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Chemistry

Science Review of Metalloids

150 150 Deborah

Overview

Metalloids are elements that have some properties of metals and some of nonmetals.  They are on the periodic table along the dividing line between metals and nonmetals. The most commonly recognized metalloids include the elements boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te).

Metalloids

Elements are commonly classified as either metals, nonmetals, or metalloids.  Most metalloids are brittle (a non-metallic property), act as semiconductors of electricity, and have a metallic luster (also a metallic property).  They are solid at room temperature. In chemical reactions, they often act more like nonmetals, but they form alloys like metals. Whether elements are classified as metalloids or not depend upon the chemist’s decision.   For example, polonium (Po), and astatine (At) are sometimes included in the list of metalloids, because of their chemical properties and their location on the periodic table.

Semiconductor Properties

Metalloids are good semiconductors, which mean that they are between the electrical conductivity of metals and materials used for insulation.  Semiconductors can conduct electricity under some conditions, so electrical current can be controlled. Semiconductor chips, transistors, and other electronic parts form integrated circuits for everything from computers to cell phones.  The metalloids, especially silicon, boron, germanium, and compounds of arsenic and antimony, are natural semiconductors. Silicon and germanium revolutionized the electronics and computer industries.

Alloys

The metalloids are often too brittle to be used as pure substances, but form many useful alloys.  For example, boron is used in alloys with steel and with nickel for welding components.  Germanium is alloyed with silver to make tarnish-resistant sterling silver.  Pewter is an alloy of tin and antimony.

Other Uses of Metalloids

Many compounds of metalloids are highly toxic, such as those containing arsenic and antimony.  However, other compounds can be used as disinfectants and antiviral agents.  Compounds of boron are used as catalysts in many chemical reactions.  Many compounds are used to form glassware, especially in chemical and industrial uses, such as optical fibers.   Silicon and boron compounds are also used in fireworks, as they are less toxic than some other compounds.

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Science Review of Mercury

150 150 Deborah

Overview

The element mercury (Hg) has atomic number 80, and is considered a transition metal.  It is liquid at room temperature.  Although mercury is highly toxic, it is a good electrical conductor, and is used in some electrical switches.

Properties of Mercury

Mercury is the only metal that is liquid at room temperature.  It is silvery, so it is sometimes called “quicksilver.”  Mercury liquid is much denser than water and has a higher surface tension, so that it beads up but does not wet the surface of the substance it floats upon.  Like other transition metals, it forms compounds at more than one oxidation level.  It has 7 different isotopes.

History of Mercury

Mercury is a relatively uncommon element, and is rarely found free in nature.  It is produced from a red ore called cinnabar, mercury (II) sulfide (HgS).  Since ancient times, cinnabar was heated to release the mercury vapor. When the vapor cooled, it formed liquid mercury.  Cinnabar ore was used in cave paintings in Spain and France because of its red color.  Mercury itself was highly prized in ancient China, and believed to have mystical properties.  It has been found in Egyptian tombs, and was used during Roman times to purify gold and silver.  Alchemists believed that all metals contained some mercury, so that by extracting mercury from them, the metal could be transformed into precious metals such as gold.

Uses of Mercury

Mercury is used now in electrolytic cells, some electrical switches and relays, because of its conduction properties, as well as in fluorescent and mercury-vapor lamps.  It was once widely contained in fever thermometers and blood-pressure monitors, but those uses were outlawed because of the metal’s toxicity.  It is still used in some scientific monitors, however.

Mad Hatter or Mercury Poisoning?

Mercury and its compounds are highly toxic.  During Roman times, prisoners were sentenced to work in cinnabar mines and in processes involving mercury production because the poison would shorten their lives.  During the 1800’s the makers of hats prepared beaver pelts and felting using mercury nitrite.  Workers inhaled mercury vapor from working in close quarters, resulting in a neurological illness with tremors and dementia, called “mad hatter disease”.  In the late 1800s, a new process that did not use mercury was developed, although the phrase “mad as a hatter” remained.  In modern times, mercury poisoning is often caused from eating fish that have high levels of mercury from pollution.

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Science Review of Transition Metals

150 150 Deborah

Overview

The transition metals occupy a large area on the periodic table.  They all appear to have similar metallic properties, because of the way their electron shells are filled.   Many transition metals form brightly colored compounds, especially in solution.  The transition metal group includes common elements such as iron (Fe), copper (Cu), nickel (Ni), gold (Au), and silver (Ag).

