Chemical bonds Chemical bond: force of attraction between two or more atoms/ions Types of bonds in crystals: Ionic bond: electrostatic attraction between two oppositely charged ions. This type of bond involves electron transfer and is non-directional. The ionic bond predominates between elements whose electronegativity differs by 2 or more Covalent bond: forms between elements with high but nearly equal electronegativity. This type of bonding involves sharing of valence electrons In some minerals, other (less important) bond types include: Hydrogen bonds: when H forms a covalent bond its single electron is largely confined to the overlap zone leaving the proton unshielded and able to attract negative ions. Metallic bonds: the outer shell of atoms in metals contain loosely held valence electron which can be shared among neighbor metal atoms forming a weak bond Van der Waals bonds: weak bond observed between electrically neutral molecules which may develop a slight positive charge at one end and a slight negative charge at the other end forming a weak dipole. The attraction between these molecules forms a very weak Van der Waal bond.
Li: 1s 2 2s 1 F: 1s 2 2s 2 2p 5
Covalent bonding Cl atoms each with 7 electrons in M shell L K M L K M Cl (Z = 17) can achieve the inert gas configuration of Ar (Z = 18) with 8 electrons in its valence (M) shell by sharing one electron from each atom forming the stable molecule Cl 2 Covalent bonds form in elements, e.g., O 2 and carbon compounds and between elements with identical or similar electronegativity Covalent bonds are strong and are directional Shared electrons
Covalent bonding in diamond Hybrid orbitals Carbon: 1s 2s 2p 1s 2(sp 3 ) The two 2s and the two 2p orbitals hybridize to form four equivalent sp 3 orbitals pointing to the corners of a tetrahedron C-C-C angle = 109 o 28 Bonds are directional and very strong
Bonding in graphite A different type of hybrid orbital occurs in graphite Carbon: 1s 2s 2p 1s 2(sp 2 ) 2p Three (2sp 2 ) orbitals are coplanar & 120º apart, forming sheets.
Covalent vs ionic bonding
What if electronegativity has a value slightly lower than 2? E.g., between Si and O? Half covalent - half ionic
Intermediate bonding techniques Figure 3.9 of Nesse s book
Abundance of elements The most abundant elements in the Earth are O and Fe (both close to 32%), Mg (~15%), Si (~14%), Ni (~1.8%), Ca (1.7%), and Al (1.6%). The majority of the Earth s Fe and Ni are in the core. The remaining rocky part of the Earth, the mantle and crust, consists of ~44% O, ~23% Mg, 21% Si, 8% Fe, 2.5% Ca, and 2.4% Al. As a consequence, the outer part of the Earth consists principally of compounds known as silicates. (White s book Chap 1)
Abundance of elements earth s crust
Table of atomic and ionic radii. As a general rule, the fewer the number of electrons the smaller the ionic radius. Also, group 1 elements will generally be the largest and group 7 the smallest. 15 C 4+ 42 Si 4+ Units are picometers (1 pm = 10-12 m). (1Å = 10-10 m) Also check White s book (chapter 1)
Sizes of atoms and ions
Crystal structure (Chap. 4) How are mineral structures illustrated?
Atoms arrangements in molecules In halite structure each Na + ion is surrounded by 6 Cl - anions and each Cl - anion is surrounded by 6 Na + cations. The Cl ions are located at the corners of a regular octahedron as are the Na ions: The ionic radius of Na + is 99 picometers (0.99Å) and the radius of Cl - is 181 pm (1.81Å) Halite Cl Na Cl Cl Cl
Space lattice and unit cell of NaCl: On a model of the structure of halite we can identify all the points where a Cl ion is in contact with a Na ion directly above. If we remove the ions we are left with a space lattice (scaffolding) composed of identipoints as shown on the right. The smallest unit in this lattice which can, by translation, build up the entire lattice is known as the unit cell. In this case, the unit cell is a cube with 8 identipoints at the corners and 6 identipoints in the middle of each face. Identipoint at corner of unit cell Identipoint at center of face of unit cell From Bloss (1971) Crystallography and crystal chemistry (Fig 6.6)
Ball and stick model of a unit cell of halite with Na ions at the corners of the cube and in the middle of each cube face. This unit cell is called face-centered isometric. Ions shown reduced in size in order to show their spatial distribution. a = 5.64 Å (dimension of unit cell).
Silicate Mineral Structures The basic building block of silicate structures is the silica tetrahedron [SiO 4 ] 4-. Silicate minerals are classified on the basis of the linkages of these tetrahedra. Ball and stick model of tetrahedron Polyhedral model of tetrahedron Si has a coordination number of 4 Si-O bond is ~50% ionic and ~50% covalent