Showing posts with label PERIODIC TABLE. Show all posts
Showing posts with label PERIODIC TABLE. Show all posts
HOW WAS THE EARTH FORMED? - BASIC INFORMATION ON THE FORMATION OF PLANET EARTH
The solar system formed about 4.6 billion years ago. The Earth and other terrestrial planets are believed to have formed by gathering together the so-called planetesimals.
Planetesimals are formed by the coalescence of fine- or coarse-grained mineral matters, metals, and gases of various kinds. As planetesimals stuck together mostly by gravity and the body thus formed grew larger, it became a precursor of terrestrial planet.
Some of these bodies were smashed by other bodies, and their fragments became meteorites. Hence, studies of meteorites would provide a lot of insight into the formation and the earlier state of the Earth.
The planet Earth was thus formed. Heat was created as the coalescence (of planetesimals) proceeded due to gravity, and heat also came from radioactivity of several radioactive elements such as aluminum-26. So the newly formed body was heated and the core was melted.
As the material becomes liquid (as a result of melting), the materials contained in the liquid separate out according to their densities. The more dense material would sink closer to the bottom (core).
Thus, the present layer structure of the Earth formed. The innermost core is a dense solid of about 1,200 km radius, whose density is about 12.6 g per cubic centimeter (12.6 ×106 kg/m3).
It is made of mostly iron metal and a small amount of nickel. By the way, the density of iron metal is only 7.8×106 kg/m3 under the ordinary pressure. The next layer is the outer core (up to 3,500 km from the center of the Earth), which is liquid and has a density of 9.5–12×106 kg/m3. The chemical composition seems to be about the same as that of the inner core.
There is an abrupt change in density in the next layer, mantle. The width of mantle is about 2,900 km (3,500–6,380 km from the center). Its density ranges from 4 to 5.5 ×106 kg/m3. The mantle is made of mostly magnesium–iron silicates (silicon oxides). The outermost layer is the thin crust of about 35–45 km on the land portion, and about 6 km under the ocean portion.
IRON (FE) CHEMICAL INFORMATION - THE CHEMISTRY OF IRON
What Is Iron? All you need to know about the element Iron (Fe)
Iron, a familiar metal, tends to rust, as everybody has seen. What happens chemically when iron rusts? Iron atoms in the metallic iron carries no electric charge Fe(0), in which 26 electrons (negatively charged) are orbiting around a nucleus that contains 26 protons (with positive charge) and 30 neutrons (with no electric charge).
This applies to an isotope 26Fe56, the most abundant isotope of element iron. But iron atom can lose its electrons. This process (loss of electrons) is called “oxidation.” Iron becomes either Fe(II) by losing two electrons or Fe(III) by losing three electrons (under normal conditions), though it can take Fe(I) (under special conditions).
Fe(0) is said to have been oxidized to Fe(II) or Fe(III). [Fe(II) means an iron atom that carries two positive charges; this is so because there are now only 24 electrons (negative charges), but there are still 26 positive charges at the nucleus.]
For this to happen you have to have a chemical entity that removes the electrons from the iron atom. Such an entity is called an oxidant or oxidizing agent. The iron atom is said to be a reductant or reducing agent in this process, for a chemical reaction in which a chemical entity (oxidant) gains electron(s) is called “reduction.”
Hence, oxidation and reduction reactions occur simultaneously and are like “head and tail” of a coin. Iron (Fe) reacts with oxygen in the air and is oxidized first to Fe(II) and then Fe(III) ending up with iron oxide Fe2O3. [Fe(II) can also be expressed as Fe2+ or FeII, and such a state is called an “oxidation state”; in this case, the oxidation state of iron atom is +2 or II. Likewise, Fe(III)=FeIII=Fe3+.
We will usually use Roman numerals to express the oxidation states in this book]. Here oxygen (O2) in the air is the oxidizing agent. In the process, oxygen O2 which is in “zero” oxidation state gains four electron and is reduced to two of O−II (−2 oxidation state); therefore, the chemical reaction is + → III ( −II ) 2 2 3 4Fe(0) 3O 2Fe O . In this chemical reaction, 12 electrons are exchanged between four iron atoms and three oxygen molecules.
When this is not purely oxide and contains hydroxide Fe(OH)O or Fe2(OH)2O2, it shows that rust color, brown. Pure oxide Fe2O3 forms an ore called “hematite,” which is red. The red bed is found in many geological locations.
These descriptions suggest that iron, when forming chemical compounds, takes the form of Fe(II) or Fe(III). And it can go back and forth between Fe(II) and Fe(III) readily. Fe(II) gives off an electron to become Fe(III), and Fe(III) becomes Fe(II) when it accepts an electron. This kind of process is also called “electron transfer” reaction.
Hence, iron (in the form of Fe(II) and Fe(III)) can readily undergo an “electron transfer” reaction or alternatively an “oxidation–reduction” reaction, because the process of Fe(II)s becoming Fe(III) is an oxidation and the reverse (Fe(III) → Fe(II)) is a reduction reaction.
Some of you might have experienced, as this author has, to have your toilet bowl and others stained brown by your well water. The water underground can contain (depending on the location and other conditions) iron compounds; the iron is in the form of Fe(II), which is dissolved in water and almost colorless. It remains as Fe(II) in the underground, because no oxidant such as oxygen in the air is available.
