Period 2 element

A period 2 element is one of the chemical elements in the second row (or period) of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The second period contains more elements than the previous period, with eight elements: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. This situation can be explained by modern theories of atomic structure.

History

Electronic structure

A period 2 element is one of the chemical elements in the second row (or "period") of the periodic table of the chemical elements. This second period contains more elements than period 1, with eight: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon. In a quantum mechanical description of atomic structure, this period corresponds to the filling of the 2s and 2p orbitals. Period 2 elements obey the octet rule in that they need eight elements to complete their valence shell. The maximum number of electrons that these elements can accommodate is ten, two in the 1s orbital, two in the 2s orbital and six in the 2p orbital.

Period trends

Diatomic molecules

Elements

Lithium

Lithium (Li) is the chemical element with atomic number 3, occurring in two isotopes: ⁶Li and ⁷Li. At standard temperature and pressure, lithium is a soft, silver-white, highly reactive alkali metal. With a density of 0.564 g·cm⁻³, lithium is the lightest metal and the least dense solid element. The most common naturally occurring form of lithium is lithium-7, symbol ⁷Li, which comprises of 92.5% of lithium abundance and has three protons and four neutrons. Lithium-6, symbol ⁶Li, is also stable, containing three protons and three neutrons. The two make up all natural occurrence of lithium on Earth, although further isotopes have been synthesised. In ionic compounds, lithium loses an electron to become positively charged, forming the cation Li⁺.

According to theory, Lithium is one of the few elements synthesised in the Big Bang, making it a primordial element. Lithium is the 33rd most abundant element on earth, occurring in concentrations of between 20 and 70 ppm by weight, but due to its high reactivity it is only ever found naturally in compounds. The most abundant source of lithium-containing compounds are granitic pegmatites, with spodumene and petalite being the most commercially-viable mineral sources for the element. Commercially, the metal is isolated electrolytically from a mixture of lithium chloride and potassium chloride.

Lithium salts are used in the pharmacology industry as mood stabilising drugs. They are used in the treatent of bipolar disorder, where they have a role in treating depression and mania and may reduce the chances of suicide. The most common compounds used are lithium carbonate, Li₂CO₃, lithium citrate, Li₃C₆H₅O₇, lithium sulphate, Li₂SO₄, and lithium orotate, LiC₅H₃N₂O₄·H₂O. Lithium is also used in batteries as an anode and its alloys with aluminium, cadmium, copper and manganese are used to make high performance parts for aircraft, most notably the external tank of the Space Shuttle.

Beryllium

Beryllium (Be) is the chemical element with atomic number 4, occurring in the form of ⁹Be. At standard temperature and pressure, beryllium is a strong, steel-grey, light-weight, brittle, bivalent alkali earth metal, with a density of 1.85 g·cm⁻³. It also has one of the highest melting points of all the light metals. Beryllium's most common isotope is ⁹Be, which contains 4 protons and 5 neutrons. It makes up almost 100% of all naturally occurring beryllium and is its only stable isotope; however other isotopes have been synthesised. In ionic compounds, beryllium loses its two valence electrons to form the cation, Be²⁺.

Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further to create larger nucleii, like carbon, nitrogen or oxygen. Beryllium is a component of 100 out of 4000 known minerals, such as bertrandite, Be₄Si₂O₇(OH)₂, beryl, Al₂Be₃Si₆O₁₈, chrysoberyl, Al₂BeO₄, and phenakite, Be₂SiO₄. Precious forms of beryl are aquamarine, bixbite and emerald. The most common sources of beryllium used commercially are beryl and bertrandite and production of it involves the reduction of beryllium fluoride with magnesium metal or the electrolysis of molten beryllium chloride, containing some sodium chloride as beryllium chloride is a poor conductor of electricity.

Due to its stiffness, light weight, and dimensional stability over a wide temperature range, beryllium metal is used in as a structural material in aircraft, missiles and communication satellites. It is used as an alloying agent in beryllium copper, which is used to make electrical components due to its high electrical and heat conductivity. Sheets of beryllium are used in X-ray detectors to filter out visible light and let only X-rays through. It is used as a neutron moderator in nuclear reactors because light nuclei are more effective at slowing down neutrons than heavy nuclei. Beryllium's low weight and high rigidity also make it useful in the construction of tweeters in loudspeakers.

