Last Time We saw that minerals are crystals, and crystals are made of unit cells, arrangements of atoms that, when stacked in 3 dimensions, form the complete crystal

Last Time The edges of the unit cells, and therefore the crystal, are parallel to a system of crystal axes. Positive a is to the front, b to the right, and c is up. The angle between b and c is called a, the angle between a and c is called b, and the angle between a and b is called g The most pronounced zone is oriented vertically and an axis parallel to that zone is called c

Last Time Six or seven crystal systems cover all possibilities Once c is selected, other axes are drawn parallel to a prominent face Last Time Six or seven crystal systems cover all possibilities =

Last Time We also worked out a notation for the orientation of a crystal face, Miller Indices, and learned how to plot them on a plane http://www.gly.uga.edu/Schroeder/geol6550/millerindices.html

Lecture 2 Crystal Chemistry Part 1: Composition of the Earth Elements, and Ions

Chemical Layers of the Earth SiO2 – 45% MgO – 37% FeO – 8% Al2O3 – 4% CaO – 3% others – 3% Fe – 86% S – 10% Ni – 4% Earth radius ~4000 miles, core about half way down (1800 miles)

Composition of the Earth’s Crust Most common silicates are from these O alone = 94 vol. % of crust Perhaps useful to think of the crust as a packed O array with interspersed metal cations in the interstices! Analogy works for minerals too (they make up the crust)

Chemistry Review Bohr model for the atom 1. Nucleus = p + n. (#protons = Atomic # identifies “element”) (nucleus ~ all mass) Nucleus gives elements their properties p + n (variable)  atomic weight (isotopes) At. Wt. is real # due to average of isotopes 2. Electrons e- spin around atom and give it its’ size Atomic radii in the range 0.5-2.5 Å (1 ångström = 1×10−10 m) e- in special shells w/ particular energy levels- quantized Beryllium 1s2 2s2

The Atom The Bohr Model The Schrödinger Model Nucleus Hydrogen Hydrogen The Bohr Model The Schrödinger Model Nucleus - contains most of the weight (mass) of the atom - composed of positively charge particles (protons) and neutrally charged particles (neutrons) Electron Shell - insignificant mass - occupies space around the nucleus defining atomic radius - controls chemical bonding behavior of atoms

Electrons are in shells. Octet Rule (Sienko & Plane p 55: When atoms combine, the bonds formed are such that each atom is surrounded by 8 e-

2. The s orbitals fill a shell first, then the p-orbitals. p-orbitals s orbitals have up to 2 e- p orbitals have up to 6 e- s - orbitals 2. The s orbitals fill a shell first, then the p-orbitals. 1. Within each shell, electrons move in orbitals, volumes where an electron is most probably located. p-orbitals First they occupy separate sub-orbitals, then when all are occupied they pair. d-orbitals have up to 10 e- of Nesse

Filling up the Orbitals Electrons occupy orbitals in order of energy level Order controlled by the energy of the orbitals Cl =1s22s22p63s23p5 =17 e- Notice 4s fills before 3d because 4s has a lower energy also 5s fills before 4d

Problem 1 a. What is the electron configuration of a neutral atom of Calcium, element 20: Answer: Ca = 1s2 2s2 2p6 3s2 3p6 4s2 b. What is the electron configuration of a neutral atom of Aluminum, element 13 Answer: Al = 1s2 2s2 2p6 3s2 3p1 Beyond Calcium, d-orbitals fill in complex ways, in order of orbital energy. C. The order of increasing energy is 1s 2s 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f Nesse p.41

This is especially true for the outermost shell Characteristics of an atom depend a lot on electron configuration Atoms with a different numbers of protons & electrons, but with similar electron configurations, have similar properties This is especially true for the outermost shell  Periodic Table

Structure of the Periodic Table # of Electrons in Outermost Shell Noble Gases metals Anions --------------------Transition Metals------------------ NOTE : Primary Shell being filled Column number gives electrons in outermost shell; predicts Ion valence

inert gases (VIII) have 8e- ... filled s & p It is the outermost shell or valence electrons that predicts bonding behavior Similar outermost shell configurations  Groups in the Periodic Table columns alkali metals (I) have one e- in outer shell halogens (VII) have 7 e- inert gases (VIII) have 8e- ... filled s & p

Problem 2 A neutral atom of Calcium has the electronic configuration of: 1s2 2s2 2p6 3s2 3p6 4s2 Question: Which are the valence electrons? Answer: the 4s2. The Calcium can lose these rather easily, yielding a Ca++ ion.

