Hydrogen is the chemical element with the symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.

The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is problematic in metallurgy because it can embrittle many metals, complicating the design of pipelines and storage tanks.

Hydrogen gas is flammable: 2 H2 + O2 → 2 H2O + 572 kJ The enthalpy of combustion is −286 kJ/mol:Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%. The explosive reactions may be triggered by heat, or sunlight; the hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.

H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.

The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy than the para form, it is unstable and cannot be purified. At low tem

Cephalopod dermal structures

Cephalopods exhibit various dermal structures on their mantles and other parts. These may take the form of conspicuous warts, papillae or scales, though in many species they are microscopic tubercles; the most elaborate forms are found among the oceanic squid of the order Teuthida. Most cephalopod dermal structures take the form of tubercles, these are the only cartilaginous dermal structures. All three main types of cartilage found in vertebrates are represented among the different squid species: hyaline and fibrocartilage. Tubercles of hyaline cartilage are associated with cranchiid or glass squid; the vast majority of cephalopod dermal structures have a thin, overlying epidermal layer, though this is damaged or missing in captured specimens. Other cephalopods with prominent dermal structures include: Brachioteuthis spp.. Among octopuses, Ocythoe tuberculata is noted for the ridges on its mantle. Different cephalopod dermal structures are hypothesised to play roles in buoyancy and pseudoskeletal support.

Two fundamentally different buoyancy mechanisms associated with dermal structures have been proposed. The mantle of Cranchia scabra is covered in multi-pointed cartilaginous tubercles. An anti-predator function has been proposed in the past, but this is thought unlikely given the small size and transparent nature of the tubercles; the tubercles of this species are covered by a thin, epidermal sheath, lost during capture. It has been speculated that in the live animal the interstitial space is filled with a buoyant fluid and acts as a "buoyancy vest"; the hard tubercles may serve to maintain the shape of this structure. It has been estimated that in a C. scabra measuring 10 cm in mantle length, the buoyancy vest could contribute an additional 4% to the animal's total buoyant fluid sufficient to achieve neutral buoyancy. A similar mechanism may be utilised by the much larger Galiteuthis glacialis, which has a similar combination of tubercles and overlying epidermal sheath; the overlapping "scales" of Lepidoteuthis grimaldii are dermal cushions with a vacuolate internal structure that are continuous with a vacuolate underlying layer of mantle tissue.

Ammonium ions are present in the mantle of this species at a measured concentration of 172 mM. Structurally similar dermal cushions are found in Pholidoteuthis adami, it has been proposed that these two species achieve buoyancy by means of the fluid stored in their vacuolate dermal cushions and upper mantle layer. Given their spongy form, these cushions may play a secondary protective role; the complex dermal structures of Pholidoteuthis massyae may play a role in reducing hydrodynamic drag. More they may be involved in maintaining laminar flow by preventing or delaying boundary layer separation along the mantle, it is possible that a similar locomotory mechanism is present in Mastigoteuthis cordiformis and Mastigoteuthis hjorti, though the small size of the tubercles in these species may preclude such a function. In the cranchiids Leachia cyclura and Liocranchia reinhardti, the dermal tubercles are not distributed throughout the mantle but arranged in discrete cartilaginous bands. A role in buoyancy control is therefore unlikely.

One possibility is that these rigid bands play a pseudoskeletal role, maintaining the shape of parts of the mantle during swimming contractions or providing attachment points for mantle muscles or visceral tissue. The dense tuberculate ridges found on the arms and dorsal mantle of Histioteuthis meleagroteuthis may provide insertion points for muscles, are most important in juvenile animals, which lack well-developed musculature. In the two other Histioteuthis species with tuberculate ridges—H. Meleagroteuthis and H. miranda—these structures have the same function. The mantle of Asperoteuthis acanthoderma is covered in minute spaced tubercles of hyaline-like cartilage. In a 1990 study of dermal structures in squid, Clyde F. E. Roper and C. C. Lu wrote that they were "unable to suggest a function" for the tubercles of this species, but that due to their small size and spacing they were unlikely to be involved in buoyancy or locomotion

