Oxygen

Oxygen (O)

Introduction

Oxygen is a nonmetallic chemical element with atomic number 8 in the periodic table. Occurring with 467,100 ppm or 46% in Earth’s crust, it’s the most abundant element both in the thin outer layer of our planet and the human body. As a member of the oxygen family of periodic table elements, this chalcogen occurs in three aggregate states (gas, solid, and liquid form), and displays high reactivity with all the other chemicals.

The element 8 is found in almost all biomolecules of all living organisms. Oxygen’s greatest role is supporting life itself. Without oxygen, there would be no life in any form on Earth and vice versa – without life, there would be no oxygen in the atmosphere. Since it’s highly soluble in water, it makes life possible in rivers, lakes, seas, and oceans.

Fact Box

Chemical and Physical Properties of Oxygen

The symbol in the periodic table of elements: O

Atomic number: 8

Atomic weight (mass): 15.999 g.mol-1

Group number: 16 (The Chalcogens)

Period: 2 (p-block)

Color: Colorless gas / Pale-blue solid and liquid form

Physical state: Nonmetal / Gas at room temperature (also: solid and liquid form)

Half-life: From 580(30)×10−24 seconds to 122.24 seconds

Electronegativity according to Pauling: 3.5

Density: 1.429 kg/m3 at 20°C

Melting point: −218.79°C, −361.82°F, 54.36K

Boiling point: −182.962°C, −297.332°F, 90.188K

Van der Waals radius: 0.074 nm

Ionic radius: 0.14 nm (-2)

Isotopes: 16

Most characteristic isotope: 16O, 17O, and 18O.

Electronic shell: [He] 2s22p4

The energy of the first ionization: 1314 kJ.mol-1

The energy of the second ionization: 3388 kJ.mol-1

The energy of the third ionization: 5300 kJ.mol-1

Discovery date: In 1774 by Joseph Priestley and Carl W. Scheele (independently)

Being one of the chalcogens, oxygen represents an element essential for life. All members of Group 16 have two elections in the outer s-orbital, and 4 electrons in the p-orbitals. However, the chemical properties of oxygen are very different from the properties of the other chalcogens (sulfur, selenium, tellurium, and polonium).  

To begin with, oxygen is classified under the symbol O with atomic number 8 in the periodic table of elements. It has an atomic mass of 15.999 g.mol-1 and electron configuration [He] 2s22p4. Oxygen is colorless, tasteless, odorless, and water-soluble gas. It’s twice more soluble than nitrogen. The solid and liquid forms of oxygen are pale-blue substances in color. 

When exposed to lower temperatures and higher pressure, solid oxygen transforms its physical aspect from blue monoclinic crystals to red, orange, black, and even a metallic appearance. In this regard, we can distinguish six phases of solid oxygen:

  • α-phase: Solid oxygen adopts light sky-blue color and forms a monoclinic crystal structure;
  • β-phase: Solid oxygen adopts pale blue to pink color and forms rhombohedral crystal structure;
  • γ-phase: Solid oxygen adopts pale blue color and forms cubic crystal structure;
  • δ-phase: Solid oxygen adopts orange color when exposed to a pressure of 9 GPa;
  • ε-phase: Solid oxygen adopts dark-red to black color when exposed to a pressure greater than 10 GPa;
  • ζ-phase: Solid oxygen adopts metallic luster when exposed to a pressure greater than 96 GPa.

Furthermore, this member of the oxygen family of elements (the chalcogens) in the periodic table has an electronegativity of 3.5 according to Pauling, whereas the atomic radius of oxygen according to van der Waals is 0.074 nm. Also, this element reaches its boiling point at −182.962°C, −297.332°F, 90.188K, while the melting point is achieved at −218.79°C, −361.82°F, 54.36K.

Oxygen has a cubic lattice structure and paramagnetic properties, i.e. it possesses small susceptibility to a magnetizing force in its gaseous, solid, and liquid aggregate state. There are two allotropes of oxygen, the molecular oxygen O2 (dioxygen) and ozone O3 (triatomic molecule). While the oxygen itself doesn’t burn, it supports the combustion of the other chemicals.                      

How Was Oxygen Discovered?

