Hydrogen Compounds & Isotopes

Hydrogen

• Hydrogen is the very first element of the periodic table; it has particular properties and is discussed apart from other elements
• Hydrogen’s electron configuration is 1s¹
• Hydrogen is the most abundant element of the universe, and the second most abundant on Earth, where it occurs in the largest amount in water
• Elementary hydrogen exists as apolar, diatomic H₂ molecules which are stable and not very reactive under ordinary conditions
• Hydrogen has a very small size and low molecular mass, therefore its diffusion rate is high
• Hydrogen is one of the less dense gases, for this reason it was used for filling dirigible balloons (Zeppelin)
• At higher temperatures, hydrogen readily reacts with many elements; some reactions are explosion-like chain reactions
• The 2:1 ratio mixture of hydrogen and oxygen are called detonating gas (or oxyhydrogen) which explodes immediately on firing: 2 H₂ + O₂ → 2 H₂O
• Mixtures of hydrogen and chlorine (chlorine detonating gas) can be stored only in darkness, because a very violent reaction happens even upon light irradiation: H₂ + Cl₂ → 2 HCl
• Elementary hydrogen is an excellent reducing agent and suitable for preparing pure metals from oxides: CuO + H₂ → Cu + H₂O
• Hydrogen is used as a reducing agent in organic chemistry for saturation of double bonds, and by industry for the synthesis of ammonia, methanol and hydrochloric acid, and food industry for the hydrogenation of vegetable oils (production of margarine)
• Hydrogen cell engines don't emit hazardous combustion products
• Hydrogen can be prepared from acids with metals of negative standard electrode potential: Zn + 2 HCl = ZnCl₂ + H₂
• Industry produces hydrogen from the reaction of carbon or hydrocarbons with water at high temperature in the presence of catalysts (so-called water gas reaction)
• Hydrogen has an average electronegativity of 2.2
• The electron configuration of hydrogen suggests oxidation states of +1 and -1; +1 is much more frequent
• Hydrogen is in the s-block, and is similar to the halogens rather than to alkali metals
• Hydrogen typically occurs in covalent molecules
• Removal of one electron from hydrogen atom results in hydrogen ion (H⁺), this particle consists of a single proton, and doesn't exist alone, but is always connected to another molecule/atom
• Alkali and alkaline earth metals form ionic hydrides with hydrogen in which the counter ion of the metal is hydride anion (H⁻); they are very reactive, strong reducing agents and bases
• They react with water violently accompanied by hydrogen gas formation: CaH₂ + 2 H₂O = Ca(OH)₂ + 2 H₂
• Other metals can also form ionic hydrides but the so-called interstitial hydrides are more likely
• Neutral hydrogen atoms are built into the clearance of the metallic lattice in interstitial hydrides
• Interstitial metal hydrides are used in hydrogenation reactions instead of elementary hydrogen gas which would be dangerous and otherwise less reactive
• Non-metals form covalent hydrogen compounds exclusively, where oxidation state of H is +1.
• Covalent hydrogen compounds are diverse, having reactivity of covalent hydrogen compounds is diverse and their acid-base properties are also different
• Halogens and the elements of the oxygen group form acidic hydrides (H₂S, HCl), while hydrides of the elements in the nitrogen group are basic (NH₃, PH₃)
• The most abundant isotope of hydrogen is ¹H; ²H (0.015 %), ³H (10⁻¹⁰ %) occur in small amounts
• ²H and ³H are two-and threefold of the atomic mass of ¹H
• Isotopes of hydrogen were provided with different names and even with own symbols: ¹H is called as protium (H), ²H is deuterium (D), and ³H is tritium (T)
• The common hydrogen atom or protium can be investigated by nuclear magnetic resonance (NMR) spectroscopy, because it has only one proton in its nucleus
• Oxide of deuterium (D₂O), called heavy water, has a higher density, viscosity, melting and boiling point than water, but their chemical properties are the same.
• Deuterium absorbs neutrons very efficiently, thus heavy water is used in nuclear plants as moderator
• Tritium is radioactive and is detected in water analysis for the determination of the origin of water
• Tritium is also applied in self-powered lighting devices (e.g. 'Exit' box).
• The inner wall of these devices is covered by phosphorus, the electrons emitted by the radioactive decay of tritium make phosphorous glow (phosphorescence), thus providing a long lasting (several years) and non-battery-powered light

Medical and biological importance of hydrogen and its compounds

• Hydrogen is an organogenic element which can be found in every biomolecule
• From the point of view of Life, its most important compound is water, which is discussed with the oxygen group
• The hydrogen atom attached to an element of high electronegativity (N, O, F) has a key role in the strongest secondary bond called hydrogen bond
• It has fundamental significance in the formation of the structures of biomolecules (e.g. proteins, nucleic acids, see bioorganic chemistry)

