Metals, Non-metals and Metalloids

Introduction

There are well over a hundred elements, and learning each one separately would be overwhelming. Chemists make the task easier by sorting elements into groups that behave in similar ways. The oldest and most useful division is into metals and non-metals . A small number of elements sit on the border and share features of both; these are called metalloids . Knowing which class an element belongs to lets you predict a great deal about how it looks, feels and reacts.

Core explanation

Metals. About three-quarters of all elements are metals. Typical physical properties of metals are:

- a shiny (lustrous) surface when freshly cut or polished; - good conduction of heat and electricity; - malleability — they can be hammered into sheets; - ductility — they can be drawn into wires; - high density, high melting point and high boiling point (with exceptions); - sonority — they ring when struck.

Iron, copper, aluminium, gold and magnesium are typical metals. All metals except mercury are solids at room temperature.

Non-metals. There are only about twenty non-metals, but they include many of the most important elements for life, such as carbon, hydrogen, oxygen and nitrogen. Typical properties are:

- a dull surface (if solid); - poor conduction of heat and electricity — they are insulators; - brittleness when solid, so they shatter rather than bend; - generally low melting and boiling points; many are gases at room temperature.

Metalloids. Elements such as boron, silicon, germanium, arsenic and antimony have a mixture of properties. Silicon, for example, is shiny like a metal but brittle like a non-metal, and it conducts electricity only moderately. Materials like this are called semiconductors , and their conductivity increases as the temperature rises.

Chemical differences. The classes also differ in their reactions. Metal oxides are usually basic : they react with acids. Non-metal oxides are usually acidic : many dissolve in water to form acidic solutions. For example, magnesium oxide neutralises acids, while carbon dioxide forms a weakly acidic solution in water.

Exceptions. No single property is a perfect test. Graphite, a form of carbon, is a non-metal but conducts electricity. Mercury is a metal but is a liquid. Sodium is a metal but is soft enough to cut with a knife and has a low density. That is why chemists look at several properties together before deciding.

Step-by-step reasoning

To classify an unknown element from data:

1. Check electrical conductivity of the solid. Good conduction suggests a metal. 2. Check the appearance and whether it bends or shatters. Shiny and bendable suggests a metal; dull and brittle suggests a non-metal. 3. Look at the oxide. A basic oxide points to a metal; an acidic oxide points to a non-metal. 4. If the evidence is mixed, for example shiny but brittle with moderate conduction, classify it as a metalloid.

Visual explanation

On most periodic tables, a thick staircase line runs diagonally from boron down to astatine. Metals lie to the left of this line and non-metals to the right, with hydrogen as a special case at the top left. The metalloids sit along the staircase itself, like houses built right on a boundary.

Real-world analogy

Sorting elements is like sorting animals into mammals and birds. Most animals fit neatly, but the platypus lays eggs like a bird while having fur and producing milk like a mammal. Metalloids are the "platypuses" of the element world: they do not fit neatly on either side.

Real-world example

Electrical wiring uses copper, a metal, because it conducts electricity very well and is ductile. The plastic coating contains mainly carbon and hydrogen, both non-metals, which do not conduct. The computer chip the wire powers is made from silicon, a metalloid, whose controllable conductivity makes it perfect for electronics.

Why?

Why do metals conduct electricity while most non-metals do not? In a metal, some electrons are not tied to any one atom and can move freely through the whole solid. When a voltage is applied, these mobile electrons flow as a current. In most non-metals, all the electrons are held tightly in bonds, so there are no free charges to carry a current.

Common misconception

"All metals are hard, heavy and magnetic." In fact, only a few metals, such as iron, cobalt and nickel, are strongly magnetic. Sodium and potassium are soft and less dense than water. Being a metal depends on the full set of properties, especially conductivity, not on hardness or magnetism.

Worked example

Question: Element X is a grey, shiny solid. It shatters when hit with a hammer, conducts electricity weakly, and its conductivity increases when warmed. Classify X.

Reasoning: Shininess suggests a metal, but brittleness suggests a non-metal. Weak conductivity that rises with temperature is the behaviour of a semiconductor.

Answer: X is a metalloid (for example, silicon or germanium).

Quick check

1. Name the only metal that is a liquid at room temperature. Answer: Mercury.

Exam focus

Be ready to list at least four physical properties of metals and non-metals and to classify an element from a table of data. Electrical conductivity is the most reliable single test, but remember graphite as the key exception. State that metal oxides are basic and non-metal oxides are acidic.

Advanced insight

The division between metals and non-metals is not fixed for all conditions. Under enormous pressure, hydrogen is predicted to become metallic, and this may be what happens deep inside Jupiter. Tin, meanwhile, exists as a shiny metallic form above about 13 °C and as a brittle grey, non-metallic form at lower temperatures. Classification describes typical behaviour under ordinary conditions.

Summary

Elements are classified as metals, non-metals or metalloids. Metals are shiny, malleable, ductile, good conductors and form basic oxides. Non-metals are dull, brittle when solid, poor conductors and form acidic oxides. Metalloids such as silicon have intermediate properties and are semiconductors. No single property decides the class, so several are considered together.

Practice questions

1. Give three physical properties that distinguish a typical metal from a typical non-metal. Answer: Metals are shiny, good conductors of electricity and malleable; non-metals are dull, poor conductors and brittle when solid. 2. Why is graphite an unusual non-metal? Answer: It conducts electricity, which is normally a property of metals. 3. Sulfur dioxide dissolves in water to give an acidic solution. What does this suggest about sulfur? Answer: Sulfur is a non-metal, because non-metal oxides are usually acidic. 4. State two metalloids and one use of a metalloid. Answer: Silicon and germanium (also boron, arsenic, antimony); silicon is used to make computer chips and solar cells.