Curiously enough, the story of oxygen’s discovery begins – not with a chemist, as expected – but with a painter. Namely, at the beginning of the 16th century, the Italian artist, engineer, scientist, and polymath of the High Renaissance, Leonardo Da Vinci, tried to design an aid that would help a man to breathe underwater. His inspiration for this invention came after he observed that a fraction of air is consumed in both respiration and combustion. 

In 1604, when the Polish alchemist, physician, and pioneer of chemistry Michael Sendivogius (Michał Sędziwój) traced the path to the discovery of the element that means life. In his work “De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromt” (also known as: “The New Light of Alchemy”), Sendivogius observes a new substance that is a part of the air. According to the words of Sendivogius, this substance is “the food of life” because “it gives life to everything”. 

By conducting thermal decomposition of potassium nitrate (saltpeter), the Polish alchemist managed to isolate the life-giving substance, but the results of his work remained unconfirmed until the 18th century.

Almost two centuries later, oxygen was discovered approximately at the same time by two independent discoverers: Joseph Priestly and Carl W. Scheele.

The Discovery of Carl W. Scheele 

In 1772, the Swedish chemist Carl Wilhelm Scheele attempted an experiment by exposing a compound containing potassium nitrate, manganese oxide, and mercury oxide to heat. Scheele’s findings from this chemical trial confirmed the presence of a combustion-enhancing gas in the compound. 

Eagerly trying to find out more about this new substance, the Swedish chemist continued to study the gas. For this, he conducted another experiment. This time, Sheele put a live mouse in a jar filled with oxygen and closed the lid tightly, projecting it would survive for only 15 minutes. But, the mouse remained alive even after the time Sheele projected. 

As a result of this experiment, Sheele observed that the new gas plays a significant biological role. However, he published his findings much later, in 1777. In the meantime, the English chemist Joseph Priestley made an independent discovery of oxygen and published his findings before Sheele did. By this act, he was officially recognized as the first discoverer of oxygen. 

The Discovery of Joseph Priestley

By conducting thermal decomposition of mercuric oxide, the English chemist and natural philosopher Joseph Priestley succeeded in isolating the oxygen gas from the compound in 1774. He was also quick to publish his findings the same year, which brought him fame as the discoverer of the chemical element that gives life – oxygen.  

How Did Oxygen Get Its Name?

In the period between 1775 – 1780, the French chemist Antoine-Laurent Lavoisier discarded the phlogiston theory and studied more closely the role of oxygen in respiration and combustion. Upon analysis, he observed that oxygen has a distinct affinity of forming acids with the other chemical elements. 

For this reason, Antoine Lavoisier considered the Greek words ‘oxys’ (acid) and ‘genes’ (forming) to be the most suitable choice for the name of the new element oxygène, since it carries the meaning of ‘acid former’. 

Where Can You Find Oxygen?

The protoatmosphere on early Earth wasn’t anything like today. It was believed that it was made up of hydrogen and helium – the elements born with “The Big Bang”. After this point, three major oxygenation processes have occurred in the Universe. Those processes were triggered by the volcanic activity and other internal processes of the Earth which led to the formation of the second atmosphere rich in oxygen. The oxygen we breathe today comes from the lowest level of the atmosphere.

The atmospheric oxygen had been incorporated in the Earth’s crust by several mechanisms:

  1. By a chemical reaction of oxygen-rich sulfates in undersea hot springs with the iron contained in the seafloor sediments;
  2. By absorption of the oxygen-rich water in the Earth’s crust;
  3. By precipitation of the product formed by a chemical reaction between oxygen-rich seawater and iron;
  4. By the process of photosynthesis supported by cyanobacteria that replaced the carbon dioxide concentrations with pure oxygen.

The rock formations on our planet are 48% oxygen by composition. In the rocks, oxygen is combined with metals and nonmetals in the form of oxides which are acidic or basic by nature. The largest portion of the oxygen concentration on Earth occurs in a form of silicon dioxide, commonly known as sand. Furthermore, oxygen can be found as a constitutional element of almost every living cell or molecule, i.e. in the human body, the plants, the animals, surface waters, underground streams, minerals, in the air – literally everywhere around us (and within us).  

Purified oxygen is obtained by air liquefaction, while the oxygen gas is mainly obtained through fractional distillation of liquefied air. Oxygen can also be synthesized in a laboratory by electrolysis of water by heating potassium chlorate (KClO3).

