heated, or wired into a circuit.
- Lustre — a freshly cut or polished metal shines; a non-metal stays dull. (Sulphur and phosphorus are matte even when pure.)
- Malleability — a metal can be hammered or rolled into thin sheets without cracking; a non-metal shatters into fragments under the same blow. Gold foil thin enough to see through is the extreme case of malleability.
- Ductility — a metal can be drawn out into thin wire; non-metals cannot be stretched this way at all.
- Conductivity — metals carry heat and electric current well, because they have mobile electrons free to move through the lattice; non-metals are generally poor conductors, since their electrons are mostly locked into bonds.
- Sonorousness — struck metals ring with a clear sound (this is literally why bells are cast in bronze); non-metals give a dull thud.
- Melting and boiling point — metals typically need very high temperatures to melt or boil; non-metals typically melt or boil at comparatively low temperatures.
- Density and hardness — metals are usually dense and hard; non-metals are usually lighter and softer.
None of these eight properties has a numerical formula to plug values into — this is a qualitative, descriptive topic, and the "core idea" is the table below together with the exceptions that follow it.
| Property | Typical metal | Typical non-metal |
|---|---|---|
| Lustre | Shiny | Dull |
| Malleability | Beaten into sheets | Brittle, breaks |
| Ductility | Drawn into wires | Not ductile |
| Conductivity | Good conductor | Poor conductor |
| Sonorous | Yes | No |
| Melting/boiling point | Generally high | Generally low |
| Density/hardness | Generally high | Generally low |
Why it works
The pattern is not a coincidence of chemistry vocabulary — it traces back to how electrons are held inside each kind of element, even though CBSE Class 10 does not require the mechanism in depth.
- Metals hold their outermost electrons loosely, and those electrons drift freely through the whole piece of metal rather than staying attached to one atom. That mobile "sea" of electrons is what lets a metal conduct heat and electricity well, and it is also why a metal can be hammered into a sheet or drawn into a wire without breaking — the atoms slide past one another while the shared electron sea holds everything together.
- Non-metals hold their electrons tightly around individual atoms in fixed bonds. With no free-moving electrons, most non-metals conduct poorly, and because the bonding is rigid rather than sliding, hammering a non-metal shatters the fixed structure instead of reshaping it — hence brittleness rather than malleability.
- Named exceptions exist because "metal" and "non-metal" are broad classes, not identical twins. Sodium and potassium are metals with unusually large, loosely packed atoms, which weakens their bonding enough to make them soft and low-melting. Mercury's bonding is weak enough that it stays liquid at room temperature. Graphite is a non-metal whose carbon atoms are arranged in flat sheets with one loosely held electron per atom free to move within each sheet, giving it the electrical conductivity normally reserved for metals. Diamond, the other common form of carbon, locks every electron into four rigid bonds per atom, producing the opposite extreme — the hardest known natural substance despite being a non-metal.
The exam-relevant takeaway is simply this: memorise the seven-item checklist as the default rule, then memorise the six named exceptions as deliberate departures from it — CBSE questions are usually built to test exactly whether you know which is which.
Worked example
Question: A student is given four unlabelled samples — A (shiny, can be hammered into a thin sheet, conducts electricity), B (dull grey, brittle, conducts electricity), C (dull, brittle, non-conductor), D (shiny, liquid at room temperature). Classify each and identify B and D by name if possible.
Step 1. Sample A shows shine, malleability, and conductivity together — all three are classic metallic properties with no exception involved. A is a typical metal (for example, copper or aluminium).
Step 2. Sample B is dull and brittle, which points toward a non-metal, but it also conducts electricity — the signature exception to "non-metals are poor conductors." Among common non-metals, only graphite conducts electricity, because of its sheet-like structure with mobile electrons within each layer. So B is graphite, a non-metallic form of carbon.
Step 3. Sample C is dull, brittle, and a poor conductor, matching the typical non-metal profile with no exception needed (for example, sulphur).
Step 4. Sample D is shiny (lustrous, hence metallic-looking) but liquid at room temperature. The only metal that stays liquid at ordinary room temperature is mercury (Hg), because its bonding between atoms is unusually weak. So D is mercury.
Answer: A is a typical metal, B is graphite (a non-metal that conducts electricity), C is a typical non-metal, and D is mercury (a metal that is liquid at room temperature).
Competitive stretch
A related question style gives a fifth sample: E (dull, extremely hard, non-conductor, cannot be scratched by steel). Applying the checklist, "dull" and "non-conductor" point to a non-metal, and extreme hardness rules out the typical soft non-metal pattern — this matches diamond exactly, the exception non-metal that is the hardest known natural substance. Recognising diamond from hardness alone, without being told it is carbon, is a common twist CBSE adds to the same four-sample format.
Common traps
- Saying "all metals are hard and have high melting points" and forgetting Na and K (soft, low melting point) and Hg (liquid at room temperature).
- Assuming "non-metals never conduct electricity" — graphite is the standard counterexample and appears in nearly every classification question.
- Confusing malleability (can be flattened into sheets) with ductility (can be drawn into wires) — they are related but distinct properties and should be stated separately in a full answer.
- Thinking diamond, being a form of carbon (a non-metal), must be soft like other non-metals — diamond is actually the hardest known natural substance.
Ask the tutor: Not sure why graphite conducts electricity while diamond does not, even though both are pure carbon — ask your AI tutor to walk through the two carbon structures side by side.
Why is mercury a metal if it does not look or behave like other metals at all?
Mercury is still classified as a metal because it shares the deeper chemical behaviour of metals, such as how it forms compounds and reacts, not just its physical appearance. Physically it is the odd one out: its atoms bond to each other only weakly, so the solid structure that gives other metals their high melting point simply does not form at room temperature, and mercury stays liquid. It is one of the six exceptions CBSE wants you to name directly rather than reason out.
If graphite conducts electricity, why is it still called a non-metal and not a metal?
Classification into metal or non-metal depends on a whole cluster of properties and on chemical behaviour, not on any single test. Graphite fails almost every other metallic test — it is dull, brittle, and does not form the typical metallic bonding pattern throughout its structure — and its chemistry matches carbon's non-metallic behaviour. Conductivity happens to be shared with metals here because of graphite's unusual sheet-like arrangement of carbon atoms, but that one shared property is not enough to move it into the metal class. This is exactly why it is taught as a named exception rather than reclassified.
How can sodium be a metal if it is soft enough to cut with a knife, unlike iron?
"Metals are generally hard" is a description of the typical case, not an unbreakable rule, and sodium and potassium are the two standard exceptions to it. Their atoms are unusually large and loosely packed compared with a metal like iron, which weakens the bonding holding the solid together and makes it both soft and low-melting. Sodium still qualifies as a metal because of its lustre when freshly cut, its conductivity, and its typical metallic chemical reactions — softness is simply one property where it departs from the general trend, and CBSE expects you to know this by name rather than derive it.