What is the difference between metals and nonmetals?

The quick answer: it is about position

Everything follows from where an element sits. Metals occupy the left and centre of the periodic table — roughly three-quarters of all elements — while nonmetals crowd into the top-right corner, beyond the metalloid staircase. That geography is not decoration: the further right you go, the more tightly each atom holds its electrons, and the more an element behaves as a nonmetal.

The electron reason: metals have few electrons in their outer shell and lose them easily, arriving at a stable structure; nonmetals are a few electrons short of a full outer shell and gain or share electrons to complete it. Lose-versus-gain is the single difference from which every other difference in the table below follows.

The differences, side by side

For NCTB and board-style questions, these are the differences you are expected to state clearly — usually "any two" with an example of each.

• Physical state: metals are solid at room temperature (mercury is the lone exception); nonmetals are gases, brittle solids or, for bromine, a liquid.
• Lustre: metals are shiny when freshly cut; nonmetal surfaces are dull (iodine's grey shine is the notable exception).
• Conduction: metals conduct heat and electricity well (copper, aluminium); nonmetals are mostly insulators (sulfur, phosphorus), with graphite the famous carbon exception.
• Malleability: metals can be hammered into sheets and drawn into wires; solid nonmetals shatter into powder.
• Ion formation: metals lose electrons to form positive ions (Na → Na⁺); nonmetals gain electrons to form negative ions (Cl → Cl⁻).
• Oxides: metal oxides are usually basic or amphoteric ($\mathrm{Na_{2}O}$, $\mathrm{Al_{2}O_{3}}$); nonmetal oxides are acidic ($\mathrm{CO_{2}}$, $\mathrm{SO_{2}}$) — which is why burning coal feeds acid rain.

Metalloids: the border guards

Between the two territories runs a staircase — boron, silicon, germanium, arsenic, antimony, tellurium. These metalloids are hybrids: they look somewhat metallic but conduct electricity only moderately, and only under the right conditions. That "in-between" conductivity is exactly what makes silicon and germanium the raw material of every computer chip: semiconductors can be switched on and off, which pure conductors cannot.

So the exam-safe phrasing is: metal and nonmetal are the two ends of a gradient, and the periodic table is honest about the blur in the middle. If a question asks whether silicon is a metal, the full-credit answer notes it is a metalloid with properties of both.

Reading the table like a map

Once you see the pattern, the table stops being 118 separate facts. A metallic character runs from top-right (weakest) to bottom-left (strongest): francium's corner is the most metallic real estate in the table. Within a period, moving right, metallic character falls; within a group, moving down, it rises. That single trend, together with valence electrons, predicts how nearly any pair of elements will interact.

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