Oxygen in Everyday Life

Apart from being used by humans, animals, and plants in the process of respiration, oxygen also has a wide application in the following instances:

  • Medical liquid oxygen is the most essential medical aid that is administered to patients in case of an emergency to support the breathing;
  • Oxygen concentrators are medical devices that deliver supplemental oxygen to patients with oxygen levels below normal. They are used as a part of the hospital and home therapy for COPD, asthma, cystic fibrosis, sleep apnea, pulmonary emphysema, and other breathing-related disorders;
  • An air and oxygen mixture supports the underwater breathing of scuba divers; 
  • Hydrogen peroxide has a wide application as an antiseptic, bleaching agent, as well as an oxidizer. It’s typically used in medicine and households;
  • Oxygen is also used in steel production, mining, and the mining and manufacture of stone and glass products;
  • Oxygen gas is used in water treatment, as an oxidizer in rocket fuel, as well as in the manufacturing process of iron, steel, and oxidation-controlled chemicals such as acetylene, ethylene oxide, and methanol;
  • Carbon and sulfur are isolated from iron ore using oxygen;
  • Burning acetylene with O2 produced a very hot flame which is used in oxyacetylene welding;
  • Diethyl ether is an oxygen compound frequently used as an anesthetic, boiling solvent, or starting fluid;
  • Being 1.658  times denser than air, and having a boiling point of −112 °C (−170 °F) at atmospheric pressure, ozone is frequently applied as a powerful oxidizing agent;
  • Oxygen-15 isotope is used in radiology as a part of positron emission tomography, or PET imaging. 

Oxygen and Health

According to physiology, almost 60% of the human adult body is water (H2O). Due to the presence of oxygen atoms in the molecule of water, we can freely say that 2/3 of the human body is made up of oxygen. 

Oxygen is the most important chemical element for humans, animals, and plants. It is a major constituent of the DNA and supports the respiration process as the most vital function of life. Each of the organs or tissues of the body require different oxygen concentration to maintain proper functioning. 

The other vitally significant functions of oxygen in our body include:

  • Conversion of the food into energy for the body;
  • Support of the cellular mechanisms;
  • Boosting of the immune system.

The Brain and the Oxygen Levels

O2 is especially important for the function of the brain, which requires a constant supply of oxygen and glucose to perform its role. Without a steady flow of oxygen to the brain, all the systems of the body would be endangered because the brain could not translate the sensory data adequately and relay them to the corresponding organs and tissues. For this, our brain consumes 25% of all oxygen intake by an adult person. 

What Is Cerebral Hypoxia?

In cases when the regular oxygen flow to the brain is interrupted, cerebral hypoxia (or brain hypoxia) can occur. The oxygen in our body can be depleted by: 

  • Cigarette smoke inhalation (or any other smoke);
  • Carbon monoxide poisoning;
  • Cardiac arrest;
  • Drowning;
  • Choking;
  • All other instances where the respiration is obstructed. 

The individuals affected by this dangerous and life-threatening condition may experience:

 

  • Dizziness;
  • Blurring of vision;
  • Poor judgment;
  • A decline in cognitive functions;
  • Memory loss;
  • Nausea;
  • Severe headache;
  • Loss of consciousness;
  • Complete unawareness of their surrounding;
  • Unresponsiveness;
  • Failure of the respiratory function.

Types of Cerebral Hypoxia

There are four types of cerebral (brain) hypoxia:

  • Hypoxic hypoxia – A condition when the amount of oxygen supply for tissues is reduced by the partial pressure of oxygen in arterial blood.
  • Anemic hypoxia – When there is less hemoglobin concentration in the blood due to anemia, anemic hypoxia occurs.
  • Histotoxic hypoxia – This type occurs when there’s enough oxygen supply in the body, but the tissues are unable to use it.
  • Stagnant hypoxia – This condition occurs as a result of slow blood flow and, consequently, lowered supply of oxygen to the organs and tissues.

To increase oxygen levels, a person must frequently ventilate the spaces where one stays, lead an active lifestyle, eat healthily, and cut down on cigarettes. 

How Dangerous Is Oxygen?

While O2 is non-toxic, O3 can lead to adverse health effects if inhaled. According to the New York State Department of Health, the United States, the individuals who have been exposed to high levels of ozone may experience the following symptoms:

  • Irritation of the eyes, nose, and throat;
  • Chest pain and shortness of breath;
  • Fatigue;
  • Coughing;
  • Inflammation of the airways.