Beryllium and beryllium compounds are classified by the International Agency for Research on Cancer as Group 1 carcinogens; they are carcinogenic to both animals and humans. Chronic berylliosis is a pulmonary and systemic granulomatous disease caused by exposure to beryllium. Between 1% - 15% of people are sensitive to beryllium and may develop an inflammatory reaction in their respiratory system and skin, called chronic beryllium disease. The body's immune system recognises the beryllium as foreign particles and mounts an attack against them, usually in the lungs where they are breathed in. This can cause fever, fatigue, weakness, night sweats and difficulty in breathing.

Boron

Boron (B) is the chemical element with atomic number 5, occurring as ¹⁰B and ¹¹B. At standard temperature and pressure, boron is a trivalent metalloid that has several different allotropes. Amorphous boron is a brown powder formed as a product of many chemical reactions. Crystalline boron is a very hard, black material with a high melting point and exists in many polymorphs: Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the most common. Boron has a density of 2.34⁻³. Boron's most common isotope is ¹¹B at 80.22%, which contains 5 protons and 6 neutrons. The other common isotope is ¹⁰ at 19.78%, which contains 5 protons and 5 neutrons. These are the only stable isotopes of boron; however other isotopes have been synthesised. Boron forms covalent bonds with other nonmetals and has oxidation states of 1, 2, 3 and 4.

Beryllium does not occur naturally as a free element, but in compounds such as borates. The most common sources of boron are tourmaline, borax, Na₂B₄O₅(OH)₄·8H₂O, and kernite, Na₂B₄O₅(OH)₄·2H₂O. it is diffucult to obtain pure boron. It can be made through the magnesium reduction of boron oxide, B₂O₃. This oxide is made by melting boric acid, B(OH)₃, which in turn is obtained from borax. Small amounts of pure boron can be made by the thermal decomposition of boron bromide, BBr₃, in hydrogen gas over hot tantalum wire, which acts as a catalyst. The most commercially important sources of boron are: sodium tetraborate pentahydrate, Na₂B₄O₇ · 5H₂O, which is used in large amounts in making insulating fiberglass and sodium perborate bleach; boron carbide, a ceramic material, is used to make armour materials, especially in bulletproof vests for soldiers and police officers; orthoboric acid, H₃BO₃ or boric acid, used in the production of textile fiberglass and flat panel displays; sodium tetraborate decahydrate, Na₂B₄O₇ · 10H₂O or borax, used in the production of adhesives; and the isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons.

Boron is an essential plant micronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development. However, high soil concentrations of over 1.0 ppm can cause necrosis in leaves and poor growth. Levels as low as 0.8 ppm can cause these symptoms to appear in plants particularly boron-sensitive. Most plants, even those tolerant of boron in the soil, will show symptoms of boron toxicity when boron levels are higher than 1.8 ppm. In animals, boron is an ultratrace element; in human diets, daily intake ranges from 2.1–4.3 mg boron/kg body weight (bw)/day. It is also used as a supplement for the prevention and treatment of osteoporosis and arthritis.

Carbon

Carbon is the chemical element with atomic number five, occurring as ¹²C, ¹³C and ¹⁴C. At standard temperature and pressure, carbon is a solid, occurring in many different allotropes, the most common of which are graphite, diamond, the fullerenes and amorphous carbon. Graphite is a soft, hexagonal crystalline, opaque black semimetal with very good conductive and thermodynamically stable properties. Diamond however is a highly transparent colourless cubic crystal with poor conductive properties, is the hardest known naturally occuring mineral and has the highest refractive index of all gemstones. In contrast to the crystal lattice structure of diamond and graphite, the fullerenes are molecules, named after Richard Buckminster Fuller whose architecture the molecules resemble. There are several different fullerenes, the most widely known being the "buckeyball" C₆₀. Little is known about the fullerenes and they are a current subject of reseach. There is also amorphous carbon, which is carbon without any crystalline structure. In mineralogy, the term is used to refer to soot and coal, although these are not truly amorphous as they contain small amounts of graphite or diamond. Carbon's most common isotope at 98.9% is ¹²C, with six protons and six neutrons. ¹³C is also stable, with six protons and seven neutrons, at 1.1%. Trace amounts of ¹⁴C also occur naturally but this isotope is radioactive and decays with a half life of 5730 years; it is used for radiocarbon dating. Other isotopes of carbon have also been synthesised. Carbon forms covalent bonds with other non-metals with an oxidation state of -4, -2, +2 or +4.