A substance is classified from the pattern of its properties, not from any single one. A metal can have an unusual property and still be a metal, and a non-metal can share one metallic property and still not be a metal.
These questions are common in periodic patterns because they test whether you can reason, not just recall a list.
What is the usual pattern for a metal?
Most metals share these properties, all explained by metallic bonding:
- conduct electricity and heat, including as solids
- malleable and ductile (can be hammered and drawn)
- shiny when freshly cut
- usually high melting points and high densities
- form positive ions in compounds, and their oxides are usually basic
Non-metals are usually the opposite: brittle as solids, poor conductors and forming negative ions or sharing electrons.
Which properties are the strongest evidence?
Two properties separate metals from non-metals most reliably: conducting electricity when solid, and malleability. A conducting solid that also bends without shattering is a metal. A substance that is brittle and conducts only when molten or in solution is an ionic compound, not a metal.
Weaker evidence comes from melting point, density and appearance, where exceptions are well known.
Worked example
The substances below are invented for practice. Classify each as a metal or a non-metal and explain.
| Substance | Conducts when solid | Malleable | Melting point | Other |
|---|---|---|---|---|
| W | yes | yes | 1 085 °C | shiny, dense |
| X | yes | no, brittle | above 3 000 °C | slippery layers |
| Y | yes | not tested | −39 °C | shiny liquid |
| Z | yes | yes, soft | 64 °C | low density, floats on water |
W: conducts and is malleable with a high melting point. A metal, and a typical one.
X: conducts when solid, but it is brittle and has slippery layers. The pattern does not support metallic bonding. A non-metal exception, like graphite, which has delocalised electrons between its layers.
Y: a liquid at room temperature, but it conducts and is shiny. One odd property, several metal-like ones. A metal with an exception to the solid rule, the same as mercury.
Z: soft with a low melting point and low density, but it conducts and is malleable. A metal, matching the Group I pattern.
The working rule: list the properties, decide which are strong evidence, then explain any property that does not fit.
What mistake goes wrong here?
Students classify from a single property.
Mistaken answer: “X conducts electricity when solid, so it is a metal.”
Conductivity alone is not enough. Graphite conducts and is a non-metal.
The correction: check the second property. Malleability, the type of ion formed or the behaviour of the oxide will usually settle it. The same thinking stops the reverse error: “Z has a low melting point, so it is not a metal.”
Check yourself
1. A solid conducts electricity, is hard and brittle, and shatters when hit. Can you say it is a metal?
Show answer
Not from the evidence given. Brittleness is not a metal property. It could be a conducting non-metal such as graphite, so more data is needed.
2. Why is sodium still classed as a metal even though it is soft enough to cut?
Show answer
It conducts electricity, is malleable, is shiny when freshly cut and forms a 1+ ion. Softness and a low melting point are Group I group properties and do not change the classification.
3. Give one property that is the exception for mercury, and one that shows it is still a metal.
Show answer
Exception: it is a liquid at room temperature. Evidence it is a metal: it conducts electricity (or is shiny, or forms positive ions).
Where this leads next
Next, interpreting a table with an unfamiliar element uses this classification habit on data you have not seen. Later, metals and reactivity puts the metal class to work in displacement and extraction questions.
If classification still depends on luck, a teacher in online one-to-one Chemistry tuition can build the routine with you. Practise it in the periodic patterns practice set first.