In more severe cases, extreme exposure to O3 may result in:

  • Asthma, 
  • Pulmonary emphysema;
  • Chronic bronchitis.

Carbon Dioxide (CO2) Poisoning

Carbon dioxide intoxication and poisoning occur upon exposure to high levels of CO2. Upon intoxication with carbon dioxide, unconsciousness occurs in less than a minute when the CO2 concentration rises over 10%. 

What are the Symptoms of Carbon Dioxide Poisoning?

The individual affected by carbon dioxide poisoning typically displays the following symptoms:

  • Difficult and laborious breathing;
  • Muscle cramps and twitching;
  • Elevated blood pressure;
  • Increased pulse rate;
  • Loss of judgment;
  • Disorientation;
  • Headache;
  • Dizziness;
  • Death.

Environmental Effects of Oxygen

Air is a mixture of gases combined in different proportions. In essence, there is 78.1% nitrogen and 20.9% oxygen in the air. The rest are gases present in trace amounts in the air, such as carbon dioxide, water vapor, ozone, nitrogen oxides, sulfur dioxide, dust and smoke particles, ammonia, etc. Even a presence of some of the rare gases (neon, xenon, helium) can be detected as a result of the industrial processes. 

How Do Trees Produce Oxygen?

Oxygen (O2) in our planet’s atmosphere adopts an unstable atomic structure. For this reason, it must be constantly renewed. This is achievable by the process of photosynthesis, when the green plants, trees, algae, and some types of bacteria use sunlight to synthesize oxygen and energy stored in glucose from carbon dioxide and water. 

The sunlight energy is stored in small organelles labeled as chloroplasts. These organelles are made up of light-absorbing pigment, i.e. chlorophyll which reflects the green-light waves, thus giving the plants and trees their green color. 

During photosynthesis, the trees and the plants absorb carbon dioxide (CO2) from the soil and air. The water becomes oxidized in the cells of the green plants and trees (it loses electrons), while the CO2 is reduced (it gains electrons). By this process, water is converted into oxygen, and carbon dioxide is turned into glucose. After that, the plants and the trees fill the glucose molecules with energy and release the oxygen back into the Earth’s atmosphere.

The Ozone Hole and the Greenhouse Gasses

Ozone (O3) is a pale-blue poisonous gas with a pungent odor. In the stratosphere, it forms a protective shield for the Earth from the harmful ultraviolet radiation coming from the Sun. On the other hand, ozone changes its role at lower altitudes and turns into one of the major components of smog. 

Unfortunately, today we are aware of extensive damage to the ozone layer, labeled as “the ozone hole”. Namely, in the 1970s, the Earth’s protective layer has become visibly thinner which allows for greater amounts of ultraviolet radiation to reach our planet’s surface, thus increasing global warming. This is due to the increased concentration of chlorofluorocarbons (carbon, chlorine, and fluorine molecules) which have been released by the always-expanding industries. 

To heal the ozone layer, many governments took a proactive approach in dealing with pollution, mainly by banning greenhouse gases, such as carbon dioxide and sulfur dioxide, and deforestation.  

Isotopes of Oxygen

There are 16 isotopes of oxygen with atomic mass ranging from 11O to 26O, of which 13 are radioisotopes. Among them, only three forms of this chemical element are stable: 16O, 17O, and 18O. These three stable isotopes make up the naturally occurring form of oxygen. 

With 99.762% occurrence in nature, 16O is the most abundant oxygen isotope. Oxygen-16 is the primary isotope because it represents a primary product of stellar evolution. It is formed by the helium fusion process and neon burning process of massive stars. When hydrogen is burned into helium during the CNO cycle, the oxygen-17 isotope is produced. The third stable isotope of oxygen, 18O, is formed when nitrogen-14 from CNO burning fuses with a helium-4 nucleus.

Having a half-life of 122.24 seconds, the oxygen-15 isotope has a wide application in radiography. It’s synthetically produced by deuteron bombardment of nitrogen-14 in a cyclotron. 

