Non-metals
The elements of air and life: few in number, but they make up almost all of you.
Most of the periodic table is metal, yet most of you is not. Water, protein, fat, DNA and the air in your lungs are built almost entirely from a small cluster of elements in the top right of the table. This page covers the seven that the site's dataset labels simply as non-metals: hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium.
They are not the only non-metals. The halogens in group 17 and the noble gases in group 18 are non-metals as well, but each of those groups has such a strong family character that it has its own page. The seven here do not belong to a tidy column, but they share a way of behaving that is roughly the reverse of everything a metal does.
What they have in common
Non-metals have outer shells that are close to full, so instead of giving electrons away they either take them from metals or share them with other non-metals. Sharing produces covalent bonds and small molecules. Hydrogen, nitrogen and oxygen exist as pairs of atoms, written as H2, N2 and O2, while phosphorus and sulfur form larger clusters such as P4 and S8. The forces between these molecules are weak, so many non-metals are gases or low-melting solids.
They are poor conductors of heat and electricity because their electrons are locked into bonds and cannot roam. Solid non-metals tend to be dull and brittle: sulfur crumbles into yellow powder and white phosphorus is waxy. Their densities are low.
When non-metals burn in oxygen, the oxides they form dissolve in water to give acids. Carbon dioxide makes rain slightly acidic, sulfur dioxide and nitrogen oxides from burning fuels make it much more so. This acidic behaviour is the clearest chemical contrast with metals, whose oxides are basic.
How they change across the block
Across the second row, from carbon to oxygen, the atoms get smaller, hold their electrons more tightly and become more eager to grab electrons from others. Going down a column, atoms get larger and their non-metal character fades. Below oxygen is sulfur, still a clear non-metal, and below that selenium, which is starting to show metalloid tendencies. Below nitrogen is phosphorus, and below phosphorus the metalloid arsenic.
Boiling points reflect the size of the molecules and how strongly they attract one another. Hydrogen boils at about minus 253 degrees Celsius, nitrogen at about minus 196 degrees and oxygen at about minus 183 degrees, so all three are gases in any place humans live. Sulfur melts at about 115 degrees Celsius and selenium at about 221 degrees.
Carbon and phosphorus are missing melting points in the dataset for a good reason. Both exist in several structural forms, called allotropes, that behave differently. Diamond, graphite and graphene are all pure carbon, and at ordinary pressure carbon turns to vapour before it melts. Phosphorus comes as waxy white phosphorus, which catches fire in air, and as far safer red phosphorus, among other forms.
Where you meet them
You meet these elements in every breath. Air is roughly four fifths nitrogen and one fifth oxygen. Your body is mostly water, which is hydrogen and oxygen, and the rest is built on chains of carbon atoms with nitrogen, oxygen, phosphorus and sulfur attached. DNA is held together by a backbone of phosphate groups, and bones are a calcium phosphate mineral.
In industry, hydrogen is combined with nitrogen to make ammonia for fertilisers, and it is being developed as a clean fuel. Carbon in the form of coal, oil and gas still supplies most of the world's energy. Sulfur is turned into sulfuric acid, one of the most widely produced chemicals on Earth, and is used to toughen rubber. Phosphorus compounds go into fertilisers, matches and detergents. Selenium is a trace nutrient in tiny amounts, colours glass red and was once the light-sensing element in photocopiers.
Odd ones out
Hydrogen is the strangest member. It is placed at the top of group 1 with the alkali metals because it has a single outer electron, yet it is a non-metal gas that usually shares that electron rather than losing it. It can also gain an electron to form a negative ion when combined with very reactive metals.
Carbon breaks the rule about conductivity. Graphite, one of its forms, conducts electricity well enough to be used as electrodes and brush contacts in motors. Diamond, by contrast, is the hardest natural substance and an electrical insulator. Selenium is the other rule-bender. Its grey form conducts better when light falls on it, which is the property photocopiers and early light meters relied on, and it is close enough to the metalloids that some tables put it there.
The numbers
Melting points rise down the list, from Hydrogen at -259 °C to Selenium at 221 °C.
| No. | Element | Mass (u) | Melts (°C) | Boils (°C) | Density | State |
|---|---|---|---|---|---|---|
| 1 | Hydrogen (H) | 1.01 | -259 | -253 | 0.08988 | Gas |
| 6 | Carbon (C) | 12.01 | – | – | 1.821 | Solid |
| 7 | Nitrogen (N) | 14.01 | -210 | -196 | 1.251 | Gas |
| 8 | Oxygen (O) | 16.00 | -219 | -183 | 1.429 | Gas |
| 15 | Phosphorus (P) | 30.97 | – | – | 1.823 | Solid |
| 16 | Sulfur (S) | 32.06 | 115 | 445 | 2.07 | Solid |
| 34 | Selenium (Se) | 78.97 | 221 | 685 | 4.81 | Solid |
Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.
Did you know?
- Air is roughly four fifths nitrogen and one fifth oxygen by volume.
- Hydrogen boils at about minus 253 degrees Celsius, about twenty degrees above the coldest temperature possible.
- Graphite conducts electricity even though it is a non-metal, while diamond, also pure carbon, is an insulator.
- White phosphorus ignites on contact with air, so it is stored under water.
- Sulfuric acid, made from sulfur, is one of the most widely produced industrial chemicals in the world.
- The halogens and the noble gases are also non-metals, and each has its own page on this site.
Frequently asked questions
What are the properties of non-metals?
Non-metals are usually dull, brittle solids or gases that are poor conductors of heat and electricity. They have low densities and low melting points compared with metals, they tend to gain or share electrons rather than lose them, and their oxides dissolve in water to form acids.
Why are most non-metals gases?
Non-metal atoms bond with each other to form small molecules such as O2 and N2. The molecules themselves are held together strongly, but the forces between separate molecules are weak, so little heat is needed to pull them apart. Larger molecules, such as the S8 rings in sulfur, attract each other more strongly, which is why sulfur is a solid.
Is hydrogen a metal or a non-metal?
Hydrogen is a non-metal. It is a colourless gas that does not conduct electricity and it usually forms covalent bonds by sharing its one electron. It sits at the top of group 1 only because it has a single outer electron, not because it behaves like the alkali metals below it.
Which non-metal conducts electricity?
Carbon in the form of graphite is the only common non-metal that conducts electricity well. Each carbon atom in graphite bonds to three neighbours in flat sheets, leaving one electron per atom free to move along the sheet. Selenium and some forms of phosphorus conduct weakly, especially when warmed or lit.
Filter the bubbles to this family, or colour them by melting point to watch the trend.
Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).