The World of Non-Metals
Defining Properties of Non-Metals
Non-metals are defined by a set of physical and chemical properties that are, for the most part, the complete opposite of metals. While there are always exceptions in science, the following properties are the most common identifiers for non-metallic elements.
Physical State: Non-metals exist in all three states of matter at room temperature:
- Gases: Oxygen ($O_2$), Nitrogen ($N_2$), Hydrogen ($H_2$), Chlorine ($Cl_2$), and the Noble Gases like Helium (He).
- Liquid: Bromine (Br) is the only non-metal that is a liquid at room temperature.
- Solids: Carbon (C) (as diamond or graphite), Sulfur (S), Phosphorus (P), and Iodine (I).
Appearance (Dullness): Solid non-metals lack the shiny, reflective quality known as luster that metals possess. For example, a piece of coal (mostly carbon) is black and dull, while a piece of sulfur is a powdery yellow. Iodine crystals have a slight metallic sheen, which is a notable exception.
Malleability and Ductility (Brittleness): Non-metals are neither malleable (cannot be hammered into thin sheets) nor ductile (cannot be drawn into wires). Instead, they are brittle. If you hit a lump of sulfur with a hammer, it will shatter into a fine powder. This is because their atomic structure lacks the "sea of electrons" that allows metal atoms to slide past one another.
Conductivity: This is one of the most defining characteristics. Non-metals are typically very poor conductors of heat and electricity. This is because their electrons are tightly bound to their atoms and are not free to move around and transfer energy. This property makes them excellent insulators. The plastic coating on electrical wires is made from carbon-based (non-metal) compounds, and it's there to protect you because it does not conduct electricity.
Locating Non-Metals on the Periodic Table
The periodic table is organized in a way that clearly shows the division between metals, metalloids1, and non-metals. Non-metals are located on the right-hand side of the periodic table.
The group of elements known as the Halogens2 (Group 17) and the Noble Gases3 (Group 18) are all non-metals. Hydrogen, the first element, is also a non-metal, though it behaves uniquely. The other non-metals, like Carbon, Nitrogen, Oxygen, Phosphorus, Sulfur, and Selenium, are found in the upper-right portion of the table. A distinctive "stair-step" line separates the metals from the non-metals, with the elements bordering this line (Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, Polonium) being the metalloids, which have properties of both metals and non-metals.
| Element (Symbol) | State at Room Temp. | Appearance | Common Use |
|---|---|---|---|
| Hydrogen (H) | Gas | Colorless | Rocket fuel, making ammonia |
| Carbon (C) | Solid | Black (graphite) or clear (diamond) | Pencils, diamonds, all living things |
| Nitrogen (N) | Gas | Colorless | $78\%$ of air, fertilizers |
| Oxygen (O) | Gas | Colorless | Respiration, combustion |
| Sulfur (S) | Solid | Brittle yellow solid | Gunpowder, sulfuric acid |
| Chlorine (Cl) | Gas | Yellow-green gas | Water purification, bleach |
The Chemical Behavior of Non-Metals
Chemically, non-metals tend to gain or share electrons when they react with other elements. This is in direct contrast to metals, which tend to lose electrons.
Forming Ions: Non-metal atoms have a high number of electrons in their outer shell. To achieve a stable electron configuration, they gain electrons to form negatively charged ions called anions4. For example, a chlorine atom ($Cl$) gains one electron to become a chloride anion ($Cl^-$). An oxygen atom ($O$) gains two electrons to become an oxide anion ($O^{2-}$).
Forming Covalent Bonds: Non-metals often react with other non-metals by sharing electrons. This sharing creates a strong link called a covalent bond5. The molecules we breathe are perfect examples: an oxygen molecule ($O_2$) is two oxygen atoms sharing two electrons each. A water molecule ($H_2O$) is formed when two hydrogen atoms share their electrons with one oxygen atom.
Acidity: Many non-metal oxides, when dissolved in water, form acidic solutions. For instance, carbon dioxide ($CO_2$) gas dissolves in water to form carbonic acid ($H_2CO_3$), which is what gives soda its fizz and slight tang. Sulfur dioxide ($SO_2$) can dissolve in water vapor in the atmosphere to form sulfurous acid, a component of acid rain.
Non-Metals in Action: From Air to Life
The importance of non-metals cannot be overstated. They are not just a category on a chart; they are the building blocks of our world and our very existence.
The Air We Breathe: The Earth's atmosphere is approximately 78% nitrogen gas ($N_2$) and 21% oxygen gas ($O_2$), both non-metals. We could not live without them.
The Basis of Life: All known life forms are carbon-based. Carbon's unique ability to form long chains and complex structures (organic chemistry) is the foundation of proteins, DNA, carbohydrates, and fats. The other key elements of life are also non-metals: oxygen, hydrogen, nitrogen, phosphorus, and sulfur.
Everyday Materials:
- Insulation: The poor conductivity of non-metals makes them perfect for insulation. Homes are insulated with materials like fiberglass (made from silicon, oxygen, and other elements) to trap heat. Styrofoam cups (made of carbon and hydrogen) keep hot drinks hot by not conducting the heat to your hand.
- Fertilizers: Nitrogen, phosphorus, and potassium are key nutrients for plant growth. Most fertilizers are primarily composed of compounds containing these non-metal elements.
- Technology: Silicon and germanium (metalloids with non-metal properties) are semiconductors, meaning their conductivity can be controlled. This property is the basis of every computer chip, transistor, and solar cell.
Common Mistakes and Important Questions
A: This is a trick question! It depends on the form. The general rule is that non-metals like carbon are poor conductors. This is true for diamond. However, graphite, another form of carbon, is a notable exception. In graphite, the carbon atoms are arranged in layers that allow electrons to move freely, making it a good conductor of electricity. This is why graphite is used in electrodes.
A: While the majority of gaseous elements are non-metals, not all gases are elements. Air is a mixture of elemental gases and compounds. Furthermore, some compounds formed from metals can be gases (e.g., hydrogen chloride, HCl). But among pure elements, the gases are exclusively non-metals or noble gases.
A: Hydrogen is a special case. It has one electron in its outer shell, similar to the alkali metals in Group 1. This is why it is placed above them. However, it behaves more like a non-metal because it typically gains or shares an electron to achieve a stable configuration, rather than losing its one electron (though it can do that too under certain conditions). Its unique behavior places it in a category of its own.
Footnote
1 Metalloids: Elements that have properties of both metals and non-metals (e.g., Silicon, Boron).
2 Halogens: The elements in Group 17 of the periodic table (F, Cl, Br, I, At). They are very reactive non-metals.
3 Noble Gases: The elements in Group 18 of the periodic table (He, Ne, Ar, Kr, Xe, Rn). They are very unreactive non-metals.
4 Anion: A negatively charged ion formed when an atom gains one or more electrons.
5 Covalent Bond: A chemical bond that involves the sharing of electron pairs between atoms.