Nuclide[2]

[n 1]

Z N Isotopic mass (Da)[3]

[n 2]

Half-life

[resonance width]

Decay

mode

[n 3]

Daughter

isotope

[n 4]

Spin and

parity

[n 5][n 6]

Natural abundance (mole fraction)
Excitation energy Normal proportion Range of variation
11O[4] 8 3 [~3.4 MeV] 2p 9C 3/2−, 5/2+
12O 8 4 12.034262(26) > 6.3(30)×10−21 s

[0.40(25) MeV]

2p (60.0%) 10C 0+
p (40.0%) 11N
13O 8 5 13.024815(10) 8.58(5) ms β+ (89.1%) 13N (3/2−)
β+, p (10.9%) 12C
14O 8 6 14.008596706(27) 70.620(13) s β+ 14N 0+
15O 8 7 15.0030656(5) 122.24(16) s β+ 15N 1/2−
16O[n 7] 8 8 15.99491461960(17) Stable 0+ 0.99757(16) 0.99738–0.99776
17O[n 8] 8 9 16.9991317566(7) Stable 5/2+ 3.8(1)×10−4 (3.7–4.0)×10−4
18O[n 7][n 9] 8 10 17.9991596128(8) Stable 0+ 2.05(14)×10−3 (1.88–2.22)×10−3
19O 8 11 19.0035780(28) 26.470(6) s β 19F 5/2+
20O 8 12 20.0040754(9) 13.51(5) s β 20F 0+
21O 8 13 21.008655(13) 3.42(10) s β 21F (5/2+)
22O 8 14 22.00997(6) 2.25(9) s β (78%) 22F 0+
β, n (22%) 21F
23O 8 15 23.01570(13) 97(8) ms β (93%) 23F 1/2+
β, n (7%) 22F
24O 8 16 24.01986(18) 77.4(45) ms β (57%) 24F 0+
β, n (43%) 23F
25O 8 17 25.02934(18) 5.18(0.35)×10−21 s n 24O 3/2+#
26O 8 18 26.03721(18) 4.2(3.3) ps 2n 24O

Source: Wikipedia

List of Oxygen Compounds

Oxygen adopts the oxidation state of -2 in almost all compounds, except in few peroxides where the oxidation state of oxygen is -1. The oxygen compounds in an oxidation state of -1 are labeled as peroxides. When exposed to higher temperatures, this chemical element forms oxides with all elements, except with the noble gases krypton, neon, helium, and argon. 

In the reactions with metals, oxygen forms basic anhydrides or basic oxides that are soluble in H2O and form hydroxides in a further reaction. 

Being a highly reactive chemical element, this chemical element forms a vast number of organic compounds, such as glycerol, acetic anhydride, citric acid, formaldehyde, glutaraldehyde, and acetamide. In continuation, oxygen creates numerous chemical bonds that also form: 

 

  • Arsenates;
  • Bicarbonates;
  • Chloryl compounds;
  • Esters;
  • Ethers;
  • Ferrates;
  • Fulminates;
  • Hydrates;
  • Hydroxides;
  • Hypochlorites;
  • Nitrogen-oxygen compounds;
  • Organic hydroxy compounds‎;
  • Oxyanions;
  • Perbromates;
  • Peroxides;
  • Phosphates;
  • Phenols;
  • Phenolates;
  • Silicate esters;
  • Superoxides;
  • Tellurates.

