Introduction To Crystal Chemistry PDF

INTRODUCTION TO CRYSTAL CHEMISTRY Atoms, Elements, Ions and Bonding What is Crystal Chemistry?  study of the atomic structure, physical properties, and chemical composition of crystalline material  basically inorganic chemistry of solids Chemical Layers of the Earth Bulk Earth Composition...

INTRODUCTION TO CRYSTAL CHEMISTRY Atoms, Elements, Ions and Bonding What is Crystal Chemistry?  study of the atomic structure, physical properties, and chemical composition of crystalline material  basically inorganic chemistry of solids Chemical Layers of the Earth Bulk Earth Composition 35% 30% 10% 10% 15% Average composition of the Earth’s Crust (by weight, elements, and volume)  Crustal composition  98.5% of the crust is comprised of just 8 elements.  Oxygen is (by far!) the most abundant element in the crust.  This reflects the importance of silicate (SiO2-based) minerals.  As a large atom, oxygen occupies ~93% of crustal volume.  The Atom  Nucleus - contains most of the weight (mass) of the atom - composed of positively charge particles (protons) and neutrally charged particles (neutrons)  Electron Shell The Bohr Model - insignificant mass - occupies space around the nucleus defining atomic radius - controls chemical bonding behavior of atoms  Elements & Isotopes  Elements are defined by the number of protons in the nucleus (atomic number).  In a stable element (non-ionized), the number of electrons is equal to the number of protons  Number of protons = Number of electrons  Number of protons = atomic number  Number of protons + Number of neutrons = atomic weight  Isotopes are atoms of the same element with differing numbers of neutrons. i.e. the number of neutrons may vary within atoms of the same element  Bohr Atom  Electrons orbit around the nucleus in different shells, labeled from the innermost shell as K, L, M, N, etc.  Each shell is associated with a principal quantum number, n, where nK = 1, nL = 2, nM = 3, nN = 4, etc.  Bohr Atom  The number of electrons in each shell is controlled by this principal quantum number by the following relationship # electrons = 2n2  K-shell can contain 2 electrons, the L-shell, 8 electrons, the M- shell, 18 electrons, and the N- shell, 32 electrons.  Electron Shells, Subshells Principal Quantum Sub-shell designation Maximum number of Number (n) electrons 1 (K) 1s 2 2 (L) 2s 2 8 2p 6 3 (M) 3s 2 3p 6 18 3d 10 4 (N) 4s 2 4p 6 32 4d 10 4f 14  Filling up the K-shell completely filled Orbitals Controlled by the energy of the L-shell completely orbitals filled 3p-orbitals completely filled # of Electrons in Outermost Shell Noble Gases Anions --------------------Transition Metals------------------ Primary Shell being filled  Ions  Cations – elements prone to give up one or more electrons from their outer shells; typically a metal element  Anions – elements prone to accept one or more electrons to their outer shells; always a non-metal element  Ionization Potential – measure of the energy necessary to strip an element  Electronegativity – measure strength with which a nucleus attracts electrons to its outer shell  Valence State (or oxidation state) – the common ionic configuration(s) of a particular element determined by how many electrons are typically stripped or added to an ion  First Ionization Potential Noble gas-high ionization potential-stable structure Group I-very low First I. P. Group II-Low first and second I. P. Group VII-Very High First I. P. Ionization Potential – measure of the energy necessary to strip an element of its outermost electron  Transition elements have d-orbital electrons in their outermost shells, and have low to high first ionization potentials their behavior is somewhat variable.  Elements in the third column tend to become +3 ions  The 4th column tend to become +4 ions (Ti  But the 5th through 11th column show variable ions.  Electronegativity Electronegativity – measure strength with which a nucleus attracts electrons to its outer shell Valence States of Ions common to Rock-forming Minerals Cations Anions Anionic Groups – tightly bound ionic complexes with net negative charge  Chemical Bonding ◼ Bonding forces are electrical in nature (related to charged particles) ◼ Bond strength controls most physical and chemical properties of minerals (in general, the stronger the bond, the harder the crystal, higher the melting point, and the lower the coefficient of thermal expansion) ◼ Five general types bonding types: ◼ Ionic ◼ Covalent ◼ Metallic ◼ van der Waals ◼ Hydrogen ◼ Commonly different bond types occur in the same mineral  Ionic Bonding  Common between elements that will...  easily exchange electrons so as to stabilize their outer shells (i.e. become more inert gas-like)  create an electronically neutral bond between cations and anions  Example: NaCl  Properties of Ionic Bonding  Dissolve easily in polar solvents like water (H2O is a polar solvent because the hydrogen ions occur on one side the water molecule and give it a slight positive charge while the other side of the water molecule has a slight negative charge)  Tend to form crystals with high symmetry  Moderate hardness and density  High melting temperatures  Generally poor conductors of heat and electricity (they are good materials for thermal and electrical insulation)  Covalent Bonding  formed by sharing of outer shell electrons  Strong chemical bond  Produces minerals that are insoluble, high melting points, hard, nonconductive (due to localization of electrons), have low symmetry (due to directional bonding  common among elements with high numbers of vacancies in the outer shell (e.g. C, Si, Al, S  Covalent Bonding  Elements near the right hand side of the periodic table tend to bond to each other by covalent bonds to form molecules that are found in crystal structures.  