Atomic number 7

Nitrogen

N
+ phase
− phase

Shells Orbitals 2s 2p

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7 electrons in 2 shells: 2, 5.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Nonmetal
Period: 2
Group: 15
Block: p
Standard atomic weight: 14.007(1) Da
Electron configuration: [He] 2s² 2p³
Phase at 25 °C: gas
Melting point: −210 °C
Boiling point: −195.795 °C
Density: 1.2506 g/L
Electronegativity (Pauling): 3.04
Discovered: 1772, Daniel Rutherford

Neighbours in the table

C O

A nitrogen atom holds seven electrons in two shells, two in the first and five in the second, [He] 2s² 2p³. The free element does not travel as single atoms. It goes about as N₂, two atoms that put three electrons each into the bond between them, and that triple bond is the reason nitrogen is so unreactive. At room temperature the gas reacts with almost nothing: with lithium, which gives Li₃N, with certain transition-metal complexes, and inside nitrogen-fixing bacteria.

Break that bond and the five outer electrons prove flexible. Nitrogen forms compounds in every oxidation state from −3 to +5. The oxidation state is a bookkeeping number, the charge the atom would carry if every bond were ionic, not a real charge. One end of that range is ammonia; the other is nitric acid. That acid has an anhydride, N₂O₅: the oxide that becomes the acid on contact with water.

A jar of the gas tells the senses nothing: nitrogen is colourless, odourless and tasteless. Cool it to −195.795 °C and it condenses to a liquid that looks like water. That low boiling point, together with a low price, is why liquid nitrogen is the coolant chemistry laboratories keep on hand.

Where it is found

Nitrogen is an atmospheric element before it is anything else. Earth’s air is 78.1% nitrogen by volume: more than 20 million tons of it over every square mile of ground, some 4,000 trillion tons in all. That is local rather than universal: the atmosphere of Mars is 2.6% nitrogen. Pure nitrogen is not mined but distilled, by liquefying air and separating the liquid.

The rock underneath holds little: the estimated crustal abundance is 19 milligrams per kilogram, and seawater about 0.5 milligrams per litre. Where the climate is dry, decaying organic matter leaves sodium nitrate and potassium nitrate as saltpetre beds, dug up and spread as fertiliser. Nitrogen’s name comes from the Latin nitrum and the Greek nitron, “native soda”, plus genes, “forming”.

Nitrogen was found in air by subtraction. In 1772 the Scottish physician Daniel Rutherford took the oxygen and carbon dioxide out of a sample of air; what remained supported neither combustion nor life. Scheele, Cavendish and Priestley were working on the same gas, which they called burnt air. Lavoisier proposed the name azote, “without life”, which the element’s own biochemistry refutes.

Why it matters

Nitrogen sets the limit on how much a crop can grow. It is part of chlorophyll, of amino acids and so of proteins, and of DNA and RNA. Plants cannot take up N₂; something has to break the triple bond first. Bacteria break it with nitrogenases, enzymes containing iron and molybdenum; the best known are the rhizobia in the root nodules of legumes. Lightning breaks it crudely, forcing nitrogen and oxygen into oxides that wash into the soil.

Industry breaks the same bond in a factory. The Haber process combines nitrogen with hydrogen to make ammonia, the largest single use of the element; the ammonia industry is its largest consumer. Ammonia in turn becomes urea for fields, plastics and animal feed, and, through the Ostwald process, nitric acid.

The remaining uses ask nothing of nitrogen except that it do nothing. Sealed over canned food and luncheon meat it stops them oxidising, so they keep their colour and flavour longer. The same inertness shields semiconductor lines and welds, and pressurises aircraft tyres. Cold, it becomes a tool in its own right. From dewars, the insulated vessels laboratories store it in, liquid nitrogen freezes food, preserves blood and freezes lesions off skin in a dermatologist’s office.

