Nitrogen

Nitrogen: Basics

Nitrogen has the atomic number 7. It has 7 electrons and 7 protons. Its atomic mass is 14, so the number of neutrons it has is 7.

Nitrogen Periodic Table Tile | Science Notes

Nitrogen exists in the environment as nitrogen gas, with the molecular formula N₂.

Fig. 2: Lewis Structure of Nitrogen | StudySmarter Originals

Above is the lewis structure of nitrogen molecule. Nitrogen atoms are bonded with triple covalent bonds in N₂. Nitrogen has the electronic configuration [He] 2s² 2p³. There are 5 electrons in its outermost shell. It completes its octet by forming a triple bond with another nitrogen atom. 1 lone pair of electrons remains which is not participating in any bonding, and is shown by the two dots. Due to the presence of triple bonds, nitrogen is relatively less reactive.

At standard temperature and pressure (STP) conditions i.e., 0 ^oC temperature, 1 bar pressure, nitrogen is found in a gaseous state. It is an odourless, tasteless, colourless gas, and is non toxic.. The behaviour of nitrogen gas is close to an Ideal Gas, which means it is relatively more difficult to compress and liquify it. To liquify nitrogen, it is required to cool it below -195.8 ^oC, which is below the boiling point of other gases found in the air.

Let us now see what applications find nitrogen useful.

Nitrogen: Uses

Nitrogen finds many industrial and commercial uses.

Fertilisers

Nitrogen is at the core of internal functions of plants and trees. nitrogen forms the chlorophyl, which is resposible for photosynthesis, and gives it the green colour. Nitrogen is also the primary component in protoplasm - the living matter in cells.

Fertilisers produced industrially are rich in ammonium phosphate, ammonium nitrate, and urea - all of which are compounds of nitrogen. Nitrogen helps crops grow faster and also helps produce more crops, as it keeps the soil healthy.

Food preservation

Bags of chips, food cans, cut fruit packages are all sucked of oxygen and filled with nitrogen to preserve food. Nitrogen provides a modified atmosphere which prevents the oxidation of food, which is what causes it to become stale.

Medicine

Nitrous oxide is used for general anaesthesia, dental anaesthesia, and procedural sedation. Nitrogen is present in every major drug class, even antibiotics. Nitrogen is also used to preserve sperm, egg, blood and other biological specimen. This type of preservation is called cryopreservation.

Construction

Liquid nitrogen is used to cool metal in order to shrink it. This is useful in "shrink-fitting", where tolerances are tight. The metal piece is cooled and shrunk, fitted, and returned to normal temperature and size for a secure fit.

Liquid nitrogen may also be used to break metal in some cases. As cooling metal with liquid nitrogen makes it extremely brittle, it breaks easily.

Nitrogen is also used as a purging gas in welding. Welding of metal involved localised melting of metal pieces to join them together. When the metal is hot, it is susceptible to oxidation, which could damage the component. Nitrogen is used to isolate the heated component from air and also cool it to prevent oxidation.

Nitrogen: Properties

Nitrogen has high electronegativity

Nitrogen has the electronegativity of 3.04 measured on the Pauling scale. Normally, high electronegativity corresponds with high reactivity, but in nitrogen, the presence of triple bonds negates this and makes it relatively less reactive.

Nitrogen has very low reactivity

The low reactivity of nitrogen is due to the presence of triple bonds in its molecule. Triple bond is the strongest covalent bond. Nitrogen has a bond energy of +944 kJ mol^-1. To make nitrogen participate in chemical reactions, its bonds need to be broken, and that means that the reaction will have a very high activation energy.

Another factor contributing to the low reactivity of nitrogen is that the molecule is not polar. Since the molecule consists of 2 like molecules, there is no polarity, and the molecule is not easily polarised either. This makes electrophiles and nucleophiles difficult to react with nitrogen.

Isotopes and allotropes of nitrogen

Isotopes are various forms of the same element that contain the same number of protons and electrons, but different number of neutrons. Did you know that nitrogen has 2 isotopes? Nitrogen occurs naturally as N-14 and N-15. Although, only N14 is talked about because it is the most abundant out of the two (99.63%).

An allotrope is a physical form in which an element can exist purely. Nitrogen only exists as N₂, and therefore has no allotropes.

Isolation and purification of nitrogen

Nitrogen has many industrial applications, and that has required the development of methods to isolate and purify nitrogen from the rest of the components of air. This is done by fractional distillation. Fractional distillation takes advantage of the fact that different gases have different boiling points. Air is cooled to a temperature below the boiling point of all gases, then it is slightly warmed up. Nitrogen evaporates first since it has the lowest boiling point of the rest of the gases in air. The evaporated nitrogen is captured and stored.