A Carbon atom can share electrons with 4 other Carbon atoms to form covalent bonds Tendencies for Ionic vs. Covalent Pairing Ionic Pairs Covalent Pairs  Ionic-Covalent The degree of ionic character (exchange rather than sharing) can be estimated Gradation from the contrasting electronegativity (ability to attract electrons) of the elements involved (electronegativity value is not a fixed number and can change)  Metallic Bonding  positively charge atomic nuclei share electrons in their electron clouds freely.  In a sense, each atom is sharing electrons freely with other atoms, and some of the electrons are free to move from atom to atom.  Since some of the electrons are free to move, metallically bonded materials have high electrical conductivity  Properties of metallic bonding  Low to Moderate hardness.  Usually very malleable and ductile.  Good thermal and electrical conductors.  Soluble only in acids.  Crystals with high symmetry.  van der Waals (Residual) Bonding  created by weak bonding of oppositely dipolarized electron clouds  commonly occurs around covalently bonded elements  produces solids that are soft, very poor conductors, have low melting points, low symmetry crystals  Hydrogen Bonding  These occur in the special case of hydrogen, because H has only one electron.  When Hydrogen gives up this electron to become H+1ion or shares its single electron with another atom in a covalent bond, the positively charged nucleus of the hydrogen atom is exposed, giving that end of the H ion a residual +1 charge.  This is what causes the H2O molecule to be a polar molecule Different Bonding in minerals  Graphite – covalently bonded sheets of C loosely bound by van der Waals bonds.  Mica – strongly bonded silica tetrahedra sheets (mixed covalent and ionic) bound by weak ionic and hydrogen bonds  Cleavage planes commonly correlate to planes of weak ionic bonding in an otherwise tightly bound atomic structure Summary of Bonding Characteristics  Coordination Number  The ion geometry can be simply considered to be spherical  Spherical ions will geometrically pack (coordinate) oppositely charged ions around them as tightly as possible while maintaining charge neutrality  For a particular ion, the surrounding coordination ions define the apices of a polyhedron  The number of surrounding ions is the Coordination Number  The coordination number depends on the relative size of the atoms or ions  Atomic and Ionic Radii  The size of an atom or ion depends on the size of the nucleus and the number of electrons.  Generally atoms with higher numbers of electrons have larger radii than those with smaller numbers of electrons.  Thus ions will have radii different from the atoms because ions will have either gained or lost electrons.  As the charge on the ion becomes more positive, there will be less electrons and the ion will have a smaller radius.  As the charge on the ion becomes more negative, there will be more electrons and the ion will have a larger radius.  As the atomic number increases in any given column of the Periodic Table, the number of protons and electrons increases and thus the size of the atom or ion increases.  Atomic and Ionic Radii  The radii increase with increasing total number of electrons downward in table.  Ionic radius also increases with increasing coordination number, the electron cloud is drawn out by the presence of more surrounding ions.  Atomic and Ionic Radii  As the charge becomes more positive the radius of the cation decreases.  This is because there are fewer electrons in the outer shells of the ions.  The sizes of anions are relatively large because there are more electrons in their outermost shells.  Atomic and ionic radii also depend on the type of bonding that takes place between the constituents, and on the coordination number. Thus, atomic and ionic radii will vary somewhat as a function of the environment in which the atoms or ions are found.  12-fold coordination  Coordination number, C.N. depends on the relative size of the ions (Rx / Rz = 1).  If all of the atoms in a crystal are the same size, then there are two ways to pack the atoms to form a crystal structure.  In this case, the maximum number of atoms that be coordinated around any individual is 12.  8-fold / Cubic coordination No rattle limit = 0.732  6-fold / Octahedral coordination No rattle limit = 0.414  Other coordinations  4-fold coordination  “No rattle limit” = 0.225  Rx / Rz = 0.225  3-fold coordination  “No rattle limit” = 0.155  2-fold coordination “ No rattle limit