Classifying elements

Open a periodic table and 118 elements stare back — far too many to learn one at a time. Chemists solve this the way a librarian sorts books: by shared properties, into a small number of shelves. The first and broadest shelving splits every element into metals and non-metals, with a small third shelf, metalloids, for the elements that refuse to sit neatly on either side. This split is not arbitrary — it tracks real, testable differences in how elements look, conduct, and behave — and a handful of memorable facts about which elements are gases, liquids, or oddly low-melting solids at room temperature ride along with it. This article builds that classification from the ground up, using only the elements named in the CBSE Class 8 chapter "Nature of Matter," and shows how to sort any of them correctly and explain why the sorting works.

The core insight

Elements are classified as metals, non-metals, or the small in-between group metalloids, based on their properties — not on which state (solid, liquid, gas) they happen to be in at room temperature, even though the two patterns often line up.

The core idea

Two main groups, one exception category. Every element you will meet at this level falls into one of three bins:

Metals — gold, silver, magnesium, iron, aluminium are the named examples in this chapter. Metals are, in general, shiny, malleable (can be hammered into sheets), ductile (can be drawn into wires), and good conductors of heat and electricity.
Non-metals — carbon, sulfur, hydrogen, oxygen are the named examples. Non-metals are typically dull-looking as solids, brittle rather than malleable, and poor conductors of electricity (with graphite, a form of carbon, as a well-known exception).
Metalloids — silicon and boron are the named examples. Their properties sit intermediate between metals and non-metals: neither list describes them well. At Class 8 level you are only expected to name this category and its two examples, not to explain which specific properties make an element a metalloid or where metalloids sit on the periodic table — that detail is left for higher grades.

A second, independent fact: physical state at room temperature. This is a separate axis of classification from metal/non-metal, but the chapter gives useful, testable facts that connect the two:

Most elements are solids at room temperature — this is the default expectation.
Eleven elements are gases at room temperature, and in this chapter's list, every one of them is a non-metal — for example, oxygen, helium, and nitrogen.
Only two elements are liquid at room temperature: mercury, which is a metal, and bromine, which is a non-metal.
Gallium and caesium are solids at room temperature, but only just — they melt at around $30\ \degree\text{C}$ (about $303\ \text{K}$ ), close enough to a warm room that they are worth remembering as near-liquids.

The pattern worth noticing: "gas at room temperature" points only toward non-metals among the elements named here, but "liquid at room temperature" does not point toward either group exclusively — it happens for one metal (mercury) and one non-metal (bromine). These physical-state facts are memorable and testable, but they are a side observation, not the definition of metal versus non-metal.

Figure

Three shelves labelled metals non-metals and metalloids holding the named example elements, with a smaller strip below listing the room temperature state facts

Why it works

This topic is descriptive, not formula-driven, so the reasoning is about what basis the classification rests on rather than a calculation.

The real basis is properties, not appearance or state. Metals and non-metals are separated by chemical and physical behaviour: how readily the element reacts, whether it conducts electricity and heat well, whether it is shiny and can be hammered or drawn into shapes (metals), or whether it is dull-looking, brittle, and a poor conductor as a solid (non-metals). None of this depends on whether the element happens to be a gas, liquid, or solid on a particular day — physical state at room temperature is a separate fact about an element, not the rule that defines its category.

Why the two axes still look connected. Among the elements named in this chapter, every gas at room temperature happens to be a non-metal. This is not a coincidence baked into the definition of "non-metal" — it reflects how weakly the atoms of these particular non-metals attract each other, so they stay spread out as a gas rather than clumping into a solid. But it is a pattern observed in this specific list of examples, not a universal law, which is exactly why mercury (a metal that is liquid) and bromine (a non-metal that is also liquid) coexist on the "liquid" list — one metal, one non-metal, side by side, proving that "liquid at room temperature" cannot be used the way "gas at room temperature" can in this chapter's examples.

Why metalloids need a third box. If metal and non-metal properties were opposites on a strict either/or switch, every element would sort cleanly. Silicon and boron show that properties can shade gradually from one behaviour to another — some metallic traits, some non-metallic traits, in the same element. Rather than forcing a bad fit into one of the two main groups, chemists name the in-between zone explicitly as metalloids, which keeps the two main categories clean and honest.

