States of Matter


Structure of an Atom

protonsneutronsquarksatomic numberatomic mass numbereight most abundant elements in the Earth"s crust . Calcium is atomic number 20. Therefore, There are 20 neutrons in the nucleus of a Calcium atomThere are 20 protons in the nucleus of a Calcium atomA Calcium atom weighs 20 atomic mass unitsIf we could weigh individual atoms of calcium we would find some variation in their mass; this must be due to variations in the number of neutrons in their nucleus as all must have the same number of protons - 20. Atoms which have the same atomic number butdifferent mass numbers are isotopes.For example, all calcium atoms have an atomic number of 20 but there are three isotopes of calcium - 39, 40, and 42. (How many neutrons does each isotope possess?)A calcium atom is found with a mass of 42. Therefore, there are 20 protons and 22 neutrons in its nucleusthere are 20 neutrons and 22 protons in its nucleusthere are 20 protons and 42 neutrons in its nucleus In addition to the particles in the nucleus, there are negatively charged particles - electrons which "orbit" the central nucleus. Recognition that the distribution of electrons around the nucleus is not random was one of the great accomplishments of physicists in the 1920s - quantum mechanics.Electrons occupy "levels" which are separated from each by some volume of space which electrons can pass through but not remain. In fact, much of the volume of an atom consists of empty space. Electrons are "attracted" to the nucleus (positive and negative charges attract).The primary division of these levels is called the principal quantum number. The first level (K) is closest to the nucleus.As the principal number increases 1..2..3..4 the number of electrons that can reside in the level increases. LevelElectronsK 2L 8M 18N 32The maximum occupancy = 2n2 where n is the level .. 1, 2, 3, 4, etc. In a neutral atom the number of negative particles equals the number of positive particles (electrons = protons).A atom of calcium has 20 neutrons. Therefore, its mass number is 40there are 20 protons in its nucleusthere are 20 electrons surrounding is nucleusall of the above are correctA useful exercise is to start with the first element (Hydrogen) with its one proton and one electron and add protons and electrons to build up the remaining chemical elements. The build-up process works as follows. Start with a single electron - it occupies the emptylevel closest to the nucleus. The second electron goes into the first level (atomic Helium) and theshell K) is filled. Actually, there are sub levels within the main level (except for theK-shell). Sub ShellMax Electronss 2p 6d 10f 14For element 11 (Sodium - Na) there are 11 electrons and 11 protons. Two electronsare in the K shell, 8 in the L shell and one in the M shell. The outer most electron(s) are termed the valence electrons. If one electron is removed from a sodium atom there is one more negative charge than positive charge : -1there is one more positive charge than negative charge : +1Main LevelOrbitals K
1s (max = 2) Total = 2 L2s (max = 2), 2p (max = 6) Total = 8 M3s (max = 2), 3p (max = 6), 3d (max = 10) Total = 18
is a charged atom; that is, there is an excess of positive (cation)or negative charge (anion). If an electron is removed a cation is formed. If an electron is added an anion is formed. Of the eight most abundant elements in the Earth"s crust only Oxygen forms anions; the rest form cations by loosing one or more electrons. When there are 8 outer most electrons in the s and p sub shells theatom has a tendency to resist change and ions are formed with great difficulty if at all.An atom of element Z has 40 protons. Therefore, the outer most, valence electrons are in which main shell?KLMNHow many of the 40 electrons are in the outer most level?12102240

Chemical Bonding

Most 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 Crust

Eight elements make up about 99% of the weight of the continental crustOxygen ~ 50%Silicon ~ 25%AluminumIronCalciumSodiumMagnesiumPotassium

Mineral Structures

Think 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.

Classification of Minerals

The broadest classification of the nearly 3,500 known minerals is based on chemical composition. Thus, we recognize Native Elements (individual chemical elements), Carbonates (containing the CO3 group, Silicates (containing Silicon and Oxygen) and other broad chemical groups. Chemically, the silicates are very complicated and not much progress was made in understanding until a structural classification was devised. The most common "structural element" is the silicon/oxygen tetrahedron. The simplest structural class of silicates consists of those compounds (minerals) which consist of isolated single tetrahedra - the Nesosilicates. Tetrahedra can share oxygens between themselves. Two, three and four oxygens per tetrahedron can be shared and in some structures two or more sharing schemes exist. The most common minerals are listed below. (keep in mind that all of these contain Si and O and many also contain Al).MineralStructural TypeCompositionOlivine Isolated Fe and Mg richPyroxene Single Chains Ca, Fe, and Mg richAmphibole Double Chains Ca, Fe, Mg and K rich (with "OH")Mica Sheet Silicates K, Na rich (with "OH") Plagioclase Tectosiicates Ca and Na rich Alkali Feldspar Tectosilicates Na and K rich Quartz Tectosilicates SiIn addition, the following mineral groups are important - especially in the sedimentary rocksCarbonatesSulfatesSalts Silicate Structural Classification.


Use the section on Properties to help answer the following questions.Distinguish between fracture and cleavageWhat is luster?Quartz has a hardness of 7 and Talc has a hardness of 1. Is Quartz seven times harder than Talc? Explain what type of scale Moh"s Scale is and how to use it. How is a diamond "cut"?What is specific gravity? The specific gravity of the continental crust is about 2.7 whereas the specific gravity of the oceanic crust is 3.0. Quartz and Feldspar are the two most abundant minerals in the Earth"s crust. How does this fit with the list of most abundant elements in the Earth"s crust?Continental Crust - Alkali FeldsparOceanic Crust - Plagioclase_________________________________________________________________________________________________| ?SUBJECT=OnlinePhysical" > jbutler