The Periodic Table and Transition Metals

Transition metals are also called transition elements.  They are in Groups 3B through 2B, or 3 -12 of the periodic table, occupying the space between the alkali earth metals on the left and the true metals on the right.  They are a very large group of 38 elements.  Also, chemists usually consider the elements in the lanthanide and actinide series as inner transition elements.  There are so many of them that they are usually in a special configuration under the main periodic table.

Properties of Transition Metals

Transition metals are usually solids (except for mercury which is liquid at room temperature), often with high melting and boiling points.  They are shiny, metallic, dense, and are often excellent conductors of both heat and electricity.  Some transition metals, such as gold and platinum, do not react easily with oxygen, so they resist oxidation, while iron, another transition metal, rusts easily.  Technically, transition metals are in the d-block of the periodic table.

Compounds

By definition, transition metals form various types of compounds by giving up different numbers of electrons from their incomplete d shells.  For example, sometimes iron gives up 2 electrons and sometimes it gives up 3 when it is oxidized.  Trace amounts of transition metals in compounds color paint pigments.  Traces of iron color citrine yellow and jade green, and traces of chromium color rubies red.  Some of the most unusual colors are caused when transition metals are dissolved in liquid (aqueous) solution.

Types of Transition Metals

The 38 elements include some of the most well-known metals.  For example, iron (Fe) is the fourth most abundant element in the Earth’s crust and one of the most widely used in manufacturing.  Since pure iron is a soft metal, it is usually alloyed with other elements to make it harder, often as steel.  Iron is also essential to life, as it is contained in hemoglobin for vertebrates and in plant cells.  Copper (Cu) is used in electrical wiring, as it is one of the best conductors, in alloys such as bronze, and even in copper pipes.  Nickel (Ni) is used in alloys, especially with iron, in coins, and is essential to life in some enzymes.  The transition metals are also excellent catalysts.

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Science Review of Alkaline Earth Metals

150 150 Deborah

Overview

The alkaline earth metals are the next column in the periodic table from the alkaline metals. They include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They have two electrons in their outermost shell, which they give up in chemical reactions.

Beryllium (Be)

Beryllium (atomic number 4) is a brittle metallic substance that is usually found in other compounds and minerals. The mineral that contains beryllium, beryl, was known since ancient times, as the gemstones emerald and aquamarine. When used in alloys, beryllium makes other metals stronger, such as aluminum and copper. It is often used in the aerospace industry. However, beryllium dust is highly toxic to the lungs, causing a disease similar to chronic pneumonia.

Magnesium (Mg) and Calcium (Ca)

Magnesium (atomic number 12) is also reactive, especially with oxygen. It is almost never found naturally as pure magnesium, but can be produced artificially. It is an essential element to both plants and animals, as magnesium ions are used by many different enzymes. Magnesium is often alloyed with aluminum to produce strong, lightweight metals for aerospace, automotive, and electronics. Calcium (atomic number 20) is very abundant, in compounds such as calcium carbonate (natural limestone) and chalk. It is essential to life, especially in the development and maintenance of bones and teeth.

Strontium (Sr) and Barium (Ba)

Strontium (atomic number 38) is reactive with oxygen, especially in water. It is found naturally in compounds and minerals, but its radioactive isotope strontium 90 is found in fallout. Barium (atomic number 56) is so reactive with the oxygen in air that it is usually stored under oil, similar to the alkali metals. It is found in minerals such as barite, or barium sulfate (BaSO4).

Radium (Ra)

Radium (atomic number 88) is radioactive in all isotopes. The isotope with the longest half-life is about 1600 years. However, radium is a naturally-occurring product of uranium decay, so it is usually found in conjunction with uranium. It is famous as a radioactive element because of the experiments in the late 19th century and early 20th century by Marie and Pierre Curie. Radium paint was once used on watch hands to make them glow in the dark, but it was abandoned for safer alternatives. Marie Curie herself died from the effects of her experiments with radium.

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Science Review of Alkali Metals

150 150 Deborah

Overview

The alkali metals are in Group 1 of the periodic table.  They include the elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).  These highly reactive elements form compounds easily and explosively.  They all have one electron in their outermost shell, giving it up to form ionic compounds.