Properties

Properties of nitrogen Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	7
Standard atomic weight	14.007(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−210 °C	°C	Los Alamos: −210 °C
Boiling point	−195.795 °C	°C	Los Alamos: −195.795 °C
Density	1.2506 g/L	g/L
Electronegativity (Pauling)	3.04
Electron configuration	1s² 2s² 2p³
Phase at 25 °C	gas

The standard atomic weight of this element is an interval in nature, from 14.00643 to 14.00728 Da; the value shown is IUPAC's conventional abridged value.

Safety

Nitrogen gas is neither toxic nor flammable: it is already most of the air in the room, and PubChem’s safety summary describes it that way. The cautions it lists are physical: gas under pressure in a cylinder, cold burns from the refrigerated liquid, oxygen crowded out of a closed room by a large release. Nitrogen has no colour and no smell, so nothing warns of that last one. Anyone working with cylinders or dewars should read the PubChem Laboratory Chemical Safety Summary for nitrogen.

Questions

Why can't plants use the nitrogen in the air?

Air is about 78% nitrogen, but the element is there as N2, two atoms held together by a triple bond. Plants cannot use it in that form. The bond is broken for them by bacteria carrying iron-and-molybdenum nitrogenase enzymes, by lightning passing through air, and, in a factory, by the Haber process.

Is liquid nitrogen dangerous?

It is not poisonous, but it is cold enough to injure: liquid nitrogen boils at about −196 °C, and the safety data list cryogenic burns among its hazards. Laboratories keep it in insulated vessels called dewars. The gas boiling off it can also crowd the oxygen out of a closed room, and it gives no smell or colour as a warning. PubChem's safety summary for nitrogen covers handling.

Why is nitrogen used in food packaging?

Food loses colour and flavour partly by reacting with the oxygen in the air sealed in with it. Nitrogen takes that oxygen's place without reacting with the food itself, so canned foods and packaged luncheon meats sealed under nitrogen instead of air keep their colour and flavour and spoil more slowly. US regulators list nitrogen as a food ingredient used as an antioxidant, a formulation aid and a propellant.

Why is nitrogen's atomic weight not a single fixed number?

Nitrogen has two stable isotopes, nitrogen-14 at 99.636% of atoms and nitrogen-15 at 0.364%. That ratio varies a little from place to place, so the average mass of a nitrogen atom depends on where the sample came from. IUPAC therefore publishes the standard atomic weight as an interval rather than one number, alongside a conventional abridged figure.

Compounds containing nitrogen

N₂O Nitrous oxide 3 atoms, 2 bonds.
NH₃ Ammonia 4 atoms, 3 single bonds, trigonal pyramidal.
HNO₂ Nitrous acid 4 atoms, 3 bonds.
HNO₃ Nitric acid 5 atoms, 4 bonds.
KNO₃ Potassium nitrate 5 atoms in one formula unit of an ionic lattice.
AgNO₃ Silver nitrate 5 atoms in one formula unit of an ionic lattice.
NH₄Cl Ammonium chloride 6 atoms in one formula unit of an ionic lattice.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.

All 48 compounds containing nitrogen

C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
(CH₃)₃N Trimethylamine 13 atoms, 12 single bonds.
C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.
C₅H₅N₅ Adenine 15 atoms, 16 bonds.
C₃H₆N₆ Melamine 15 atoms, 15 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₅H₉N₃ Histamine 17 atoms, 17 bonds.
C₃H₈NO₅P Glyphosate 18 atoms, 17 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₅H₈NO₄Na Monosodium glutamate 19 atoms in one formula unit of an ionic lattice.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₄H₁₁N₅ Metformin 20 atoms, 19 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₁₀H₁₄N₂ Nicotine 26 atoms, 27 bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₅H₁₁I₄NO₄ Thyroxine 35 atoms, 36 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₁₇H₁₈F₃NO Fluoxetine 40 atoms, 41 bonds.
C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.
C₁₀H₁₆N₅O₁₃P₃ Adenosine triphosphate 47 atoms, 49 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.
C₃₃H₃₅FN₂O₅ Atorvastatin 76 atoms, 79 bonds.

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