Notable Compounds of Nitrogen

Let us discuss about some compounds of nitrogen that you'll be coming across very often while studying chemistry.

Ammonia

Ammonia is compound of nitrogen and hydrogen. It is a colourless gas with a pungent, suffocating smell, which is used. It is commonly used in fertilisers, as a refrigerant, and in household cleaning solutions.

Fig. 3: Dot and Cross Diagram of Ammonia | StudySmarter Originals.

Ammonia has alkaline properties, which means that when it reacts with water, it forms a solution that is basic. Recall that nitrogen has the electronic configuration of [He] 2s² 2p³. After making 3 covalent bonds with 3 hydrogen atoms to form ammonia, it has 1 free pair of electron. Nitrogen atom is able to donate this free pair of electron to electrophiles. The basic nature of ammonia comes from the ability of nitrogen to accept a proton (H⁺ ion) by donating an electron pair. This makes ammonia a Brønsted–Lowry base.

Oxides of Nitrogen (NO_x)

Oxides of nitrogen is a group of gases generally denoted as NO_x. Nitrogen forms the the following oxides -

• Nitric oxide (NO).
• Nitrogen dioxide (NO₂).
• Nitrous oxide (N₂O).
• Dinitrogen trioxide (N₂O₃).
• Dinitrogen tetroxide (N₂O₄).
• Dinitrogen pentoxide (N₂O₅).

Out of these, nitric oxide (NO) and nitrogen dioxide (NO₂) are the two most toxicologically significant compounds. Nitric oxide is a sweet-smelling, colourless gas which is used in the manufacture of explosives and fertilisers. Nitrogen dioxide is a reddish-brown gas with an irritating odour. It is an intermediate in the production of nitric acid (HNO₃).

These gases are greenhouse gases and are also highly toxic. Man made sources of these gases in the atmosphere include:

• Burning of fossil fuels - including in automobiles.
• Agricultural practices.
• Power plants, industrial boilers

NO_x emissions from automobiles

More than half of NO_x emissions are from transportation. Reducing NO_x emission from internal combustion engines in automobiles is a serious concern and has been made a priority by governments all around the world. A catalytic convertor is used in the exhaust pipe of automobiles, which uses reduction catalysts platinum and rhodium to reduce NO_x into N₂.

Role of NO_x in the formation of photochemical smog

Nitrogen oxides are majorly responsible for photochemical smog and acid rain. Nitrogen dioxide can react with oxygen in air and unburnt hydrocarbons from exhaust gases to form ozone and a compound called peroxyacetylnitrate (PAN), which is a major component responsible for a photochemical smog.

Fig. 4: General Formula of Peroxyacetylnitrate (PAN) | Wikimedia Commons.

PAN is one of a group of compounds called peroxyacylnitrates. In the general formula in the figure, the R- group is CH₃-, but can also be other hydrocarbon chain.

Role of NO_x in acid rain

NO_x, mainly NO and NO₂, released in the air due to burning of fossil fuels in vehicles and factories result in the phenomenon of acid rain. NO reacts with oxygen to produce more NO2. NO₂ reacts with moisture to produce dilute solutions of nitric acid (HNO₃) and nitrous acid (HNO₂), which come down with the rain.

$$2NO+O_{2}\rightarrow 2NO_{2}$$

$$2NO_{2}+H_{2}O\rightarrow HNO_{3}+HNO_{2}$$

Acid rain can hinder the growth of or kill plants and trees. It can free up metal ions which were previously bound to the soil. Metal ions such as calcium, potassium, and magnesium are crucial for plant growth, and acid rain washes them away. Aluminium ions are poisonous to vegetation. Acid rain frees them and enables them to be absorbed by the plants.

Acid rain also destroys monuments and building made of limestone. Limestone is calcium carbonate, and carbonates react with acid.

Water from acid rain flows down to lakes and rivers, and changes the pH of the water, which can be fatal for aquatic life.

Ammonium Ion

We discussed in ammonia how it accepts a proton i.e. an H⁺ ion by donating a pair of electron to it. This results in the formation of the ammonium ion, NH₄⁺.

Fig. 5: Ammonium Ion | StudySmarter Originals.

You can see that one of the N-H bonds is shown with an arrow. This shows the dative bond formed by donating of electrons by nitrogen to H⁺.

Ammonium ions are formed in a solution when ammonia reacts with water:

$$NH_{3 (g)}+H_{2}O_{(l)}\rightleftharpoons NH_{4(aq)}^{+}+OH^{-}_{(aq)}$$

When ammonium salts are heated with an alkali, ammonia gas is released.