Lithium (Li)

Lithium is the lightest metal, with atomic number 3.  It is so soft that it can be cut with a knife, although it is so reactive that it quickly oxidizes.  It reacts with water to form lithium hydroxide (LiOH) in solution and free hydrogen.  It is also very flammable, and is usually stored in petroleum jelly.  Since lithium is so reactive, it is almost always found in various compounds.  Lithium is used in batteries (including lithium-ion batteries), in ceramics and glass, in industrial lubricants, and as a catalyst in fusion reactions.

Sodium (Na) and Potassium (K)

Sodium (atomic number 11) and potassium (atomic number 19) are both fairly common elements.  Both are even more reactive than lithium, and are also flammable.  When sodium comes in contact with water, it produces flammable hydrogen and caustic sodium hydroxide.  Therefore, it is usually stored in mineral oil.  One of the most common compounds of sodium is table salt (NaCl), which is also an essential nutrient, necessary to plant and animal life.  Potassium, its heavier cousin, is also essential to cell functioning.  It forms similar chemical compounds to sodium.  Potassium compounds are used in fertilizer, food, various dyes, and in potash.

Rubidium (Rb) and Cesium (Cs)

Rubidium (atomic number 37), like the other alkali metals, is highly reactive, flammable, and only occurs naturally in compounds.  It is used in atomic clocks and laser cooling. One isotope of rubidium has specialized medical applications, such as PET scans.  Particular types of tumors contain more rubidium than normal tissue, so they can be detected and destroyed.    Cesium or caesium (atomic number 55) is liquid at about 83 degrees F., although it is not found as pure metal naturally.  If cesium comes in contact with water, it explodes, and is so flammable in air that it can catch fire spontaneously.  However, atomic clocks that use cesium are so accurate that they lose less than a second in 1.4 million years.

Francium (Fr)

Francium (atomic number 87) is radioactive, with a half-life of about 22 minutes.  It is extremely rare because of its short half-life, and occurs naturally as a product of the decay of the element actinium.  When francium was discovered in the laboratory, it was first mistaken for more common elements, but careful study by scientist Marguerite Perey showed it was a new element.  Although she first isolated it in 1939, it has been synthesized by scientists since 1996 by bombarding a tiny gold target with beams of oxygen atoms emitted by a linear accelerator.

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Science Review of Chemical Equations

150 150 Deborah

Overview

Chemical equations are shorthand descriptions of the reactions between substances.  They include the chemicals that react, the conditions for reaction, the products of the reaction, and the amounts of all substances involved.

Word Equations

In the simplest form, chemical equations can be expressed in words.  For example, the substance methane reacts with oxygen to form carbon dioxide and water.  The reactants are methane and oxygen and the products are carbon dioxide and water, such that methane + oxygen → carbon dioxide and water.  The arrow pointing to the products of the reaction is another way to say “forms” or “produces.”

Chemical Equations

Although word equations are an improvement over writing a paragraph describing reactions between chemicals, they still lack information, such as the chemical formulas that make up the compounds involved in the reaction and their quantities, their quantities, and details about the reactions themselves.  The word methane can be replaced by its chemical formula CH4, oxygen by its molecular state O2, the term carbon dioxide by its chemical formula CO2, and water by its chemical formula H2O. Chemical formulas already state more information than the word forms.  A molecule of methane consists of 4 atoms of hydrogen for each atom of carbon; a molecule of carbon dioxide, of two atoms of oxygen for each atom of carbon; a molecule of water, two atoms of hydrogen for each atom of oxygen.

Unbalanced Chemical Equations

After writing a word equation, make sure than the chemical formula for each substance is correct , the reactants are on the left side of the equation, and the products are on the right side of the equation, so that methane + oxygen → carbon dioxide + water becomes CH4 +O2 →CO2 +H2O.  The process is not complete.  The reactants contain one carbon atom, 4 hydrogen atoms, and two oxygen atoms.  The products contain one carbon atom, three oxygen atoms, and two hydrogen atoms.