However, once pumped out above ground and being exposed to the air, the iron soon turns into Fe(III) (through oxidation by oxygen). Fe(III) in water (neutral water, that is) is not stable, and soon reacts with water itself and forms iron hydroxide Fe(OH)3, which is brown and precipitates.
This is the brown stain. And the fact of the easy formation of iron stain suggests an easy affinity or reaction of Fe(II) with oxygen O2. This is indeed the basis of the usefulness of iron in the biological systems and our health.
LEWIS THEORY - CHEMISTRY TUTORIALS
Something unique in the electron configurations of noble gas atoms accounts for their inertness, and atoms of other elements combine with one another to acquire electron configurations like those of noble gas atoms.
The theory that grew out of this model has been most closely associated with G. N. Lewis and is called the Lewis theory. Some fundamental ideas associated with Lewis s theory follow:
1. Electrons, especially those of the outermost (valence) electronic shell, play a fundamental role in chemical bonding.
2. In some cases, electrons are transferred from one atom to another. Positive and negative ions are formed and attract each other through electrostatic forces called ionic bonds.
3. In other cases, one or more pairs of electrons are shared between atoms. A bond formed by the sharing of electrons between atoms is called a covalent bond.
4. Electrons are transferred or shared in such a way that each atom acquires an especially stable electron configuration. Usually this is a noble gas configuration, one with eight outer-shell electrons, or an octet.
Lewis developed a special set of symbols for his theory. A Lewis symbol consists of a chemical symbol to represent the nucleus and core (inner-shell) electrons of an atom, together with dots placed around the symbol to represent the valence (outer-shell) electrons.
A Lewis structure is a combination of Lewis symbols that represents either the transfer or the sharing of electrons in a chemical bond.
In Lewis theory, we use square brackets to identify ions, as we did in equation (10.1). The charge on the ion is given as a superscript.
Lewis s work dealt mostly with covalent bonding, which we will emphasize throughout this chapter. However, Lewis s ideas also apply to ionic bonding, and we briefly describe this application next.
FEATURES OF PERIODIC TABLE OF ELEMENTS
Guide In Using The Periodic Table of Elements.
In the periodic table, elements are listed according to increasing atomic number starting at the upper left and arranged in a series of horizontal rows. This arrangement places similar elements in vertical groups, or families.
For example, sodium and potassium are found together in a group labeled 1 (called the alkali metals). We should expect other members of the group, such as cesium and rubidium, to have properties similar to sodium and potassium. Chlorine is found at the other end of the table in a group labeled 17.
Some of the groups are given distinctive names, mostly related to an important property of the elements in the group. For example, the group 17 elements are called the halogens, a term derived from Greek, meaning salt former.
Each element is listed in the periodic table by placing its symbol in the middle of a box in the table. The atomic number (Z) of the element is shown above the symbol, and the weighted-average atomic mass of the element is shown below its symbol. Some periodic tables provide other information, such as density and melting point, but the atomic number and atomic mass are generally sufficient for our needs.
Elements with atomic masses in parentheses, such as plutonium, Pu (244), are produced synthetically, and the number shown is the mass number of the most stable isotope. It is customary also to divide the elements into two broad categories metals and nonmetals. In figure below, colored backgrounds are used to distinguish the metals (tan) from the nonmetals (blue and pink).
Except for mercury, a liquid, metals are solids at room temperature. They are generally malleable (capable of being flattened into thin sheets), ductile (capable of being drawn into fine wires), and good conductors of heat and electricity, and have a lustrous or shiny appearance.
The properties of nonmetals are generally opposite those of metals; for example, nonmetals are poor conductors of heat and electricity. Several of the nonmetals, such as nitrogen, oxygen, and chlorine, are gases at room temperature.
Some, such as silicon and sulfur, are brittle solids. One bromine is a liquid. Two other highlighted categories in the figure are a special group of nonmetals known as the noble gases (pink), and a small group of elements, often called metalloids (green), that have some metallic and some nonmetallic properties.
The horizontal rows of the table are called periods. (The periods are numbered at the extreme left in the periodic table inside the front cover.) The first period of the table consists of just two elements, hydrogen and helium.
This is followed by two periods of eight elements each, lithium through neon and sodium through argon. The fourth and fifth periods contain 18 elements each, ranging from potassium through krypton and from rubidium through xenon.
The sixth period is a long one of 32 members. To fit this period in a table that is held to a maximum width of 18 members, 15 members of the period are placed at the bottom of the periodic table. This series of 15 elements start with lanthanum and these elements are called the lanthanides.
The seventh and final period is incomplete (some members are yet to be discovered), but it is known to be a long one. A15-member series is also extracted from the seventh period and placed at the bottom of the table. Because the elements in this series start with actinium they are called the actinides.
The labeling of the groups of the periodic table has been a matter of some debate among chemists. The 1-18 numbering system used is the one most recently adopted. Group labels previously used in the United States
consisted of a letter and a number, closely following the method adopted by Mendeleev, the developer of the periodic table.
As seen in the figure, the A groups 1 and 2 are separated from the remaining Agroups (3 to 8) by B groups 1 through 8. The International Union of Pure and Applied Chemistry (IUPAC) recommended the simple 1 to 18 numbering scheme in order to avoid confusion between the American number and letter system and that used in Europe, where some of the Aand B designations were switched! Currently, the IUPAC system is officially recommended by the American Chemical Society (ACS) and chemical societies in other nations. Because both numbering systems are in use, we show both in the figure and in the periodic table inside the front cover.
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