Worked example

Question: A student is given six elements — iron, sulfur, mercury, aluminium, gallium, silicon — and asked to (a) sort each into metal, non-metal, or metalloid, and (b) say which of them is a solid that melts unusually close to room temperature.

Step 1. Iron is explicitly listed among the metals in this chapter, so it is classified as a metal.

Step 2. Sulfur is explicitly listed among the non-metals, so it is classified as a non-metal.

Step 3. Mercury is one of the two elements that are liquid at room temperature, and it is specifically identified as a metal — classified as a metal, with the extra note that its liquid state is the exception to remember, since most metals are solids.

Step 4. Aluminium is explicitly listed among the metals — classified as a metal.

Step 5. Gallium is not on the metal/non-metal example list given in this chapter for that purpose, but it is named specifically as a solid that melts at around $30\ \degree\text{C}$ — so for part (b), gallium is one of the two low-melting solids to flag (the other being caesium, not in this list of six).

Step 6. Silicon is explicitly named as a metalloid — classified as a metalloid, with properties intermediate between metals and non-metals.

Final answer: metals = iron, mercury, aluminium; non-metal = sulfur; metalloid = silicon; and gallium is the element from this list that is a solid melting unusually close to room temperature ( $\approx 30\ \degree\text{C}$ ).

Competitive stretch

If asked to also state which elements in this list would be expected to be gases at room temperature, the answer is none of the six — none of iron, sulfur, mercury, aluminium, gallium, or silicon is a gas at room temperature. This is worth checking explicitly rather than guessing, since the "11 gases, all non-metals" fact only tells you that gaseous elements are non-metals — it does not mean every non-metal in a list is a gas. Sulfur, a non-metal, is a solid, which is exactly why the state fact must be checked per element, not assumed from the metal/non-metal label alone.

Common traps

Assuming every element is either a metal or a non-metal with nothing in between — silicon and boron form a genuine third category, metalloids, with properties intermediate between the two main groups.
Thinking "gas at room temperature" and "non-metal" are two ways of saying the same thing — the chapter's pattern only runs one way: all 11 named gaseous elements are non-metals, but not all non-metals are gases (sulfur, carbon are solids).
Assuming all liquids at room temperature must be metals because mercury is one — bromine, a non-metal, is also liquid at room temperature, so "liquid" alone does not tell you the category.
Trying to memorise detailed metalloid chemistry or their exact periodic-table position — at this level only the naming-level fact (silicon, boron are metalloids, with intermediate properties) is expected; the deeper chemistry is higher-grade content.

Ask the tutor: Want a quick round of classifying a few more elements as metal, non-metal, or metalloid to check your sorting?

If mercury is a metal but it is liquid, how do I remember it is still a metal?

Hold onto the property-based definition, not the state. Mercury conducts electricity well, is shiny (it looks like liquid silver), and behaves chemically like other metals — those properties, not its physical state, make it a metal. Its liquid state at room temperature is simply an unusual, exam-worthy exception among metals, most of which are solid. Think of it as "a metal that happens to melt very low," the same way gallium and caesium are "metals that happen to melt only a little above room temperature."

Are all non-metals brittle and dull, then, since sulfur and carbon are solids?

At this level, brittle and dull describes typical non-metal solids reasonably well, but exceptions exist even within this short list — graphite, a form of carbon, conducts electricity, which is unusual for a non-metal. The chapter does not require you to explain graphite's exception in detail; it is enough to know that "non-metal" is a properties-based category with occasional exceptions, not a rule with zero exceptions. Focus on the named examples and their listed properties rather than assuming every single trait applies without exception.

Why does the chapter even mention gases and liquids if the real classification is metal versus non-metal?

Because the state facts are useful, testable, memorable details that happen to correlate partly with the metal/non-metal split — and noticing a correlation is good scientific practice, as long as you do not mistake it for the definition. Knowing that all 11 named gaseous elements are non-metals gives you a quick sanity check: if you are told an element is a gas at room temperature, it is very likely a non-metal in this chapter's world. But it is a clue, not a rule you can apply blindly, which is exactly why the chapter also gives you the counter-example of liquids, where one metal and one non-metal share the same state.