This Lullaby

This Lullaby is a young adult novel written by Sarah Dessen. Remy is an eighteen-year-old, about to leave for college, her father, a musician, wrote his one and only hit song the day. The song, called "This Lullaby," became popular, but he died soon after its release. Now, Remy's mother is getting married for the fifth time. After her mother's previous failed marriages, love is something. One day, she randomly meets Dexter at a car dealership, he claims to feel a connection with her the second he saw her. He is two of her least favorite traits, but he is persistent. She finds herself falling for him, she doesn't want to care about him, but somehow she just can't bring herself to get rid of him. They start dating and she is surprised by how open and honest and caring he is; when Dexter overhears Remy saying that she only wants him to be a summer fling, they break up. Remy begins to date another guy, but she finds herself always thinking about Dexter. Meanwhile, her brother is getting engaged, her mother's new husband is cheating with his secretary, her friends are all having problems of their own.

But in the end, Remy realizes that she does love Dexter, they get back together. Remy still leaves for college but in Just Listen it is revealed that Remy and Dexter are together because Remy is shown with Dexter while Remy is on fall break from college; this lullaby is only a few words A simple run of chords Quiet here in this spare roomBut you can hear it, hear itWherever you may goI will let you downBut this lullaby plays on... Note: When Truth Squad does a cover of "This Lullaby", Dexter sings the 6th line as "Even if I let you down" instead. Remy Starr – Remy is the protagonist of This Lullaby and is cool and cynical. After seeing her mother's failed marriages, she proclaims, she believes. She is obsessive about cleanliness and does not allow anyone to eat or smoke in her car. Remy is named after an expensive brand of cognac, she is scarred by her past, as her father – whom she never knew – wrote her the song "This Lullaby" the day she was born, left soon after. The song becomes a one-hit wonder, Remy hears it in her life.

She claims to have relationships all figured out, in mathematical terms. She used to be known for partying. Dexter Jones – Dexter is the lead singer of a band called Truth Squad. Gangly and sociable, he bumps into Remy at her stepfather-to-be's car dealership, claiming to have sensed a special bond with her, it is revealed he dropped out of college for music, "breaking his mother's heart", as Remy's stepfather Don says at the Fourth of July barbecue. Don's sister is Dexter's aunt. Dexter's dog is named Monkey because he got a dog instead, he is carefree and funny. During the novel, he works at a one-hour photo place. One of these photos is evidence of Remy's stepfather's affair, Remy mistakenly believes Dexter shows it to her on purpose to hurt her. Dexter hates English muffins. Barbara Starr – Barbara is Remy's mother, who happens to be a famous author, her books are the romantic type, all about exotic locations and women who have everything yet nothing. Her latest book is'The Choice', she has been married numerous times, each time changing her outward personality to match her newest husband, yet still believes in love despite her failed relationships, in direct contrast to Remy's view on love.

Christopher Starr – Chris is Remy's brother. He used to be a juvenile delinquent, before he met his girlfriend, Jennifer Anne, whom he met at his job at Jiffy Lube. Since he's shaped up considerably, he breeds lizards in his room. He is named after his mother's favorite saint and he is close to Remy because of their shared childhood. Towards the end of the book, he and Jennifer Anne state. Jennifer Anne Baker – Jennifer Anne is Chris's girlfriend, described as being small with big blond hair, whip smart, she is seen as an uptight, annoying perfectionist in Remy's eyes only, is the only one who refers to Chris as "Christopher". In the book and Chris become engaged. Remy cannot tolerate Jennifer Anne. Jennifer Anne reads many self-help books and dislikes Chris's obsession with his lizards. Don Davis – Don is Barbara's fifth husband, they met at the car dealership when Remy went to buy a new car. A lifelong bachelor, he has habits that annoy both Chris and Remy: his paintings and statues, which seem to have sexual or violent themes, his pyramids of Ensure cans.

After his marriage with Barbara, he is caught having an affair with his secretary, Patty.... Jess – Jess is Remy's oldest friend, she is very emotionless, clashes a lot with Chloe. Remy not fat, she is the "mother" of their group of friends because she's been raising her two younger brothers since her mom died in fifth grade. She fights with Chloe often, she is the most responsible of the group. Jess avoids dating problems by not dating at all for the most part, she is somewhat Chloe's polar