Halogens

• Elements in the 7th main group have a valence electron configuration s²p⁵
• These elements are highly electronegative and good oxidizing agents
• Electronegativity decreases down in the group in order of F, Cl, Br, I as the atomic size increases.
• Halogens form apolar, diatomic molecules
• As the atomic size increases in the group, the molecules are getting easier to polarize and excite; the melting points of the elements increase, their colors deepen
• Element Chlorine was a war gas during the First World War
• Liquid Bromine causes ulcerous wounds on the skin, which heal very slowly
• Iodine dissolved in alcohol is a milder oxidizing agent and is used as an external disinfectant
• Halogens dissolve well in apolar solvents
• Fluorine and chlorine dissolve in water producing water soluble acids
• Fluorine oxidizes even water: F₂ + H₂O → 2 HF + ½ O₂
• Bromine and iodine also react with water like chlorine, but these reactions occur only to very low extent
• Bromine and iodine are just slightly soluble in water
• The solubility of iodine in water can notably be increased by the addition of potassium iodide (KI)
• Iodine forms soluble triiodide complex anions with iodide: I₂ + I⁻ = I₃⁻
• The mixture of aqueous KI and iodine is called Lugol‘s solution (K[I₃])
• Iodine dissolves excellently in organic solvents exposing characteristic colors
• Oxygen-free solvent molecules (hexane, chloroform) dissolve iodine with violet color, while oxygen containing solvents (alcohol, acetone) form brown iodine solutions
• In aromatic solvents (benzene, toluene) iodine is purple-red
• In the presence of starch, iodine forms characteristic dark blue complexes (see organic chemistry), for this reason starch solution is suitable for the detection of even traces of I₂

Compounds of halogens

• Have an oxidation of -1
• Halogens form ionic compounds with metals of low electronegativity while nonmetals and electronegative metals give covalent halides
• Ionic halides contain a -1 charge and generally have high melting points and dissolve well in water (NaCl, MgBr₂) Certain polarizable metal ions (e.g. Ag⁺ silver(I), Hg₂²⁺ mercury(I), Pb²⁺ lead(II)) form precipitate with halide ions
• Heavier metal salt is less water soluble and its color is deeper; easier to polarize

Hydrogen halides

• Under normal conditions halides of hydrogen (HX) are colorless, pungent smelling, toxic gases
• Acid strength increases from HF to HI
• The aqueous solution of hydrogen chloride is called hydrochloric acid
• Molecule HF (hydrogen fluoride) has special properties, forming H-bonds
• Aqueous solution of HF is also called hydrofluoric acid or etching acid
• Pure HF has a higher boiling point than expected because of the existence of H₂F₂ dimers connected by H-bonds
• HF is a weak acid, yet very corrosive in pure form or in aqueous solution, causing painful typically white wounds
• HF attacks glass: SiO₂ + 4 HF = SiF₄ + 2 H₂O. Therefore, fluorides should be stored in plastic vessels

Noble gases

• Elements in the last group of the periodic table have s² p⁶ valence electron configuration, and are very unreactive
• The saturated, hardly polarizable valence shell is what makes noble gases very unreactive
• Noble gases occur in monoatomic form with molecular lattice in solid state of very low melting and boiling points
• Dispersion interactions between their atoms are so weak that they behave practically as ideal gases (see kinetic theory of gases)
• Helium (He, electron configuration 1s²) is the first noble gas; it is not abundant on Earth but the 2nd most abundant element in the universe
• Stars are composed mainly of hydrogen and helium
• Helium is used for the preparation of artificial air, which is applied in the pressure-bottles of divers
• Artificial air contains 20% oxygen and 80% helium (nitrogen content of air is exchanged to helium)
• When ascending from deep water to the surface quickly (pressure change), nitrogen dissolved in blood bursts and causes a fatal air embolism, is called aeremia (caisson disease)
• Helium dissolves very poorly in blood (much worse than nitrogen), therefore helium containing artificial air does not cause aeremia.
• The boiling point of helium is extremely low (4.2 K), so liquid helium is used for cooling superconductor magnets in NMR spectroscopic apparatus
• Neon (Ne), does not form compounds
• Neon is used mainly for filling neon gas lighting tubes
• Argon (Ar) is relatively abundant in Earth, it gives 0.93% of air (and produced by the liquefaction of air).
• Argon is the most frequently used inert gas
• Gas lighting tubes filled with argon have blue color.
• Krypton (Kr) and xenon (Xe) have empty d orbitals, they can react with fluorine or oxygen under special conditions, and can form compounds
• Elementary krypton is used for filling traditional light bulbs.
• Radon (Rn) is soluble both in water and in fats, is radioactive and emits α-radiation during its decomposition and accumulates locally.
• A typical source of radon is the walls of buildings made of silicate slag or other type of bricks.
• As years pass, the very small amount of radium in the stones decomposes to radon, which liberates into the atmosphere of the rooms
• Long-lasting inhalation of α-radiating radon increases the risk of lung cancer

