States of MatterSolidLiquidGasMineral
Structure of an Atomprotonsneutronsquarksatomic numberatomic mass numbereight most abundant elements in the Earth"s crust
|1s (max = 2) Total = 2||L||2s (max = 2), 2p (max = 6) Total = 8||M||3s (max = 2), 3p (max = 6), 3d (max = 10) Total = 18|
Chemical BondingMost elements in the Earth react to form compounds although there are a few which are stable as elements (gold, for example). There are several "bonding models" which need to be summarized.Ionic Bonding - Element 11 (Sodium) has a single valence electron which can be relatively easily lost as it is relatively far from the positive charges in the nucleus. Oxygen needs (atomic number 8) needs two electrons to give it full s and p subshells. The compound Na2O consists of two Sodium cations and one Oxygen anion. Each Sodium contributes a single electron to the Oxygen giving the Oxygen a charge of -2. This compound is "held together" by ionic bonds.Covalent Bonding - Carbon contains 6 electrons and 4 of them are in the outer most level (the L level). Two carbons could bond by sharing their four electrons which would create the full s and p subshells in the L main shell. Covalent bonding involves sharing electrons.Metallic Bonding - Metals are known for their ability to conduct the flow of electrons. Metallic Bonding involves a "smearing out" of the valence electrons of the metal atoms. These electrons are easily displaced.van der Waals Bonding The carbon atoms in graphite are covalently bonded to form sheets of carbon atoms. The sheets are held together by weak attractive forces. The Periodic Table provides a framework in which to place thechemical elements so that their similarities are recognized. Look at the properties of Oxygen and Silicon - the two most abundant elements in the Earth"s crust - by clicking on their symbols on the Periodic Table. The recognition that the elements could be arranged in a systematic way so as to emphasize relationships between elements, was a major break through in the history of chemistry. For example, all of the elements in the first column (the alkali metals) have a singleoutermost electron in its outermost sub shell (an s sub shell).All of these elements can lose a single electron forming a cation with a +1 charge. All of the elements in the column on the far right (inert or Nobel gases) have two s electrons and eight p electrons in their outermost level (called the valence level). Note that the Periodic Table has the shape of a distorted "H". The vertical bars (sides of the H) contain the A group elements. The central bar contains the "transition" elements. Down at the bottom of the page are two rows - the "Lanthanides" and the "Actinides". Look to see where these rows fit into the "H". If these rows which shown in proper position the Periodic Table would be less compact.
The Chemical Composition of the Continental CrustEight elements make up about 99% of the weight of the continental crustOxygen ~ 50%Silicon ~ 25%AluminumIronCalciumSodiumMagnesiumPotassium
Mineral StructuresThink about the statement that oxygen occupies 95% of the volume of the Earth"s crust. If oxygens were cubes they could be packed together to fill up space. However, the oxygens are presumed to be spheres and you cannot pack equal sized spheres to fill up all space; some open spaces will remain inside of the framework produced by the oxygens. Other ions fit into these open spaces. In general, these spaces are "regular".
One common type of space is that created when there are three oxygens on the bottom and one on the top. This is called a "tetrahedral void". (A tetrahedron is a regular solid consisting of four faces, each of which is an equilateral triangle.) The larger the ion the larger the preferred site. The coordination number of a cation is the number of nearest neighbor anions. Silicon, with one exception, prefers to "sit" in a tetrahedral void formed by packing Oxygen anions together. Thus, silicon usually has a coordination number of 4. Coordination numbers are used to produce a structural formula. Remember that the subscripts give the number of ions in one formula unit and the numbers above the chemical symbols given the coordination numbers.Quartz4SiO2Stishovite6SiO2Quartz and Stishovite are polymorphs (many forms). They have identical chemical compositions but differ in structure and hence in physical properties. An increase in pressure favors a larger coordination number. Temperature has the reverse effect. If a large meteorite were to impact quartz then stishovite might form (if the pressure were sufficiently high).Other polymorphs are:diamond and graphite (both carbon)calcite and aragonite (both calcium carbonate)
Calcite and aragonite differ in structure in that in calcite each calcium ion is surrounded by 6 nearest neighbor oxygens. This is an octahedral structure. In aragonite each calcium ion is surrounded by 9 nearest neighbor oxygens. Which of these two minerals has the highest density?aragonite calciteIn the definition of a mineral it was noted that a mineral has a fixed composition (like quartz) or a composition that is variable within a limit. Consider the two minerals Forsterite and Fayalite.Forsterite6 4Mg2SiO4Fayalite6 4Fe2SiO4Note that the two minerals differ chemically in that one has Mg (magnesium) and the other Fe (iron). The rest of the chemical formulas are identical. Note that both Mg and Fe sit in 6 fold sites (octahedral). The sizes of these two ions are nearly identical, the charge on both is +2 and the Mg-O and Fe-O bonds are dominately ionic in nature. When such similarities occur the ions may substitute for each other forming a solid solution series. The series between these two end members is called the olivine series. Compositions range from pure Forsterite to pure Fayalite.Not all solid solution series are complete; some exhibit a limited amount of substitution.Almost all of the common minerals we will work with are solid solution series. Quartz is the notable exception.