Atoms of Geologic Importance Important elements are abundant

Elements and Isotopes Elements are defined by the number of protons in the nucleus (atomic number). In a stable element (zero charge), the number of electrons is equal to the number of protons The various isotopes of a particular element are defined by the total number of neutrons in addition to the number of protons in the nucleus (isotopic number). Various elements can have multiple (2-38) stable isotopes, some of which are unstable (radioactive) Isotopes of a particular element have the same chemical properties, but different masses.

Ions and Valence States Elements can attain an inert gas configuration (octet rule) Cations – “positive ion” one or more electrons removed from outer shell, often a metal, e.g. Na+, Fe++, Fe+++, Mg++, Ca++, S+6 Anions – “negative ion” one or more electrons added to outer shell; always a non-metal element, example Cl-, S-- Valence State (or oxidation state) – the common ionic configurations of a particular element -determined by how many electrons are typically stripped or added to form an ion

Valence States of Ions and Anionic Groups common to Rock-forming Minerals Cations – metals, transition metals, semiconductors and nonmetals that have lost electrons Anions – nonmetals with extra electrons Anionic Groups tightly bound ionic complexes with net negative charge, SO4-2, CO3-2, NO3-, SiO4-4, PO4-3 +1 +2 +3 +4 +5 +6 +7 -2 -1 -----------------Transition Metals---------------

Problem 3 A neutral atom of Calcium has the electronic configuration of: 1s2 2s2 2p6 3s2 3p6 4s2 a. What is the electron configuration for the Ca++ ion? b. How many electrons does Ca++ have in its outermost shell?

Alkalis have one extra electron, can lose it to attain outer octet Alkalis have one extra electron, can lose it to attain outer octet. This results in an ion with a +1 valence, Na+ Group II alkaline earths will lose 2 e-  +2 Become a +2 valence ion, Ca++ Halogens will capture an e-  inert gas configuration.  -1 valence ion. Cl-

Properties derived from outer e- Ionization potential  energy required to remove the least tightly bound electron Electron affinity  energy given up as an electron is added to an element Electronegativity  quantifies the tendency of an element to attract a shared electron when bonded to another element.

In general, first ionization potential, electron affinity, and electronegativities increase from left to right across the periodic table, and to a lesser degree from bottom to top.

Ionic vs. Covalent Elements on the right and top of the periodic table draw electrons strongly Bonds between atoms from opposite ends more ionic, diatomics are 100% covalent Bond strength  Covalent>Ionic>metallic Affects hardness, melting T, solubility Bond type affects geometry of how ions are arranged More ionic vs. covalent = higher symmetry Go over this on a periodic table – this is electronegativity argument essentially Test them a little – SiO2, CaO, AsS (realgar), ZnS (sphalerite) more or less ionic – properties? More ionic  less strength  melts lower, more soluble Realgar much softer than Sphalerite

Atomic Radius A function partly of shielding, size is critical in thinking about substitution of ions, diffusion, and in coordination numbers

Again: Outer Electron stuff Ionization Potential – measure of the energy necessary to strip an element of its outermost electron. Low in alkali metals, e.g. Sodium Na => Na+ Electronegativity – measure strength with which a nucleus attracts electrons to its outer shell. High in the abundant element oxygen.

Electronegativity (e-neg) Metals w/ e-neg < 1.9 thus lose e- and  cations Nonmetals > 2.1 thus gain e- and  anions Metalloids intermediate (B, Si, Ge, As, Sb, Te, Po)

Small Cations (Oxide Ion) Ferric Ion Note Sizes Huge K+ needs a large space between anions Medium Cations Ferrous Ion 1.Cations are smaller than their neutral counterpart. Electrons lost, so outer electron cloud is smaller. 2.Anions are much larger than their neutral counterpart. Electrons gained, so outer electron cloud is larger. Large Cations Huge Cation

Problem 4 Write down the common cations of Silicon Si, Aluminum Al, Iron Fe (there are two, ferrous and ferric Iron), Magnesium Mg, Sodium Na, Calcium Ca, Potassium K, and Sulfur. Write down the common anions of Oxygen, Chlorine, and Sulfur.

The sizes vary with Coordination, the number of touching neighbors Unit Cell The Cations fill the space available between the Anions

Next Lecture Bonding and Ionic Radii