Balanced Chemical Equations

The Law of Conservation of Mass states that there must be the same amount and type of each substance on both sides of the equation.  However, in the unbalanced chemical equation, there are the same number of carbon atoms in CH4 and CO2, but not the same amount of hydrogen and oxygen atoms on both sides of the equation.  There are already 4 hydrogen atoms in CH4, so in order to put 4 hydrogen atoms in the products have 2 atoms of H2O.  The chemical equation is then CH4 +O2→CO2 +2 H2O.  Oxygen is still unbalanced, as there are 2 atoms of oxygen on the reactant side and 4 on the product side.  Multiply the O2 by 2 so that the entire reaction is CH4 + 2 O2→ CO2 + 2 H2O.  There is one atom of carbon, 4 atoms of hydrogen, and 4 atoms of oxygen on both sides of the chemical equation.

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Science Review of Compounds

150 150 Deborah

Overview

Chemical elements are often found within chemical compounds, in which one or more elements are bonded together.  Some elements exist freely in compounds with one another, while others are bonded by electrostatic charges and still others are bonded with molecular bonds.

Molecules

A molecule is the smallest uncharged unit of a compound, containing at least two atoms.  Unlike elements, molecules can always be divided into individual atoms by chemical changes.  For example, the smallest molecule of water (H2O) contains 2 atoms of hydrogen to every atom of oxygen, in exactly that proportion.  Molecules of water can be frozen to form solid ice or heated to form gaseous water vapor, but those physical changes do not change its chemical composition.  However, if water molecules react chemically with another substance, then new molecules of other substances are formed.

Diatomic Molecules

Some chemical elements almost always exist freely in molecules of two atoms of the element.  For example, free hydrogen exists as H2, free oxygen as O2, and free nitrogen as N2.  Other elements that exist freely as diatomic molecules include fluorine, F2, a pale yellow gas; chlorine, Cl2, a yellow-green gas; bromine, Br2, a reddish-brown liquid; and iodine, I2, a bluish-black solid.  However, when any of those elements form other compounds, they are no longer free.  For example, water contains both hydrogen and oxygen, but neither element is free, just two atoms of hydrogen combined with one atom of oxygen in a water molecule.

Ionic Compounds

The atoms in ionic compounds are bound together by the attraction between negatively charged ions, called anions, and positively charged ions, called cations.  Common salt, sodium chloride (NaCl), consists of positively charged sodium cations and negatively charged chlorine anions that form the compound.  Ionic compounds tend to be soluble, and many conduct electricity when they are in solution.

Molecular Compounds

Molecules such as water are bound together by shared electrons between elements rather than by electrostatic charges.  Bonds may be between the electrons of two or more different atoms, to form many different types of structures.

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Science Review of Hydrogen Isotopes

150 150 Deborah

Overview

The element hydrogen is the lightest of all elements in the periodic table, with one proton in its nucleus. It is also the most abundant element in the universe.  Hydrogen has 3 naturally occurring isotopes, although others have been produced experimentally.

Hydrogen

The most common form of hydrogen has one proton in its nucleus and no neutrons.  It occurs over 99% of the time, often in common compounds on Earth such as H2O, or water.  At room temperature, hydrogen gas itself is colorless, odorless, highly reactive, and flammable.  It forms explosive reactions with other elements such as the oxygen in the air and chlorine.

Deuterium

Deuterium is a stable isotope of hydrogen, containing one proton and one neutron in its nucleus. It is twice as heavy as the common form of hydrogen.  Most often on Earth, highly concentrated deuterium occurs in a form of water called “heavy water”, because water containing deuterium atoms is heavier and more viscous than ordinary water.  For example, ice made of heavy water sinks rather than floats in regular water.  Since some occurs naturally, it can be separated out by various processes and is often used commercially to cool nuclear reactors.

Tritium

The tritium atom contains one proton and 2 neutrons.  It occurs naturally in the upper atmosphere by cosmic rays striking hydrogen molecules.  Like deuterium, some tritium atoms find their way into chemical reactions with oxygen to form water.  However, tritium is radioactive, with a half-life of about 12.32 years.  Its natural decay gives scientists a way to measure the age of groundwater, since some water containing tritium falls as rain.  Water that has fallen over 100 years ago contains no radioactive tritium, but more recent rainfall does.

Hydrogen in Space

Free hydrogen exists in interstellar clouds.  Most stars consist of hydrogen plasma, used in nuclear fusion reactions.  Deuterium has been found in the solar corona, spectroscopic observations of other stars, as well as in the atmospheres of Jupiter and Venus.  Some comets, such as Halley’s Comet and Comet Hale-Bopp, have a high proportion of deuterium.  Scientists theorize that some of the water on Earth may have come from comets and asteroids striking its surface.

