Some Elements Refuse to Be Metals or Non-Metals

Because some elements genuinely will not sit in either group.

Silicon looks metallic and has a grey sheen, yet it is brittle and shatters like glass instead of bending like a metal. It conducts electricity — but poorly, far worse than copper and far better than rubber.

Calling it a metal would be wrong; calling it a non-metal would be equally wrong. So a third class exists, the metalloids, and it exists because nature did not sort itself into two neat boxes. This page covers the first part of the ICSE Class 8 Chemistry chapter on elements, compounds and mixtures: what purity means, how the three classes of matter differ, and how element symbols are written.

A pure substance is made of only one kind of particle and has a fixed composition throughout. Every part of it behaves identically, and it has sharp, definite properties such as a single melting point.

Pure substances come in two kinds.

An element is a pure substance that cannot be broken down into anything simpler by any chemical process. It is made of only one kind of atom. Oxygen, hydrogen, carbon, iron, gold, sulphur, copper.

A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass. Water, common salt, sugar, carbon dioxide, calcium carbonate.

A mixture contains two or more substances physically mixed in any proportion, with no chemical combination. It is therefore an impure substance. Air, milk, brass, sea water, soil, sand in water.

What fixed ratio actually means. Water is always ${\text{H}}_2\text{O}$. Taking hydrogen as $1$ and oxygen as $16$:

$$\text{one molecule}=2(1)+16=18$$

so by mass the hydrogen and oxygen are in the ratio

$$2:16=1:8$$

This means $18\text{g}$ of water always contains $2\text{g}$ of hydrogen and $16\text{g}$ of oxygen, and $9\text{g}$ of water always contains $1\text{g}$ of hydrogen and $8\text{g}$ of oxygen — anywhere in the world, from any source.

A mixture has no such rule. Salt water can be slightly salty or very salty and is still salt water, because nothing forced the two into a fixed proportion.

Sorting some everyday samples:

- Element — copper wire, an iron nail, a gold ornament, oxygen gas, graphite, sulphur powder
- Compound — water, common salt, sugar, baking soda, marble, carbon dioxide
- Mixture — air, milk, brass, bronze, soil, sea water, tea, sand in water

The two that catch students out. Air is a mixture, not a compound: its proportions vary from place to place, and its gases can be separated by physical means. Brass is a mixture of copper and zinc even though it looks like a single metal — an alloy is a mixture, not a compound.

By a set of physical properties, and the metalloids show some from each side.

Metals are characterised by:

- Lustre — a shiny surface when freshly cut or polished
- Malleability — they can be beaten into thin sheets without breaking
- Ductility — they can be drawn into wires
- Good conduction of heat and electricity
- Sonority — they make a ringing sound when struck
- High density and high melting point, and they are solid at room temperature

Examples: iron, copper, aluminium, gold, silver, zinc, sodium.

Non-metals are largely the opposite:

- Dull in appearance
- Brittle — they break or crumble instead of bending
- Not ductile and not sonorous
- Poor conductors of heat and electricity
- Low density and low melting point, and they may be solid, liquid or gas

Examples: oxygen, nitrogen, sulphur, phosphorus, carbon, chlorine, iodine.

Metalloids show properties of both. They often have a metallic lustre but are brittle, and they conduct electricity weakly — far less than a metal and far more than a non-metal.

The metalloids to know: silicon, germanium, boron, arsenic, antimony and tellurium.

The exceptions that every exam asks about:

- Mercury is a metal that is liquid at room temperature — the only one.
- Bromine is a non-metal that is liquid at room temperature — the only one.
- Graphite, a form of carbon, is a non-metal that conducts electricity well, which is why it is used for electrodes and in pencil leads.
- Iodine is a non-metal with lustre.
- Sodium and potassium are metals soft enough to cut with a knife, and they are less dense than water.
- Diamond, also carbon, is the hardest natural substance and yet a non-metal.

Why so many exceptions? Because these are trends, not laws. The properties describe how most metals and most non-metals behave, and the list of exceptions is exactly why the metalloid category had to be invented for the elements that sit in the middle.

Where the practical difference shows up. Aluminium is drawn into wire and beaten into foil because it is ductile and malleable; sulphur can be neither, because it simply crumbles. And silicon's weak, controllable conduction is precisely what makes it useful in electronic components — a property no metal and no true non-metal offers.

A symbol is a short chemical shorthand for an element — one or two letters standing for one atom of it.

Rule 1 — one capital letter, used where the first letter of the name is not shared with another element.

- Hydrogen — $\text{H}$
- Oxygen — $\text{O}$
- Nitrogen — $\text{N}$
- Carbon — $\text{C}$
- Sulphur — $\text{S}$
- Phosphorus — $\text{P}$
- Iodine — $\text{I}$
- Fluorine — $\text{F}$
- Boron — $\text{B}$

Rule 2 — two letters, the first capital and the second small, used when several elements share a first letter.

- Calcium — $\text{Ca}$
- Magnesium — $\text{Mg}$
- Aluminium — $\text{Al}$
- Zinc — $\text{Zn}$
- Chlorine — $\text{Cl}$
- Helium — $\text{He}$
- Neon — $\text{Ne}$
- Silicon — $\text{Si}$
- Barium — $\text{Ba}$

The second letter is always small. Writing $\text{CA}$ instead of $\text{Ca}$ is a genuine error, not a matter of neatness, because $\text{CA}$ reads as carbon followed by an unknown element.

Rule 3 — from the Latin name. A number of elements were known long before English chemistry named them, and their symbols come from the Latin name instead of the English one. This is why the symbol often looks unrelated to the word you use.

- Sodium — Natrium — $\text{Na}$
- Potassium — Kalium — $\text{K}$
- Iron — Ferrum — $\text{Fe}$
- Copper — Cuprum — $\text{Cu}$
- Silver — Argentum — $\text{Ag}$
- Gold — Aurum — $\text{Au}$
- Lead — Plumbum — $\text{Pb}$
- Tin — Stannum — $\text{Sn}$
- Mercury — Hydrargyrum — $\text{Hg}$
- Antimony — Stibium — $\text{Sb}$

Tungsten's symbol $\text{W}$ comes from its older name wolfram in the same way.

Why symbols beat words. A symbol lets a compound be written as a formula that shows exactly which elements are present and in what numbers. ${\text{H}}_2\text{O}$ says two hydrogen atoms and one oxygen atom, and it says it identically in every language. Writing two parts of hydrogen joined to one of oxygen is longer, vaguer and untranslatable.

And the symbol stands for one atom. $\text{O}$ means one atom of oxygen; ${\text{O}}_2$ means one molecule of oxygen gas, made of two atoms. That distinction matters as soon as formulas and equations begin.