The most common oxygen compounds are listed below:‎

  • Disodium hydrogen arsenate
  • Calcium bicarbonate
  • Sodium bicarbonate
  • Magnesium bicarbonate
  • N-Acetylglutamic acid
  • Cefadroxil
  • Cefatrizine
  • Cefluprenam
  • Cefoperazone
  • Melatonin
  • Methazolamide
  • Midodrine
  • Nicotinamide mononucleotide
  • Nicotinamide riboside
  • Nicotinyl methylamide
  • Allysine
  • 2-Aminomuconic semialdehyde
  • Calostomal
  • 2-Carboxybenzaldehyde
  • 4-Carboxybenzaldehyde
  • Elenolic acid
  • Glutamate-5-semialdehyde
  • Glyoxylic acid
  • 2-Hydroxymuconate semialdehyde
  • L-Aspartate-4-semialdehyde
  • 2-Methyl-3-oxopropanoic acid
  • Succinic semialdehyde
  • Traumatin
  • Chlorous acid
  • Sodium chlorite
  • Water
  • Amlodipine
  • Ethyl acetate
  • Ethyl acetoacetate
  • Ethyl acrylate
  • Ethyl benzoate
  • Calcium hypochlorite
  • Sodium hypochlorite
  • Sulfite
  • Calcium sulfite
  • Magnesium sulfite
  • Perbromate
  • Perbromic acid
  • Birnessite
  • Boehmite
  • Brucite
  • Alterite
  • Asbolane
  • Hydrogen peroxide
  • Sodium peroxide
  • Chromium oxide
  • Cobalt(III) oxide
  • Copper(I) oxide
  • Gold(III) oxide
  • Iron(III) oxide-hydroxide
  • Manganese dioxide
  • Manganese oxide
  • Manganese(III) oxide
  • Niobium monoxide
  • Silver oxide
  • Titanium dioxide
  • Vanadium(IV) oxide
  • Zirconium dioxide
  • Zinc oxide
  • Barium oxide
  • Lanthanum oxide
  • Lead dioxide
  • Lead oxide
  • Lanthanum strontium cobalt ferrite
  • Lanthanum strontium manganite
  • Lanthanum ytterbium oxide
  • Potassium oxide
  • Thulium(III) oxide
  • Tin(IV) oxide
  • Uranium hexoxide
  • Sulfur oxide
  • Sulfur dioxide
  • Sulfur monoxide
  • Sulfur trioxide.

 

Water (H2O)

Water is an integral part of Earth’s hydrosphere. The pure water molecule comprises two hydrogen atoms and one oxygen atom in its structure. This iconic compound of oxygen is held together by very strong covalent bonds. These chemical bonds have been formed by the electron exchange between the hydrogen atoms and the single oxygen atom. Namely, hydrogen provides the two additional atoms to the six electrons of oxygen’s outer shell that are needed for the oxygen to adopt the stable octet configuration. 

The solid form of H2O (ice) has a lower density than the liquid H2O. For this reason, the ice cubes float in a glass of water. Water can also occur in a form of precipitation (rain, snow, fog, hail), clouds, vapor, etc. 

5 Interesting Facts and Explanations

  1. The most iconic oxygen compounds include water (H2O), table sugar (C12H22O11), baking soda (NaHCO3), bleach (NaClO), hydrogen peroxide (H2O2), acetone ((CH3)2CO), carbon dioxide (CO2), melanin (C18H10N2O4), citric acid (C6H8O7), glycerol (C3H8O3), etc. 
  2. Chalcogens are the elements gathered in the Group 16 of the periodic table. This family of elements is made up of the elements oxygen, sulfur, selenium, tellurium, and polonium. Since all of these elements support life itself, this group is considered the most significant one in Mendeleev’s system.
  3. The deep-red and green hues of Aurora Borealis and Aurora Australis come from the oxygen atoms split by the highly energetic electrons from the solar wind. 
  4. As a result of its small atomic size, oxygen cannot bond with more than four elements and rarely adopts the central atom position in a molecule.
  5. Out of all biomolecules, only carotene and squalene do not contain oxygen. 

Chemical Property and physical property of element Oxygen

Symbol of Oxygen: O
Name: Oxygen
Atomic Number of Oxygen: 8
Group of Oxygen: Non-Metal
Crystal Structure of Oxygen: Cubic
Atomic Weight of Oxygen: 15.9994
Shells of Oxygen: 2,6
Orbitals of Oxygen: [He] 2s2 2p4
Valence of Oxygen: 2
Melting Point of Oxygen: 54.36
Boiling Point of Oxygen: 90.188
Electro Negativity of Oxygen: 3.44
Covalent Radius of Oxygen: 0.73 Å
Ionic Radius of Oxygen: 1.40 (-2) Å
Atomic Radius of Oxygen: 0.65 Å
Atomic Volume of Oxygen: 14.0 cm³/mol
Name Origin of Oxygen: Greek: oxys and genes, (acid former).
Discovered of Oxygen By: Joseph Priestly, Carl Wilhelm Scheele
Year: 1774
Location: England/Sweden
Pronounced of Oxygen: OK-si-jen
Oxydation States of Oxygen: -2
Density of Oxygen: 0.001429 g/cm³
Uses of Oxygen: Used in steel making, welding, and supporting life. Naturally occuring ozone (O3) in the upper atmosphere shields the earth from ultraviolet radiation.
Description of Oxygen: Greenish-yellow, pungent, corrosive gas. Extremely reactive. Does not occur uncombined in nature.