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Science Review of Stellar Structure and Nuclear Reactions inside Stars

150 150 Deborah

Overview

The nuclear energy that powers stars depends upon the core temperature and the available fuel. During the main-sequence phase for most stars, nuclear fusion of 4 hydrogen atoms to form 2 helium atoms provides the most energy. Other types of fusion reactions occur in stars with higher core temperatures and when hydrogen fuel is exhausted.

The Structure of a Star

A star consists of superheated gaseous plasma that is held together by gravity. As interstellar gases condense, layers form around a central core. Gases are under the highest pressure there, and the energy ignites further reactions. Heat and energy are carried by convection and radiation to the outer layers, and beyond. Turbulence within the core mixes and carries elements throughout the core and to the zones beyond. Studies in particle physics, in the measurement of stellar spectra, and mathematical modeling of nuclear fusion reactions all contribute to the study of stellar structure.

The Proton-Proton Chain Reaction

The most common reaction in the sun and other stars is called the proton-proton chain reaction. It involves 4 hydrogen atoms, which contribute 4 protons. In the process 2 protons lose their charge, becoming 2 neutrons, gamma rays, electrons, and positrons, which spin off and release energy. The electrons and positrons (which are antimatter, after all) annihilate each other, releasing more energy. Other elements that are produced by the proton-proton chain include unstable isotopes of beryllium and lithium that decay rapidly and release more energy.

The Carbon-Nitrogen-Oxygen Reaction

Stars that have core temperatures of 12000 K to 15000 K have another type of nuclear reaction, called the carbon-nitrogen-oxygen reaction. Carbon, nitrogen, and oxygen are catalysts to the proton-proton chain reaction, and those heavy elements are also produced. In the largest, hottest stars (the blue and white main-sequence stars, as well as the heaviest and brightest supergiants), nitrogen is produced and helium is released, as well as energy in the form of gamma rays, neutrinos, electron-positron pairs, and other subatomic particles.

Helium Fusion

Helium fusion is also called the triple-alpha process.  After the hydrogen in the stellar core is used up, the core collapses. The core collapse raises the temperature enough that helium nuclei fuse. Intermediate steps produce the elements beryllium, lithium, carbon, and some heavier elements. Although the hydrogen to helium process can be maintained for billions of years in main-sequence stars, helium fusion is more unstable. Stars lose their photospheres, and many explode. The most massive stars become supernovae.

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Chemistry Review of Substances and Mixtures

150 150 Deborah

Overview

Substances occur in elements or compounds, while mixtures can be any combination of one or more elements or compounds. Mixtures may occur as solutions or heterogeneous combinations.

Elements and Compounds

Elements and compounds are known as pure substances. A molecule of one of the elements or compounds only contains that substance. For example, a molecule of oxygen, O2, only contains oxygen. A molecule of water, H2O, only contains 2 atoms of hydrogen to one atom of oxygen.

Homogenous or Heterogeneous

Pure substances are always homogeneous. The ideal pure substance contains molecules of only that substance, and nothing else. Mixtures may be homogeneous (if they are solutions) or heterogeneous. If they are heterogeneous, that means that different substances are throughout the mixture.  For example, sand is a mixture that contains many different silicates, other types of rocks, organic debris such as particles of shells, and miscellaneous other items.

Solutions

Solutions are mixtures that are homogenous. Dissolve a teaspoon of sugar in a cup of hot water. Both the sugar molecules and the water molecules exist separately, as can be demonstrated if the water is allowed to evaporate. Homogenized milk is a suspension of fat globules in liquid. By definition, the milkfat globules are evenly distributed throughout the liquid rather than being allowed to rise to the top as cream.

Heterogeneous Mixtures

Suppose there is a mixture of white sugar and white sand. Both substances are solids and difficult to separate. It is also very hard to tell them apart, unlike a package of different-colored M &M’s. In some mixtures, all types of matter are solid. In others, such as soda pop, some types are solid, some are liquid, and some are bubbles of gas. Rocks and minerals are also heterogeneous mixtures. Different types of chemical compounds, water, and other solids combine to form them. A diamond is formed of crystalline carbon under conditions of extreme heat and pressure. However, diamonds in the real world contain impurities that affect their color and clarity. Boron is responsible for those that are blue; nitrogen, yellow and brown; and deformation or irradiation for other colors.

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