Inorganic chemistry- Halogen containing oxoacids and their salts

• Fluor has one oxoacid with a composition of HOF
• Perchloric acid (HClO4) can be produced in pure form; it is an extremely strong acid and oxidizing agent in pure form, and can dissolve gold
• Potassium chlorite is the most important chlorite; it is a again strong oxidizing agent; in the presence of flammable substances it explodes on shock
• Potassium chlorite is used in matches and fireworks; it is highly toxic, destroys the red blood cells and causes serious intoxication by the formation of methemoglobin
• Hypochlorous acid and its sodium salt are indispensable disinfectants in everydaylife
• Hypochlorous acid forms if chlorine gas is bubbled into water: Cl2 + H2O ⇋ HCl + HOCl
• Hypochlorous acid is a labile oxidizing agent; during its decomposition atomic (nascent) oxygen forms which is also responsible for the oxidizing effect: HOCl → HCl +, O'
• Chlorine water or chlorine gas is used for disinfection of drinking water
• If chlorine gas is introduced into a solution of a base, a similar disproportionation reaction takes place, instead of the acids their salts are yielded
• If the base is sodium hydroxide the following reaction happens: Cl₂ + 2 NaOH ⇋ NaCl + NaOCl + H₂O

Inorganic Chemisty- Medical and Biological importance of halogens

• In the body fluorine occurs as fluoride ion
• It is a component of the dental enamel in the form of fluorapatite (Ca₅(PO₄)₃F)
• The main source of fluoride is drinking water, where the optimal fluoride concentration is 1 mg/dm³
• Excess of fluoride can cause fluorosis: white or yellow spots on the surface of teeth
• Fluoride inhibits the reproduction of bacteria living in the mouth; therefore NaF is applied as an additive in toothpastes to prevent caries
• Tin fluoride SnF₂ is also for this purpose to prevent cavities, because it is efficient for a long time by adhering to the enamel of the teeth
• Chloride is one of the ions responsible for the electrical characteristics of cells
• Sodium chloride (NaCl) or table salt is an important component of blood
• The salt solution is used in blood and infusions

Inorganic chemistry- Iodine Compounds

• Certain iodine compounds are used as contrast medium in X-ray diagnostics, since iodine has high molar mass and absorbs X-ray radiation
• Radioactive isotopes of iodine (¹²³I, ¹³¹I, ¹³²I) are applied in diagnostics; they accumulate in the thyroid gland
• By measuring radioactivity, the iodine metabolism of thyroid gland can be detected
• In higher concentration, the radioactive isotopes are used for radiotherapy of thyroid gland tumors
• Potassium perchlorate (KClO₄) hinders the accumulation of iodine in the thyroid gland, and can be used for the therapy of hyperthyroidism (Basedow syndrome)

Inorganic chemistry- Other Important Compound

• Daily NaCl volumetric requirement of 0.15 mol /dm³ is isotonic with the body and can be used in infusions
• Human body requires around 5g of NaCl; it is excreted in urine and sweat
• Hydrochloric acid is a part of gastric juice, which assures the acidic medium needed for pepsin function.
• Concentrated gastric juice contains approximately 0.1 mol/dm³ of hydrochloric acid
• In cases of hypoacidity (low acid secretion) or anacidity (lack of acid), hydrochloric acid can be taken in medication form
• Hypochlorous acid (HOCl) forms in the human body in inflammations
• Neutrophil produce HOCl in order to kill pathogens captured by phagocytes
• Iodide is the only element with documented use as a medicine; its alcohol solution is an external antiseptic.
• Elemental iodine has bactericidal, fungicidal, antiviral, and antiprotozoal effects
• Injured skin is treated with alcohol-free iodine products, like Betadine
• Iodide compounds that easily decompose are complexed with a polymer called polyvinyl pyrrolidone (braunol)
• The polymer slowly decomposes with heat
• In the human body iodine is covalently bound in organic compounds, and therefore it must be in the thyroidal gland
• The two thyroid gland hormones are triiodothyronine and thyroxine, which contain significant amount of iodine

Explore covalent hydrogen compounds, deuterium oxide, and tritium applications in lighting. Learn about hydrogen bonds in biomolecules and hydrogen isotopes. Understand acid dissociation and interstitial hydrides.