Which period contains the least number of elements

Hydrogen gas is combined with nitrogen in the Haber process to synthesize ammonia NH 3 , which is widely used in fertilizers. It is also used in the manufacture of hydrogenated vegetable oils; in this reaction, hydrogen atoms add to the carbon-carbon double bonds in the vegetable oils double-bonded carbons bond to fewer hydrogen atoms than single-bonded carbons — i.

Another use for hydrogen is in rocket fuels: the Saturn V rockets that launched the Apollo lunar missions used , gallons of kerosene and , gallons of liquid oxygen in its first stage S-IC , , gallons of liquid hydrogen and 83, gallons of liquid oxygen in its second stage S-II , and 69, gallons of liquid hydrogen and 20, gallons of liquid oxygen in its third S-IVB stage; the Space Shuttle main engines use , gallons of liquid hydrogen and , gallons of liquid oxygen.

Hydrogen is lighter than air, and was used in balloons and dirigibles also known as airships or zeppelins. Dirigibles were used in city-to-city air travel in the early s, and in trans-Atlantic crossings in the s and s. During World War I, German zeppelins were used in bombing runs over England, since they could fly higher than the British planes. On May 6, , the German dirigible Hindenburg caught fire as it came in for a landing at Lakehurst Naval Air Station in New Jersey; 35 people out of the 97 aboard and one person on the ground were killed.

The exact cause of the fire is still the subject of speculation, but the disaster signaled the beginning of the end for airship travel.

Modern "blimps" use helium to provide lift, which avoids the problem of hydrogen's flammability. Molecules which contain hydrogen bonded to nitrogen, oxygen, or fluorine can attract one another through the formation of hydrogen bonds. Hydrogen bonds are a particularly strong form of dipole-dipole forces , which arise because of the unequal sharing of electrons in some covalent bonds.

If one atom in a covalent bond is more electronegative than the other, it "pulls" harder on the electrons that the two atoms share, giving the more electronegative atom a partial negative charge, and the less electronegative atom a partial positive charge.

The partially negative atom on one molecule attracts the partially positive atom on a neighboring molecule, causing the two molecules to be more attracted to each other than two nonpolar molecules which have no electronegativity differences between their bonded atoms would be.

Molecules that interact by these dipole-dipole forces tend to have higher boiling points than nonpolar molecules, because higher temperatures are necessary to overcome the attractive forces between the molecules and separate the molecules into the gas phase.

In the case of O—H, N—H, and F—H bonds, the electronegativity differences are particularly large because fluorine, oxygen, and nitrogen are the most strongly electronegative elements. The attractive forces between molecules containing these bonds are particularly strong, and are given the name hydrogen bonds.

Hydrogen bonds are not as strong as covalent bonds, but they greatly influence the physical properties of many substances. In particular, hydrogen bonds are responsible for the fact that water is a liquid at temperatures at which molecules of similar molecular mass are gases. For instance, hydrogen sulfide, H 2 S, which weighs Ice floats on liquid water because the hydrogen bonds hold the molecules into a more open, hexagonal array, causing the solid form to be less dense than the liquid form.

In living systems, hydrogen bonding plays a crucial role in many biochemical process, from the coiling of proteins into complex three-dimensional forms to the structure of the DNA double helix, in which the two strands of DNA are held together by the hydrogen bonding between their nucleic acids components. In this technique, a sample is placed in a powerful magnetic field usually produced by a superconducting magnet — see the section on Helium , which causes the hydrogen atoms in the sample to resonate between two different magnetic energy levels; pulsing the sample with a burst of radiofrequency radiation typically between to MHz causes the hydrogen atoms to absorb some of this radiation, producing a readout called an "NMR spectrum" which can be used to deduce a great deal of structural information about organic molecules.

Since almost all organic molecules contain hydrogen atoms, this technique is widely used by organic chemists to probe molecular structure; it can also be used to determine a great deal of information about extremely complex molecules such as proteins and DNA.

The technique is nondestructive, and only requires small amounts of sample. NMR spectroscopy can also be performed with the carbon isotope, and several other isotopes of other elements. This technology is also used in an important medical imaging technique called Magnetic Resonance Imaging MRI ; the water molecules in different environments in the body respond to very slightly different magnetic field strengths, allowing images of tissues and organs to be obtained.

This technique can be used in diagnosing cancers and creating images of tumors and other diseased tissues. MRI is also used to study how the brain works by looking at what areas of the brain "light up" under different stimuli.

The term "nuclear" is avoided in the medical application because of its unpleasant associations, even though the only radiation involved is similar to that of an FM radio transmitter.

Lithium is a soft, silvery metal, with a very low density, which reacts vigorously with water, and quickly tarnishes in air. The name of the element is derived from the Greek word for stone, lithos. It is found in the Earth's crust at a concentration of 20 ppm, making it the 31st most abundant element. Lithium also presents some exceptions to the "typical" Group 1A behaviors.

The lithium ion has a very high charge density because of its small size; thus, many lithium salts have significant covalent-bonding character, instead of being purely ionic. These salts dissociate less easily in water than the salts of sodium and potassium, and are therefore less soluble in water.

In addition, lithium can form bonds to carbon which have high covalent character the organolithium compounds.

Lithium was one of the three elements produced in the Big Bang, although it was produced only in trace amounts. Aluminum and magnesium alloys of lithium are strong and lightweight; aluminum-lithium alloys are used in aircraft construction, trains, and bicycles.

Lithium-based batteries have very long lifetimes particular important in implantable devices such as pacemakers and defibrillators , and are very lightweight; they are frequently used in portable electronic devices and computers.

Lithium salts such as lithium carbonate, Li 2 CO 3 are used in the treatment of bipolar disorder and some types of depression, and are also used to augment the actions of other antidepressants. Lithium deuteride LiD, see entry on Hydrogen above is used in hydrogen bombs; neutrons produced by a fission-powered explosive are absorbed by the lithium atoms, transforming them into tritium; the fusion of tritium and deuterium to form helium releases tremendous amounts of energy. Lithium hydroxide LiOH is used in confined spaces to remove carbon dioxide from the air the carbon dioxide is captured in the form of lithium carbonate ; this is particularly important in submarines and spacecraft.

Sodium is a soft, silvery metal that reacts very vigorously with water, and tarnishes easily in air. It is the fourth most abundant element in the Earth's crust, which consists of 2.

The name is derived from the English word soda, a term found in many compounds of sodium, such as washing soda sodium carbonate or soda ash , sodium bicarbonate baking soda , and sodium hydroxide caustic soda. The symbol "Na" is derived from the Latin name for the element, natrium. It is found in the minerals halite [rock salt, or sodium chloride, NaCl] and trona [sodium carbonate bicarbonate, Na 3 CO 3 HCO 3 ], and can be extracted from seawater.

Metallic sodium is usually stored in mineral oil or some other hydrocarbon, because it will react with the moisture in the air to form sodium hydroxide. A common laboratory demonstration illustrates the reactivity of sodium. A small piece of sodium placed in a dish of water skates around on the surface of the water, hissing violently, and slowly disappears. The sodium reacts with water in a single-displacement reaction, producing sodium hydroxide and hydrogen gas:.

The sodium hydroxide is soluble in water, and dissolves. This demonstration can become very dangerous if too large a piece of sodium is used, however, since enough heat can be generated to ignite the hydrogen gas.

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